IPCS INCHEM Home



    UNITED NATIONS ENVIRONMENT PROGRAMME
    INTERNATIONAL LABOUR ORGANISATION
    WORLD HEALTH ORGANIZATION


    INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY


    ENVIRONMENTAL HEALTH CRITERIA 212



    PRINCIPLES AND METHODS FOR ASSESSING ALLERGIC
    HYPERSENSITIZATION ASSOCIATED WITH EXPOSURE
    TO CHEMICALS






    This report contains the collective views of an international group of
    experts and does not necessarily represent the decisions or the stated
    policy of the United Nations Environment Programme, the International
    Labour Organisation, or the World Health Organization.



    Published under the joint sponsorship of the United Nations
    Environment Programme, the International Labour Organisation, and the
    World Health Organization, and produced within the framework of the
    Inter-Organization Programme for the Sound Management of Chemicals.



    World Health Organization
    Geneva, 1999





    The International Programme on Chemical Safety (IPCS), established in
    1980, is a joint venture of the United Nations Environment Programme
    (UNEP), the International Labour Organisation (ILO), and the World
    Health Organization (WHO).  The overall objectives of the IPCS are to
    establish the scientific basis for assessment of the risk to human
    health and the environment from exposure to chemicals, through
    international peer review processes, as a prerequisite for the
    promotion of chemical safety, and to provide technical assistance in
    strengthening national capacities for the sound management of
    chemicals.

    The Inter-Organization Programme for the Sound Management of Chemicals
    (IOMC) was established in 1995 by UNEP, ILO, the Food and Agriculture
    Organization of the United Nations, WHO, the United Nations Industrial
    Development Organization, the United Nations Institute for Training
    and Research, and the Organisation for Economic Co-operation and
    Development (Participating Organizations), following recommendations
    made by the 1992 UN Conference on Environment and Development to
    strengthen cooperation and increase coordination in the field of
    chemical safety.  The purpose of the IOMC is to promote coordination
    of the policies and activities pursued by the Participating
    Organizations, jointly or separately, to achieve the sound management
    of chemicals in relation to human health and the environment.

    WHO Library Cataloguing-in-Publication Data

    Principles and methods for assessing allergic hypersensitization
    associated with exposure to chemicals.

         (Environmental health criteria ; 212)

         1.Hypersensitivity - chemically induced  2.Immune tolerance 
         3.Autoimmunity - physiology  4.Immunologic tests  
         5.Environmental exposure   6.Occupational exposure   7.Risk
         assessment - methods
         I.International Programme on Chemical Safety II.Series

         ISBN 92 4 157212 4             (NLM Classification: QW 900)
         ISSN 0250-863X

         The World Health Organization welcomes requests for permission to
    reproduce or translate its publications, in part or in full. 
    Applications and enquiries should be addressed to the Office of
    Publications, World Health Organization, Geneva, Switzerland, which
    will be glad to provide the latest information on any changes made to
    the text, plans for new editions, and reprints and translations
    already available.

    (c) World Health Organization 1999

         Publications of the World Health Organization enjoy copyright
    protection in accordance with the provisions of Protocol 2 of the
    Universal Copyright Convention.  All rights reserved.

         The designations employed and the presentation of the material in
    this publication do not imply the expression of any opinion whatsoever
    on the part of the Secretariat of the World Health Organization
    concerning the legal status of any country, territory, city, or area
    or of its authorities, or concerning the delimitation of its frontiers
    or boundaries.

         The mention of specific companies or of certain manufacturers'
    products does not imply that they are endorsed or recommended by the
    World Health Organization in preference to others of a similar nature
    that are not mentioned. Errors and omissions excepted, the names of
    proprietary products are distinguished by initial capital letters.

    CONTENTS

    PRINCIPLES AND METHODS FOR ASSESSING ALLERGIC HYPERSENSITIZATION
    ASSOCIATED WITH EXPOSURE TO CHEMICALS

    PREAMBLE

    ABBREVIATIONS

    PREFACE

    1. THE IMMUNE SYSTEM

         1.1. Introduction

              1.1.1. Evolution and function of the adaptive immune
                        system
              1.1.2. Immunosuppression, immunodeficiency and
                        autoimmunity
              1.1.3. Allergy and allergic diseases
              1.1.4. Conclusion

         1.2. Physiology and components of the immune system

              1.2.1. T-cells
                        1.2.1.1   Balancing the immune response
              1.2.2. B-cells
              1.2.3. Macrophages
              1.2.4. Antigen-presenting cells
                        1.2.4.1   Co-stimulatory molecules in T-cell
                                  activation
              1.2.5. Adhesion molecules
              1.2.6. Fc receptors
              1.2.7. Polymorphonulear leukocytes
              1.2.8. Cytotoxic lymphocytes
              1.2.9. Mast cells
              1.2.10. Basophils
              1.2.11. Eosinophils
              1.2.12. Complement components
              1.2.13. Immunoglobulins
                        1.2.13.1  IgG
                        1.2.13.2  IgA
                        1.2.13.3  IgM
                        1.2.13.4  IgD
                        1.2.13.5  IgE

         1.3. Immunotoxicology

         1.4. Immunosuppression/immunodeficiency

              1.4.1. Biological basis of
                        immunosuppression/immunodeficiency
              1.4.2. Consequences of immunosuppression/immunodeficiency

         1.5. Immunological tolerance

              1.5.1. T-cell tolerance to self-antigens
              1.5.2. B-cell tolerance to self antigens

              1.5.3. Tolerance to non-self antigens
                        1.5.3.1   Scope
                        1.5.3.2   Mucosal defence against exogenous toxic
                                  pressures
                        1.5.3.3   Induction of oral tolerance
                        1.5.3.4   Factors determining the development of
                                  oral tolerance
                        1.5.3.5   Orally induced flare-up reactions and
                                  desensitization
                        1.5.3.6   Mechanisms of tolerance
                        1.5.3.7   Conclusions

    2. HYPERSENSITIVITY AND AUTOIMMUNITY --  OVERVIEW OF MECHANISMS

         2.1. Classification of immune reactions

              2.1.1. Type I hypersensitivity
                        2.1.1.1   Anaphylaxis
              2.1.2. Type II hypersensitivity
              2.1.3. Type III hypersensitivity -- immune complex
                        reaction
                        2.1.3.1   Arthus reaction
              2.1.4. Type IV -- delayed-type hypersensitivity
                        2.1.4.1   Mechanisms of allergic contact
                                  dermatitis
                        2.1.4.2   T-cell responses in chemically induced
                                  pulmonary diseases
              2.1.5. Type V stimulatory hypersensitivity

         2.2. Regulation of hypersensitivity

              2.2.1. Regulation of IgE synthesis by IL-4 and IFN-gamma
              2.2.2. Eosinophilia and IL-5
              2.2.3. The relationship between Th2 cells and type I
                        hypersensitivity
              2.2.4. IL-12 drives the immune response towards Th1
              2.2.5. IL-13, an interleukin-4-like cytokine

         2.3. Autoimmune reactions

         2.4. Possible mechanisms of autoimmune reactions

              2.4.1. Release of anatomically sequestered antigens
              2.4.2. The "cryptic self" hypothesis
              2.4.3. The self-ignorance hypothesis
              2.4.4. The molecular mimicry hypothesis
              2.4.5. The "modified self" hypothesis
                        2.4.5.1   Hapten-induced antibody responses to
                                  "modified self"
                        2.4.5.2   Hapten-induced autoantibodies that
                                  recognize "self" proteins
              2.4.6. Immunoregulatory disturbances
                        2.4.6.1   Errors in central or peripheral
                                  tolerance
                        2.4.6.2   Polyclonal activators

         2.5. Type I hypersensitivity diseases and allied disorders

              2.5.1. Asthma
                        2.5.1.1   Definition
                        2.5.1.2   Airways inflammation and asthma
              2.5.2. Occupational asthma
                        2.5.2.1   Occupational asthma and allergy
              2.5.3. Atmospheric pollutants and asthma
              2.5.4. Rhinitis
              2.5.5. Atopic eczema
              2.5.6. Urticaria
              2.5.7. Gastrointestinal tract diseases: mechanisms of
                        food-induced symptoms
                        2.5.7.1   Non IgE-mediated food-sensitive
                                  enteropathy
                        2.5.7.2   IgE-mediated food allergy
                        2.5.7.3   Role of gastrointestinal tract
                                  physiology in food allergy

         2.6. Type II hypersensitivity diseases

              2.6.1. Drug-induced Type II reactivity
              2.6.2. Transfusion reactions
              2.6.3. Autoimmune haemolytic anaemia
              2.6.4. Autoimmune thrombocytopenic purpura
              2.6.5. Pemphigus and pemphigoid
              2.6.6. Myasthenia gravis

         2.7. Type III hypersensitivity diseases

              2.7.1. Immune complex disease
              2.7.2. Serum sickness

              2.7.3. Allergic bronchopulmonary aspergillosis
              2.7.4. Extrinsic allergic alveolitis
                        2.7.4.1   Farmer's lung
                        2.7.4.2   Bird-fancier's lung

         2.8. Type IV hypersensitivity diseases

              2.8.1. Chronic beryllium disease
              2.8.2. Systemic autoimmune diseases
                        2.8.2.1   Systemic lupus erythematosus
                        2.8.2.2   Rheumatoid arthritis
                        2.8.2.3   Scleroderma
                        2.8.2.4   Sjögren's syndrome
                        2.8.2.5   Hashimoto's disease

    3. FACTORS INFLUENCING ALLERGENICITY

         3.1. Introduction

         3.2. Inherent allergenicity

              3.2.1. Inherent properties of chemicals inducing
                        autoimmunity

         3.3. Exogenous factors affecting sensitization

              3.3.1. Exposure
                        3.3.1.1   Magnitude of exposure
                        3.3.1.2   Frequency of exposure
                        3.3.1.3   Route of exposure
              3.3.2. Atmospheric pollution
                        3.3.2.1   Tobacco smoke
                        3.3.2.2   Geographical factors
              3.3.3. Metals
              3.3.4. Detergents

         3.4. Endogenous factors affecting sensitization

              3.4.1. Genetic influence
                        3.4.1.1   Contact sensitization
                        3.4.1.2   IgE-related allergy
                        3.4.1.3   Other genetic factors
              3.4.2. Tolerance
                        3.4.2.1   Orally induced flare-up reactions and
                                  desensitization
                        3.4.2.2   Non-specific and specific mechanisms of
                                  unresponsiveness
              3.4.3. Underlying disease
              3.4.4. Age
              3.4.5. Diet
              3.4.6. Gender

    4. CLINICAL ASPECTS OF THE MOST IMPORTANT ALLERGIC DISEASES

         4.1. Clinical aspects of allergic contact dermatitis

              4.1.1. Introduction
              4.1.2. Regional dermatitis
                        4.1.2.1   Hand eczema
                        4.1.2.2   Facial dermatitis
                        4.1.2.3   Other types of dermatitis
              4.1.3. Special types of allergic contact reactions
                        4.1.3.1   Systemic contact dermatitis
                        4.1.3.2   Allergic photo-contact dermatitis
                        4.1.3.3   Non-eczematous reactions
                        4.1.3.4   Allergic contact urticaria
              4.1.4. Allergic contact dermatitis as an occupational
                        disease
              4.1.5. Diagnostic methods
                        4.1.5.1   Patch testing
                        4.1.5.2    In vitro testing
              4.1.6. Assessment of exposure
              4.1.7. Treatment and prevention of allergic contact
                        dermatitis
                        4.1.7.1   Primary prevention
                        4.1.7.2   Secondary prevention
                        4.1.7.3   Ways of preventing contact sensitization
              4.1.8. Information needed for a preventative programme

         4.2. Atopic eczema (atopic dermatitis)

              4.2.1. Definition
              4.2.2. Epidemiology of atopic eczema
              4.2.3. Clinical manifestations and diagnostic criteria
                        4.2.3.1   Age-dependent clinical manifestations
                        4.2.3.2   Diagnosis of atopic eczema
                        4.2.3.3   Stigmata of the atopic constitution
                        4.2.3.4   Prognosis
              4.2.4. Etiology
                        4.2.4.1   Genetic influence
              4.2.5. Environmental provocation factors
              4.2.6. Pathophysiology
                        4.2.6.1   Dry skin
                        4.2.6.2   Autonomic dysregulation
                        4.2.6.3   Cellular immunodeficiency
                        4.2.6.4   Increased IgE production
                        4.2.6.5   Psychosomatic aspects
              4.2.7. Diagnostic approach
                        4.2.7.1   Medical history
                        4.2.7.2   Skin tests
                        4.2.7.3   Laboratory tests
                        4.2.7.4   Provocation tests

              4.2.8.    Therapeutic considerations
                        4.2.8.1   Avoidance of provocation factors
                        4.2.8.2   Basic dermatological therapy
                        4.2.8.3   Anti-inflammatory therapy
              4.2.9. Conclusion

         4.3. Allergic rhinitis and conjunctivitis

              4.3.1. Introduction
              4.3.2. Definition
              4.3.3. Clinical manifestations
                        4.3.3.1   Seasonal allergic rhinitis and
                                  conjunctivitis (hay fever, pollinosis)
                        4.3.3.2   Perennial allergic rhinitis and
                                  conjunctivitis
                        4.3.3.3   Prognosis
              4.3.4. Etiology
                        4.3.4.1   Allergic rhinitis and conjunctivitis
                                  caused by contact with chemicals
              4.3.5. Pathophysiology
              4.3.6. Diagnostic techniques
                        4.3.6.1   Medical history
                        4.3.6.2   Clinical examination
                        4.3.6.3   Allergy testing
              4.3.7. Therapeutic considerations

         4.4. Clinical aspects of allergic asthma caused by contact with
              chemicals

              4.4.1. Introduction
              4.4.2. Importance of occupational asthma
              4.4.3. Chemical causes of occupational asthma
                        4.4.3.1   Isocyanates
                        4.4.3.2   Acid anhydrides
                        4.4.3.3   Complex platinum salts
              4.4.4. Diagnosis of occupational asthma
                        4.4.4.1   Investigation of causes of occupational
                                  asthma
                        4.4.4.2   Serial peak expiratory flow (PEF) rate
                                  measurements
                        4.4.4.3   Immunological investigations
                        4.4.4.4   Inhalation challenge tests
              4.4.5. Outcome of occupational asthma
              4.4.6. Management and prevention of occupational asthma

         4.5. Food allergy

              4.5.1. Definitions
              4.5.2. IgE-mediated food allergy
                        4.5.2.1   Oral allergy syndrome

                        4.5.2.2   Allergic reactions after ingestion of
                                  food
                        4.5.2.3   Allergic reactions following skin
                                  contact with food
              4.5.3. Non-IgE-mediated immune reactions
                        4.5.3.1   Gluten-sensitive enteropathy (coeliac
                                  disease)
              4.5.4. Diagnosis of adverse food reactions
                        4.5.4.1   Case history and elimination diet
                        4.5.4.2   Skin tests
                        4.5.4.3   Specific serum IgE
                        4.5.4.4   IgG determination
                        4.5.4.5   Other  in vitro tests
                        4.5.4.6   Oral challenge tests
              4.5.5. Therapeutic considerations
              4.5.6. Prevalence
                        4.5.6.1   Introduction
                        4.5.6.2   Children
                        4.5.6.3   Adults
                        4.5.6.4   Conclusions

         4.6. Autoimmune diseases associated with drugs, chemicals and
              environmental factors

              4.6.1. Introduction
              4.6.2. Systemic lupus erythematosus
              4.6.3. Scleroderma:  environmental and drug exposure
              4.6.4. Silicone breast implants
              4.6.5. Toxic oil syndrome
              4.6.6. Eosinophilia-myalgia syndrome
              4.6.7. Vinyl chloride disease (occupational
                        acro-o-steolysis)
              4.6.8. Systemic vasculitis:  environmental factors and
                        drugs
              4.6.9. Conclusion

    5. EPIDEMIOLOGY OF ASTHMA AND ALLERGIC DISEASE

         5.1. Introduction

         5.2. Definition and measurement of allergic disease

              5.2.1. Asthma
                        5.2.1.1   Definition
                        5.2.1.2   Assessment
              5.2.2. Rhinitis
              5.2.3. Atopic dermatitis
                        5.2.3.1   Definition
                        5.2.3.2   Assessment

              5.2.4. Skin-prick test and serum IgE
              5.2.5. Allergic contact dermatitis

         5.3. Asthma and atopy: prevalence rates and time trends in
              prevalence rates

              5.3.1. Europe
                        5.3.1.1   Prevalences
                        5.3.1.2   Time trends
              5.3.2. Oceania
                        5.3.2.1   Prevalences
                        5.3.2.2   Time trends
              5.3.3. Eastern Mediterranean
              5.3.4. Africa
              5.3.5. Asia
                        5.3.5.1   Prevalences
                        5.3.5.2   Time trends
              5.3.6. North America
                        5.3.6.1   Prevalences
                        5.3.6.2   Time trends
              5.3.7. The International Study of Asthma and Allergies in
                        Childhood
              5.3.8. Conclusion

         5.4. Age and gender distribution

         5.5. Migration

         5.6. Viral infection

         5.7. Socioeconomic status

         5.8. Occupational exposure

              5.8.1. Chemicals with low relative molecular mass
                        5.8.1.1   Diisocyanates
                        5.8.1.2   Acrylates
                        5.8.1.3   Anhydrides
                        5.8.1.4   Solder flux
              5.8.2. Metals
                        5.8.2.1   Cobalt
                        5.8.2.2   Metal-polishing industry
                        5.8.2.3   Aluminium
                        5.8.2.4   Platinum salts
              5.8.3. Natural rubber latex
              5.8.4. Flour
              5.8.5. Animals
              5.8.6. Other agents

         5.9. Allergic contact dermatitis

              5.9.1. Epidemiology of allergic contact dermatitis
                        5.9.1.1   Nickel
                        5.9.1.2   Chromates
                        5.9.1.3   Fragrances
                        5.9.1.4   Preservatives
                        5.9.1.5   Medicines
                        5.9.1.6   Plants and woods
              5.9.2. Lack of a relationship between atopy and allergic
                        contact sensitization

         5.10. Diet

              5.10.1. Breast feeding
              5.10.2. Sodium
              5.10.3. Selenium
              5.10.4. Vitamins and antioxidants

         5.11. Number of siblings and crowding

         5.12. Indoor environment

              5.12.1. Tobacco smoke
              5.12.2. Pets
              5.12.3. Biocontaminants
                        5.12.3.1  House dust mites and insects
                        5.12.3.2  Moulds
              5.12.4. Other indoor factors

         5.13. Indoor and outdoor environmental factors

              5.13.1. Nitrogen dioxide
              5.13.2. Sulfur dioxide, acid aerosols and particulate
                        matter
              5.13.3. Volatile organic compounds, formaldehyde and other
                        chemicals

         5.14. Outdoor air pollution

              5.14.1. Pollen and dust
              5.14.2. Ozone
              5.14.3. Motor vehicle emissions

         5.15. Conclusions

    6. HAZARD IDENTIFICATION: DEMONSTRATION OF ALLERGENICITY

         6.1. Hazard and risk; allergy and toxicity

              6.1.1. Testing allergic potential and toxicity testing
              6.1.2. Databases and prior experience

         6.2. Validation and quality assurance

         6.3. Structure-activity relationships

              6.3.1. Case-Multicase system
              6.3.2. DEREK skin sensitization rulebase
              6.3.3. SAR for respiratory hypersensitivity

         6.4. Predictive testing  in vivo 

              6.4.1. Testing for skin allergy
                        6.4.1.1   Testing in guinea-pigs
                        6.4.1.2   Testing in mice
                        6.4.1.3   Predictive testing for skin allergy in
                                  humans
              6.4.2. Testing for respiratory allergy
                        6.4.2.1   Guinea-pig model
                        6.4.2.2   Mouse IgE model
                        6.4.2.3   Rat model
                        6.4.2.4   Predictive testing for respiratory
                                  allergy in humans
                        6.4.2.5   Cytokine fingerprinting

         6.5. Testing for food allergy

         6.6.  In vitro approaches

         6.7. Testing for autoimmunity

              6.7.1. Popliteal lymph node assay
              6.7.2. Animal models of autoimmune disease

         6.8. Clues from general toxicity tests

    7. RISK ASSESSMENT

         7.1. Introduction

         7.2. Risk assessment of allergy

         7.3. Factors in risk assessment of allergy

         7.4. Information aspects

              7.4.1. No information about hazard
              7.4.2. Scanty or no information about exposure
              7.4.3. Unreliable or scanty information about risk

         7.5. Conclusions

    8. TERMINOLOGY

    9. CONCLUSIONS

    10. RECOMMENDATIONS FOR PROTECTION OF HUMAN HEALTH

    11. FURTHER RESEARCH

    REFERENCES

    CONCLUSIONS

    CONCLUSIONES
    

    NOTE TO READERS OF THE CRITERIA MONOGRAPHS

         Every effort has been made to present information in the criteria
    monographs as accurately as possible without unduly delaying their
    publication.  In the interest of all users of the Environmental Health
    Criteria monographs, readers are requested to communicate any errors
    that may have occurred to the Director of the International Programme
    on Chemical Safety, World Health Organization, Geneva, Switzerland, in
    order that they may be included in corrigenda.

    *     *     *

         A detailed data profile and a legal file can be obtained from the
    International Register of Potentially Toxic Chemicals, Case postale
    356, 1219 Châtelaine, Geneva, Switzerland (telephone no. + 41 
    22 - 9799111, fax no. + 41 22 - 7973460, E-mail irptc@unep.ch).

    *     *     *

         This publication was made possible by grant number
    5 U01 ES02617-15 from the National Institute of Environmental Health
    Sciences, National Institutes of Health, USA, and by financial support
    from the European Commission.

    Environmental Health Criteria

    PREAMBLE

    Objectives

         In 1973 the WHO Environmental Health Criteria Programme was
    initiated with the following objectives:

    (i)       to assess information on the relationship between exposure
              to environmental pollutants and human health, and to provide
              guidelines for setting exposure limits;

    (ii)      to identify new or potential pollutants;

    (iii)     to identify gaps in knowledge concerning the health effects
              of pollutants;

    (iv)      to promote the harmonization of toxicological and
              epidemiological methods in order to have internationally
              comparable results.

         The first Environmental Health Criteria (EHC) monograph, on
    mercury, was published in 1976 and since that time an ever-increasing
    number of assessments of chemicals and of physical effects have been
    produced.  In addition, many EHC monographs have been devoted to
    evaluating toxicological methodology, e.g., for genetic, neurotoxic,
    teratogenic and nephrotoxic effects.  Other publications have been
    concerned with epidemiological guidelines, evaluation of short-term
    tests for carcinogens, biomarkers, effects on the elderly and so
    forth.

         Since its inauguration the EHC Programme has widened its scope,
    and the importance of environmental effects, in addition to health
    effects, has been increasingly emphasized in the total evaluation of
    chemicals.

         The original impetus for the Programme came from World Health
    Assembly resolutions and the recommendations of the 1972 UN Conference
    on the Human Environment.  Subsequently the work became an integral
    part of the International Programme on Chemical Safety (IPCS), a
    cooperative programme of UNEP, ILO and WHO.  In this manner, with the
    strong support of the new partners, the importance of occupational
    health and environmental effects was fully recognized. The EHC
    monographs have become widely established, used and recognized
    throughout the world.

         The recommendations of the 1992 UN Conference on Environment and
    Development and the subsequent establishment of the Intergovernmental
    Forum on Chemical Safety with the priorities for action in the six
    programme areas of Chapter 19, Agenda 21, all lend further weight to
    the need for EHC assessments of the risks of chemicals.

    Scope

         The criteria monographs are intended to provide critical reviews
    on the effect on human health and the environment of chemicals and of
    combinations of chemicals and physical and biological agents.  As
    such, they include and review studies that are of direct relevance for
    the evaluation.  However, they do not describe  every study carried
    out.  Worldwide data are used and are quoted from original studies,
    not from abstracts or reviews.  Both published and unpublished reports
    are considered and it is incumbent on the authors to assess all the
    articles cited in the references.  Preference is always given to
    published data.  Unpublished data are only used when relevant
    published data are absent or when they are pivotal to the risk
    assessment.  A detailed policy statement is available that describes
    the procedures used for unpublished proprietary data so that this
    information can be used in the evaluation without  compromising its
    confidential nature (WHO (1990) Revised Guidelines for the Preparation
    of Environmental Health Criteria Monographs. PCS/90.69, Geneva, World
    Health Organization).

         In the evaluation of human health risks, sound human data,
    whenever available, are preferred to animal data.  Animal and  in
     vitro studies provide support and are used mainly to supply evidence
    missing from human studies.  It is mandatory that research on human
    subjects is conducted in full accord with ethical principles,
    including the provisions of the Helsinki Declaration.

         The EHC monographs are intended to assist national and
    international authorities in making risk assessments and subsequent
    risk management decisions.  They represent a thorough evaluation of
    risks and are not, in any sense, recommendations for regulation or
    standard setting.  These latter are the exclusive purview of national
    and regional governments.

    Content

         The layout of EHC monographs for chemicals is outlined below.

    *    Summary -- a review of the salient facts and the risk evaluation
         of the chemical
    *    Identity -- physical and chemical properties, analytical methods
    *    Sources of exposure
    *    Environmental transport, distribution and transformation
    *    Environmental levels and human exposure
    *    Kinetics and metabolism in laboratory animals and humans
    *    Effects on laboratory mammals and  in vitro test systems
    *    Effects on humans
    *    Effects on other organisms in the laboratory and field
    *    Evaluation of human health risks and effects on the environment
    *    Conclusions and recommendations for protection of human health
         and the environment

    *    Further research
    *    Previous evaluations by international bodies, e.g., IARC, JECFA,
         JMPR

    Selection of chemicals

         Since the inception of the EHC Programme, the IPCS has organized
    meetings of scientists to establish lists of priority chemicals for
    subsequent evaluation.  Such meetings have been held in: Ispra, Italy,
    1980; Oxford, United Kingdom, 1984; Berlin, Germany, 1987; and North
    Carolina, USA, 1995. The selection of chemicals has been based on the
    following criteria: the existence of scientific evidence that the
    substance presents a hazard to human health and/or the environment;
    the possible use, persistence, accumulation or degradation of the
    substance shows that there may be significant human or environmental
    exposure; the size and nature of populations at risk (both human and
    other species) and risks for environment; international concern, i.e.
    the substance is of major interest to several countries; adequate data
    on the hazards are available.

         If an EHC monograph is proposed for a chemical not on the
    priority list, the IPCS Secretariat consults with the Cooperating
    Organizations and all the Participating Institutions before embarking
    on the preparation of the monograph.

    Procedures

         The order of procedures that result in the publication of an EHC
    monograph is shown in the flow chart.  A designated staff member of
    IPCS, responsible for the scientific quality of the document, serves
    as Responsible Officer (RO).  The IPCS Editor is responsible for
    layout and language.  The first draft, prepared by consultants or,
    more usually, staff from an IPCS Participating Institution, is based
    initially on data provided from the International Register of
    Potentially Toxic Chemicals, and reference data bases such as Medline
    and Toxline.

         The draft document, when received by the RO, may require an
    initial review by a small panel of experts to determine its scientific
    quality and objectivity.  Once the RO finds the document acceptable as
    a first draft, it is distributed, in its unedited form, to well over
    150 EHC contact points throughout the world who are asked to comment
    on its completeness and accuracy and, where necessary, provide
    additional material.  The contact points, usually designated by
    governments, may be Participating Institutions, IPCS Focal Points, or
    individual scientists known for their particular expertise.  Generally
    some four months are allowed before the comments are considered by the
    RO and author(s).  A second draft incorporating comments received and
    approved by the  Director,  IPCS, is then  distributed to Task Group
    members, who carry out the peer review, at least six weeks before
    their meeting.

         The Task Group members serve as individual scientists, not as
    representatives of any organization, government or industry.  Their
    function is to evaluate the accuracy, significance and relevance of
    the information in the document and to assess the health and
    environmental risks from exposure to the chemical.  A summary and
    recommendations for further research and improved safety aspects are
    also required.  The composition of the Task Group is dictated by the
    range of expertise required for the subject of the meeting and by the
    need for a balanced geographical distribution.

         The three cooperating organizations of the IPCS recognize the
    important role played by nongovernmental organizations.
    Representatives from relevant national and international associations
    may be invited to join the Task Group as observers.  While observers
    may provide a valuable contribution to the process, they can only
    speak at the invitation of the Chairperson. Observers do not
    participate in the final evaluation of the chemical; this is the sole
    responsibility of the Task Group members.  When the Task Group
    considers it to be appropriate, it may meet  in camera.

         All individuals who as authors, consultants or advisers
    participate in the preparation of the EHC monograph must, in addition
    to serving in their personal capacity as scientists, inform the RO if
    at any time a conflict of interest, whether actual or potential, could
    be perceived in their work.  They are required to sign a conflict of
    interest statement. Such a procedure ensures the transparency and
    probity of the process.

         When the Task Group has completed its review and the RO is
    satisfied as to the scientific correctness and completeness of the
    document, it then goes for language editing, reference checking, and
    preparation of camera-ready copy.  After approval by the Director,
    IPCS, the monograph is submitted to the WHO Office of Publications for
    printing.  At this time a copy of the final draft is sent to the
    Chairperson and Rapporteur of the Task Group to check for any errors.

         It is accepted that the following criteria should initiate the
    updating of an EHC monograph: new data are available that would
    substantially change the evaluation; there is public concern for
    health or environmental effects of the agent because of greater
    exposure; an appreciable time period has elapsed since the last
    evaluation.

         All Participating Institutions are informed, through the EHC
    progress report, of the authors and institutions proposed for the
    drafting of the documents.  A comprehensive file of all comments
    received on drafts of each EHC monograph is maintained and is
    available on request.  The Chairpersons of Task Groups are briefed
    before each meeting on their role and responsibility in ensuring that
    these rules are followed.

    FIGURE 

    WHO TASK GROUP MEETING ON PRINCIPLES AND METHODS FOR ASSESSING
    ALLERGIC HYPERSENSITIZATION ASSOCIATED WITH EXPOSURE TO CHEMICALS

     Members

    Professor V. Bencko, Institute of Hygiene and Epidemiology,
    Charles University, Prague, Czech Republic

    Dr K. Brockow, Clinic for Dermatology and Allergic Disease,
    Biederstein Technical University, Munich, Germany

    Professor A.D. Dayan, Department of Toxicology, Department of
    Health, St Bartholomew's Hospital Medical College, London, United
    Kingdom ( Chairman)

    Dr D. D'Cruz, Department of Rheumatology, Royal London
    Hospital, London, United Kingdom

    Professor M. Eglite, Institute of Occupational and Environmental
    Health, Medical Academy of Latvia, Riga, Latvia

    Dr M.-A. Flyvholm, Department of Allergy and Irritation, National
    Institute of Occupational Health, Copenhagen, Denmark

    Dr J. Gergely, Department of Immunology, Lorand Eötvös
    University, God, Hungary

    Dr D. Germolec, National Toxicology Program, National Institute
    of Environmental Health Sciences, Research Triangle Park, North
    Carolina, USA ( Joint Rapporteur)

    Dr H.S. Koren, National Health and Environmental Effects
    Research Laboratory, US Environmental Protection Agency, Research
    Triangle Park, North Carolina, USA

    Dr M. Lovik, National Institute of Public Health, Oslo, Norway
    ( Joint Rapporteur)

    Dr C. Madsen, Institute of Toxicology, Danish Veterinary and Food
    Administration, Söborg, Denmark

    Dr A. Penninks, Nutrition and Food Research Institute TNO, Zeist,
    Netherlands

    Professor R.J. Scheper, Institute of Pathology, Amsterdam,
    Netherlands

    Dr H. van Loveren, Laboratory for Pathology, National Institute of
    Public Health and the Environment, Bilthoven, Netherlands
    ( Vice-Chairman)

    Dr B.M.E. von Blomberg, Institute of Pathology, Amsterdam,
    Netherlands

    Dr J.G. Vos, National Institute of Public Health and the
    Environment, Bilthoven, Netherlands


     Secretariat

    Dr E.M. Smith, International Programme on Chemical Safety,
    World Health Organization, Geneva, Switzerland


     Assisting the Secretariat

    Dr H. Duhme, Institute for Epidemiology and Social Medicine,
    Münster, Germany (8-10 September 1997)

    Dr M. Kammüller, Rheinfelden, Germany (8-10 September 1997)

    Professor M.H. Karol, Department of Environmental and Occupational
    Health, University of Pittsburgh, Pittsburg, PA, USA (8-10 September
    1997)

    Dr I. Kimber, ZENECA Central Toxicology Laboratory, Alderley Park,
    Cheshire, United Kingdom (11-12 September 1997)

     Representatives of other Organizations

    Dr D. Basketter, Unilever, Sharnbrook, Bedford, United Kingdom
    (representing the European Centre for Ecotoxicology and Toxicology of
    Chemicals)

    Dr D. Metcalfe, Allergy and Immunology Institute, International
    Life Sciences Institute, Washington DC, USA

    Dr C. D'Ambrosio, Drug Allergy Unit, Catholic University of
    Sacred Heart, Rome, Italy (representing the International Union of
    Pharmacology).

    ENVIRONMENTAL HEALTH CRITERIA ON PRINCIPLES AND METHODS FOR ASSESSING
    ALLERGIC HYPERSENSITIZATION ASSOCIATED WITH EXPOSURE TO CHEMICALS

         A WHO Task Group on Principles and Methods for Assessing Allergic
    Hypersensitization Associated with Exposure to Chemicals met at the
    National Institute of Public Health and the Environment, Bilthoven,
    Netherlands from 8 to 12 September 1997. Dr E.M. Smith, IPCS, welcomed
    the participants on behalf of Dr M. Mercier, Director of the IPCS, and
    on behalf of the three IPCS cooperating organizations (UNEP/ILO/WHO).
    The Group reviewed and revised the draft and made an evaluation of the
    risks to human health and of allergic hypersensitization associated
    with exposure to chemicals.

         The main authors were

         Professor A.D. Dayan, London, United Kingdom
         Dr D. D'Cruz, London, United Kingdom
         Dr H. Duhme, Münster, Germany
         Dr M. Kammüller, Rheinfelden, Germany
         Professor M.H. Karol, Pittsburgh, PA, USA
         Professor U. Keil, Münster, Germany
         Dr I. Kimber, Macclesfield, United Kingdom
         Dr H.S. Koren, Research Triangle Park, NC, USA
         Dr C. Madsen, Söborg, Denmark
         Professor T. Menné, Hellerup, Denmark
         Professor A.J. Newman Taylor, London, United Kingdom
         Professor J. Ring, Munich, Germany
         Professor R.J. Scheper, Amsterdam, Netherlands
         Dr H. van Loveren, Bilthoven, Netherlands
         Dr B.M.E. von Blomberg, Amsterdam, Netherlands
         Professor B. Wüthrich, Zurich, Switzerland

         Contributing authors were:

         Dr D. Abeck, Munich, Germany
         Dr D. Basketter, Sharnbrook, Bedford, United Kingdom
         Dr K. Brockow, Munich, Germany
         Dr D. Germolec, Research Triangle Park, NC, USA
         Dr G. Hughes, London, United Kingdom
         Dr M. Lovik, Oslo, Norway
         Dr A. Penninks, Zeist, Netherlands
         Dr T. Rustemeyer, Amsterdam, Netherlands
         Dr E.M. Smith, Geneva, Switzerland
         Dr M. Stender, Münster, Germany
         Dr S.K. Weiland, Münster, Germany

         Dr E.M. Smith and Dr P.G. Jenkins, both of the IPCS Central Unit,
    were responsible for the scientific aspects of the monograph and for
    the technical editing, respectively.

         The efforts of all who helped in the preparation and finalization
    of the monograph are gratefully acknowledged.

         IPCS expresses its gratitude to the external reviewers who
    provided comments and other relevant material, in particular to the
    United Kingdom Department of Health, the US Environmental Protection
    Agency, the European Centre for Ecotoxicology and Toxicology of
    Chemicals (ECETOC), and to the Netherlands National Institute for
    Public Health and the Environment (RIVM) for hosting the meeting.

         Funds for the preparation, review and publication of this
    monograph were generously provided by the US Environmental Protection
    Agency, the Department of Toxicology, Department of Health, United
    Kingdom, and the Netherlands National Institute for Public Health and
    the Environment.

    ABBREVIATIONS

    APC       antigen-presenting cell
    COPD      chronic obstructive pulmonary disease
    DEREK     deductive estimation of risk from existing knowledge
    DTH       delayed-type hypersensitivity
    FcR       Fc receptor
    FEV1         forced expiratory volume in 1 second
    FVC       forced vital capacity
    HIV       human immunodeficiency virus
    ICAM      intercellular adhesion molecule
    Ig        immunoglobulin
    IL        interleukin
    LAK       lymphokine-activated killer
    LC        Langerhans cell
    LPS       lipopolysaccharide
    MALTs     mucosal-associated lymphoid tissues
    MDR       multiple drug resistance
    NCAM      neural cell adhesion molecule
    NK        natural killer
    PAM       pulmonary alveolar macrophage
    PDGFR     platelet-derived growth factor receptor
    QSAR      quantitative structure-activity relationship
    SAR       structure-activity relationship
    SLE       systemic lupus erythematosus
    TCPA      tetrachlorophthalic anhydride
    TCR       T-cell antigen receptor
    TDI       toluene diisocyanate
    Th        T helper
    TNF       tumour necrosis factor

    PREFACE

         Normal functioning of the immune system prevents serious
    illnesses, such as infections and tumours. Immunotoxicology represents
    abnormalities in the immune system produced by exposure to chemicals
    and drugs. One consequence of dysfunction of the immune system is
    partial or complete immunosuppression, resulting in reduced defences
    against these conditions. This is often termed "immunotoxicity" and
    the IPCS Environmental Health Criteria monograph 180: Principles and
    Methods for Assessing Direct Immunotoxicity Associated with Exposure
    to Chemicals (IPCS, 1996) provides an extensive review of the causes,
    consequences and detection of this type of disorder.

         Allergy is another type of adverse effect on health produced by
    harmful immune responses following exposure to certain chemicals. The
    initial exposure results in the state of allergic sensitization, in
    which the immune system is primed to respond inappropriately on
    subsequent exposure to the same agent, and allergy is the functional
    disorder caused by that response. The best-known types of allergic
    response affect the skin, i.e., allergic contact dermatitis and atopic
    eczema, and the airways, i.e., asthma and allergic rhinitis, but any
    tissue in the body may be affected.

         Allergic responses usually occur to foreign antigens, although
    self-antigens may sometimes be the targets of damaging immune
    responses. This is known as autoimmunity and may occur because the
    self-antigens have been modified by chemicals or because the latter
    have adversely affected the control mechanisms that normally prevent
    autoimmune reactions.

         Both allergic and autoimmune disorders may be caused by the
    responses of the immune system to substances of low (e.g., transition
    metals and simple organic compounds) or high relative molecular mass
    (e.g., proteins, including food components). The harmful reactions may
    occur at the site of exposure or systemically. The genetic make-up of
    the individual may be one predisposing factor.

         Once developed, sensitization persists, sometimes for life, and
    further exposure, even to a low concentration of the allergen, may
    result in serious disease. After the chemical nature of the substance,
    exposure (concentration, route, duration and frequency) is the most
    important factor in the development of sensitization, as increased
    exposure to allergens leads to increased risk of sensitization.
    Allergic disorders represent major ill-health and economic loss to the
    public and in the workplace. There are suggestions that pollution and
    other environmental factors, such as lifestyle and smoking, may be
    involved in the rising number of affected people in both developed and
    developing countries.

         The incidence of chemically induced autoimmune diseases is low,
    but they represent important adverse consequences of the use of
    certain medicines and, possibly, of exposure to various chemicals.

         The structure and functional processes of the immune system and
    the mechanisms of sensitization, allergic responses and autoimmunity
    need to be considered in relation to the corresponding disorders and
    chemicals known to produce them. This consideration will include
    factors that affect the allergenicity of substances and the
    development of sensitization and autoimmunity, such as the chemical
    nature of allergens, special features of the causal exposures, and the
    physiology of affected subjects.

         Allergic disorders are important causes of ill-health at work and
    in the community, and defining their epidemiology and the evaluation
    of methods to study their occurrence are crucial. Hazard
    identification and risk assessment are important if the incidence of
    allergy and autoimmune disorders is to be contained or reduced. Test
    methods for the prediction of some forms of sensitization and the risk
    of disease following a given exposure are now available.

         Allergic disorders of humans have been described for many years,
    but the pace of advances in knowledge of the immune system means that
    awareness and understanding of allergy and autoimmunity and their
    consequences are increasing. Our understanding of allergy is
    developing rapidly, and hypotheses about causes and mechanisms will
    change as more is learnt about normal and abnormal functioning of the
    immune system.

         Because understanding of sensitization, allergy and autoimmunity
    is still limited by the extent of knowledge of basic immunology there
    is a need for fundamental and applied research in areas of the basic
    mechanisms, detection and prevention of allergy.


    1.  THE IMMUNE SYSTEM

    1.1 Introduction

         The role of the immune system may be succinctly stated as the
    "preservation of integrity". This system is responsible for
    identifying what is "self" and what is "non-self". The great
    complexity of the mammalian system is an indication of the importance,
    as well as the difficulty, of this task. If the system fails to
    recognize as non-self an infectious entity or the neoantigens
    expressed by a newly arisen tumour, then the host is in danger of
    rapidly succumbing to the unopposed invasion. Alternatively, if some
    integral bodily tissue is not identified as self, then the host is in
    danger of turning its considerable defensive abilities against the
    tissue and an autoimmune disease is the result. The cost to the host
    of these mistakes, made in either direction, may be quite high.
    Therefore, an extremely complex array of organs, cells, soluble
    factors and interactions has evolved to regulate this system and
    minimize the frequency of either of the above-described errors. Recent
    advances in cellular and molecular biology have dramatically increased
    our understanding of the mammalian immune system. It is now possible
    to study in detail biochemical and signal transduction pathways, as
    well as the regulation of genes in lymphocytes, because of the novel
    chemical and molecular probes that have been developed. Most
    importantly, the identification and characterization of the cells,
    cell surface receptors and cytokines that participate in the immune
    response have enabled immunologists to produce transgenic and gene
    "knockout" (disrupted target gene) mice, which will allow even more
    in-depth study of critical elements in the immune response to
    antigens. Along with the increased power of experimental immunology
    has come the ability to study both the direct and indirect actions of
    drugs and environmental chemicals (i.e., xenobiotics) on immunological
    processes. Of particular importance are new insights regarding the
    interactive role of the immune system with other organ systems such as
    the nervous and endocrine systems. By way of mutual physical and
    chemical communication between these organ systems, both direct and
    indirect alteration of immunological function may occur through the
    actions of xenobiotics.

    1.1.1  Evolution and function of the adaptive immune system

         Even the most primitive species of animals display some form of
    immune system that enables identification of "non-self" and that
    provides for some rudimentary host defence against environmental
    challenges. With the emergence of the vertebrates, however, there is
    seen the evolution of an adaptive immune system that has as its
    primary physiological responsibility protection of the organism from
    microbiological challenge and tumour development. The structure and
    function of the immune system at the anatomical, biochemical and
    functional levels are broadly comparable in all mammals.

         Natural immunity is phylogenetically more ancient than the
    adaptive immune response, but nevertheless is of critical importance
    in providing resistance to infectious microorganisms, and the
    nonspecific or innate immune system acts as a first line of defence.
    Among the functions of the natural immune system is provision of a
    physicochemical barrier at external surfaces in the skin and the
    mucosal tissues of the gastrointestinal, reproductive and respiratory
    tracts, and the physical elimination of bacteria by coughing,
    sneezing, etc. The ability of these surfaces to renew themselves and
    secrete antimicrobial agents such as fatty acids and lysozyme reduces
    penetration by microbes. However, microbes that bypass these barriers
    must be dealt with by other more advanced immunological mechanisms,
    which can be either specific or nonspecific in nature. Cellular
    elements of the natural immune system include natural killer (NK)
    cells, mononuclear phagocytes, and eosinophil and neutrophil
    polymorphonuclear cells. In addition, a complex series of plasma
    proteins and glycoproteins together comprise the complement system,
    which acts together with antibody in the elimination of bacteria, but
    which can also be activated to provide natural immune function in the
    absence of, or before, a specific immune response. The adaptive immune
    system acts together with innate or natural immune mechanisms to
    provide host resistance to infectious and malignant disease.

         The adaptive immune system comprises organs, tissues, cells and
    molecules that must act in concert to provide an integrated immune
    response. The three cardinal characteristics of adaptive immunity are
    memory, specificity and the capacity to distinguish between self and
    non-self. Each of these characteristics are displayed by lymphocytes:
    the main cellular vectors of adaptive immune responses. Immunological
    memory is the ability to distinguish a foreign material as a previous
    invader and to mount a greatly increased and lasting response to that
    particular antigen. This process is the product of immunocompetent
    cell cooperativity and allows for both amplification of the immune
    response after repeated encounters with the same antigen
    (immunization) and tolerance to self tissues. In contrast, nonspecific
    or innate mechanisms do not possess individuality and do not lead to
    memory.

         Mature lymphocytes circulate throughout the body, between and
    within lymphoid tissues. If a lymphocyte encounters a foreign antigen
    in an appropriate form under suitable conditions then the cell becomes
    activated and an immune response is initiated. The primary response
    takes place in organized lymphoid tissues. It has been estimated that
    in a normal adult human the immune system is capable of recognizing
    and responding to many millions of antigens; even antigens that have
    never been encountered previously, such as for instance new synthetic
    chemicals. This enormous repertoire is provided by the clonal
    diversity of lymphocytes; these cells being clonally distributed with
    respect to antigen specificity. Thus, each clone of mature lymphocytes
    differs one from another in terms of the antigenic structures that

    will induce activation. Antigen recognition is effected via
    specialized membrane receptors that have diversified among lymphocytes
    during development of the immune system by a process of somatic
    recombination of antigen receptor genes. It is the possession of these
    receptors by lymphocytes that confers specificity to immune responses.

         Recognition of antigen by lymphocytes in primary lymphoid tissues
    results in rapid cellular activation and the stimulation of division
    and differentiation. Division provides for a selective expansion in
    numbers of those lymphocytes that are able to recognize and interact
    with the inducing antigen. Selective clonal expansion forms the basis
    of immunological memory. After first encounter with antigen,
    responsive lymphocytes have increased in number such that if the
    individual is exposed subsequently to the same antigenic material then
    an accelerated and more aggressive response will be mounted. These are
    the central events necessary for adaptive immunity and those that are
    made use of in vaccination against infectious microorganisms.

         All lymphocytes involved in adaptive immune responses interact
    specifically with antigen, and they divide and differentiate in
    response to antigenic challenge. These cells may be subdivided into
    two main populations, T-lymphocytes and B-lymphocytes, that differ
    with respect to their origins and development pathways, the way in
    which antigen is recognized, and the effector cells into which they
    ultimately differentiate. Both populations arise in the bone marrow
    from primitive precursors, but thereafter follow discrete
    developmental pathways. Cells committed to becoming T-lymphocytes
    (pre-T-cells) require passage through and differentiation within the
    thymus to achieve immunological maturity. The thymus serves also to
    identify and destroy most of those T lymphocytes that display membrane
    receptors which would permit interaction with self antigens. When they
    leave the thymus the mature antigen-sensitive T-lymphocytes join the
    recirculating pool.

         Bone marrow derived B-cells also join the recirculating pool
    where, with T-lymphocytes, they seek antigen for which they have
    complementary membrane receptors. B-lymphocytes recognize antigen
    usually in its native form. Activation triggers B-lymphocyte
    differentiation and division. The end-cell of B-lymphocyte
    differentiation is the plasma cell that possesses the synthetic and
    secretory machinery to manufacture and export large amounts of
    antibody. The antibody secreted by an individual plasma cell is of a
    single specificity and matches identically the specificity of the
    membrane receptor on the B-lymphocyte from which the plasma cell
    differentiated. The purpose of antibody is essentially to form a
    bridge between the inducing antigen and biological mechanisms that
    serve to eliminate it. The interaction of antibody with antigen
    facilitates the activation of complement (lysis of bacteria) and
    phagocytosis by mononuclear phagocytes and neutrophils (intracellular
    killing of bacteria) and results in the clearance of pathogenic

    bacteria. The importance of B-lymphocytes and the antibodies that
    derive from their activation is protection against extracellular
    infection by bacteria and parasites.

         The existence of T-lymphocytes was recognized for many years
    before the true nature of their role in adaptive immune responses was
    appreciated. Cell-mediated immune responses effected by T-lymphocyte
    participate in host defence against all types of infectious organisms,
    but of greatest evolutionary significance is immunity against viruses.
    Humoral immunity effected by antibody is of relevance only in the
    viraemic stage of viral infections. Viruses are obligate intracellular
    parasites and once inside the infected host cell are protected from
    antibody-mediated mechanisms.

         The overall purpose of these host defence mechanisms is to
    provide the organism with resistance to a challenging microbial
    environment and to confer protection from the internal development of
    non-self neoplasms or tumours. When normal immune function is absent
    or compromised, the consequences for human health are serious.
    Consideration of immunosuppression and immunodeficiency illustrates
    the evolutionary importance of immune function.

    1.1.2  Immunosuppression, immunodeficiency and autoimmunity

         Active immune function is clearly beneficial for health, whereas
    the consequences of a compromised immune system are adverse health
    effects.

         Immunodeficiency disorders can be congenital or acquired.
    Congenital immunodeficiency is comparatively rare, but is frequently
    very serious and can be fatal. Examples include a complete, or almost
    complete, failure of the immune system to develop due to the absence
    or aberrant maturation of lymphocyte or leukocyte progenitors,
    resulting in severe combined immunodeficiency disease or reticular
    dysgenesis. Without appropriate treatment these conditions are fatal,
    children succumbing to overwhelming infection.

         Acquired immunodeficiency can be secondary to malnutrition,
    severe stress, treatment with immunosuppressive drugs or with cancer
    chemotherapeutic agents, exposure to certain environmental chemicals
    or infection, such as infection with the human immunodeficiency virus
    (HIV), the cause of acquired immunodeficiency syndrome (AIDS). In all
    instances immunosuppression is associated with reduced host resistance
    and more persistent infection, often with unusual microorganisms that
    are resisted well by immunocompetent individuals. Immunodeficiency is,
    in addition, associated with an increased incidence of malignant
    diseases that are known or suspected to be associated with oncogenic
    viruses.

         The benefits that derive from active immune function do not come
    without a cost, however. While the adaptive immune system acts as a
    "friend" in providing host defence, it may also act as a "foe", being
    instrumental in the pathogenesis of certain diseases. The immune
    system can, for instance, turn on the host if the fine discrimination
    between self and non-self breaks down. The result is the development
    of autoimmune responses and autoimmune disease. The mechanisms by
    which autoimmunity develops are multifactorial, complex and remain
    poorly understood. The majority of cases are idiopathic, although
    diseases such as systemic sclerosis have been associated with organic
    chemicals and silica.

    1.1.3  Allergy and allergic diseases

         Allergy may be defined as the adverse health effects resulting
    from hypersensitivity caused by exposure to an exogenous antigen
    (allergen) resulting in a marked increase in reactivity and
    responsiveness to that particular antigen on subsequent exposure.
    Allergy is not necessarily, or usually, the consequence of perturbed
    immune function, but the result of an immune system response to an
    antigen (in this case allergen) in such a way that a temporary or
    long-lasting disease results. The immunological processes that are
    involved in the development of allergic responses and allergic disease
    are in principle and practice no different to those that provide
    protective immunity and host resistance against potential pathogens.

         Allergy normally develops in two phases. The first phase is
    induced following initial encounter of the susceptible individual with
    the allergen. A primary immune response is mounted that results in a
    state of heightened responsiveness to that particular antigen
    (specific sensitization). In immunological terms sensitization to an
    allergen does not differ from immunization to a pathogenic
    microorganism. Following second or subsequent exposure of the now
    sensitized individual to the inducing allergen a more vigorous and
    accelerated secondary immune response is provoked and it is at this
    stage that adverse health effects are normally first recognized. The
    aggressive secondary immune response against the allergen causes local
    tissue disruption and inflammation that is recognized clinically as
    allergic disease.

         Individuals vary widely in terms of allergic responsiveness and
    susceptibility to allergic disease. There are a number of factors of
    importance here including opportunities for encounter with the
    inducing allergen, the route, the dose or concentration of allergen,
    extent and duration of exposure and genetic predisposition. The latter
    is incompletely understood but clearly impacts significantly upon
    susceptibility. Respiratory allergy (including hay fever and asthma)
    to protein aeroallergens is associated frequently with atopy; a
    genetic predisposition for increased production of IgE, the class of
    antibody that causes respiratory hypersensitivity to proteins. In
    addition, the immunological repertoire of individuals and the ability
    of their immune system to recognize and respond to certain antigenic
    structures will also influence susceptibility.

         Allergic diseases are widespread and can be caused by allergens
    encountered in the external environment, home or work. They range from
    comparatively mild inflammatory responses localized to a single site
    to systemic anaphylactic responses that may prove fatal. Allergic
    disease, as well as representing an important and widespread health
    problem, is also of great economic significance with respect to the
    cost of health care and time lost from work. It has been recognized
    that some forms of allergy are increasing in prevalence, compounding
    the health impact of these diseases. The incidence of asthma, for
    instance, has grown significantly in some developed countries, an
    increase that may be attributable to changing allergen exposure
    patterns, alterations in lifestyle, environmental pollution or to a
    combination of all of these factors.

         In the context of occupational and environmental health the two
    most important allergic diseases caused by exposure to chemicals are
    allergic contact dermatitis and respiratory hypersensitivity. The
    former is very common and can be induced by industrial chemicals,
    metals and natural products. Sensitization results from dermal
    exposure of the susceptible individual to the inducing allergen.
    Allergic contact dermatitis reactions are provoked subsequently when
    the now sensitized individual is exposed for a second time to the
    inducing allergen at the same or different skin site. Many hundreds of
    contact allergens, varying enormously in potency, have been
    identified.

         Although from the occupational and environmental health
    standpoint allergic contact dermatitis and respiratory
    hypersensitivity represent the most important types of allergy induced
    by chemicals, it should not be forgotten that exposure to xenobiotics
    has been implicated in other forms of allergic disease. Certain drugs
    are associated with systemic allergic reactions that are sometimes
    reminiscent of autoimmune diseases. In addition, food components and
    food additives are implicated in adverse reactions, which in some
    cases take the form of an allergic response.

    1.1.4  Conclusion

         An active adaptive immune system is essential for health and
    survival in a hostile microbiological environment. A price paid for
    the host resistance provided by the immune system is that some immune
    responses, often to benign antigens, result in the adverse health
    effects of allergic disease.

    1.2  Physiology and components of the immune system

         Immunity refers to all those physiological mechanisms/processes
    that enable an animal (i.e., the host) to recognize materials as
    foreign to "self" and to neutralize, eliminate or metabolize them,
    with or without injury to its own tissue. The immune system of higher
    animals is therefore capable of distinguishing between self materials
    from which they are constituted and "non-self" (i.e., those that are

    foreign or antigenic). It probably evolved to confer a selective
    advantage to organisms that could withstand colonization and microbial
    invasion. The immune response must decipher sometimes quite subtle
    differences between self and non-self, without error, to both provide
    protection and avoid self-attack. Accomplishment of this selective
    process requires the concerted action of a number of cell types.
    Mammals have developed a highly complex, intertwined and redundant
    system composed of layers of protective mechanisms to cope with more
    sophisticated environmental threats.

         The immune system comprises both lymphoid organs and specialized
    cells. Erythrocytes, myeloid cells, megakaryocytes (which mature to
    form platelets) and lymphocytes arise from a totipotent or pluripotent
    stem cell in the yolk sac of the developing fetus and, later, the
    fetal liver. In adult mammals, the stem cells are manufactured in the
    bone marrow and progress via different pathways of differentiation to
    become mature cells that may carry out specialized functions, such as
    antibody production or phagocytosis (Abramson et al., 1977). The
    thymus and bone marrow are the primary lymphoid organs that serve to
    nurture the development of stem cells into mature effector cells.
    Mature lymphocytes traffic to the secondary lymphoid organs, the lymph
    nodes, spleen and mucosal-associated lymphoid tissues (MALTs), and
    form immune-reactive units that respond vigorously to antigens. The
    design of these secondary organs is such that the specialized
    populations of lymphocytes reside in proximity, can interact with each
    other, and can regulate the antigen-driven immune response required.
    The lymph nodes, which are situated throughout the body, filter out
    antigens draining from the peripheral bodily tissues. The spleen
    monitors the blood and functions as a factory for red blood cell
    turnover. The MALTs provide a frontline defence for microbes that are
    ingested. Lymphocytes that reside in the spleen can, upon encountering
    antigen, respond  in situ or migrate to the site of infection via the
    blood, colonizing a sensitized response unit in a local lymph node.
    The virgin stem cell is believed to receive different maturational
    stimuli in the microenvironment of the bone marrow, with stromal cell
    contact and lymphokine exposure inducing entry into one of several
    pathways of development. Functional lymphocytes are continuously
    formed from stem cells and pass from the bone marrow through the
    bloodstream to the lymphoid organs. The migratory pattern of the
    lymphocyte determines its lifespan and behaviour, as described in
    greater detail below for T-cells, B-cells and other immunocompetent
    cells.

    1.2.1  T-cells

         Stem cells that enter the thymus gland, formed from the third and
    fourth pharyngeal pouches in mammals, rapidly divide, acquire their
    antigen specificity and are selectively deleted if they bear any
    self-reactivity. The "educated" daughter cells, termed thymus-derived
    or T-lymphocytes, then leave the thymus and travel to other lymphoid
    tissues, persisting for weeks or even years. As stem cells pass

    through the thymic subcapsular region, cortex and medulla, they
    display plasma membrane-bound surface molecules that define their
    function. It is possible to experimentally identify and isolate
    subpopulations of T-lymphocytes by exploiting the differential
    expression of these marker glycoproteins, using alloantisera or
    monoclonal antibodies and immunostaining techniques. Murine
    T-lymphocytes possess both the Thy-1 marker and the T-cell antigen
    receptor (TCR)-CD3 complex, and fall into two major classes, either
    T-helper/inducer cells expressing CD4 or T-suppressor/cytotoxic cells,
    which display CD8.

         Studies in inbred mice show that the T-cell antigen receptor only
    recognizes antigen processed and presented on major histocompatibility
    complex (MHC) molecules from the same thymic environment. MHC proteins
    are products of the immune response (Ir) genes, which are primarily
    responsible for tissue graft and organ transplantation rejection. In
    general, CD4+ T-cells complex with antigen associated with MHC Class
    I molecules, which are only found on certain cells of the immune
    system, while CD8+ T-cells only see antigen when associated with MHC
    Class I molecules, located on all nucleated cells. T-cell selection of
    this type is termed positive and deletion of clones reactive to self
    is termed negative selection (Zinkernagel & Doherty, 1975). Upon
    contact with antigen, mature T-cells may either respond clonally in an
    antigen-specific manner and initiate an immune response, or become
    inactivated and eliminated in a process which is not well understood,
    potentially leaving the animal unable to recognize the antigen. This
    latter phenomenon is referred to as T-cell anergy.

         The majority of lymphocytes in the peripheral blood and lymph
    nodes and about one half of the cells in the spleen are T-cells.
    Thymectomized animals or naturally occurring athymic or nude mice
    (because they are also hairless) and children with Di George syndrome
    are immunocompromised hosts that lack cell-mediated immune function
    and responses to T-dependent antigens (Sell, 1987). The endocrine
    function of the thymus has been recognized through partial recovery of
    T-cell function in thymectomized animals given cell-free thymic
    extracts, suggesting thymic hormones may, to some extent, replace
    thymus-driven T-cell maturation (Law et al., 1968). However, the
    thymic microenvironment appears necessary for proper selection and
    differentiation of the T-cell repertoire. Imbalances in the function
    of mature T-cell subpopulations may also occur clinically, as shown by
    HIV infection of CD4+ T cells, resulting in decreased T-helper cell
    levels (Stahl et al., 1982; Lane & Fauci, 1985), and systemic lupus
    erythematosus in which lowered CD8+ T-suppressor cell activity is
    thought to contribute to elevated antibody production and to
    exacerbate the autoimmune state.

    1.2.1.1  Balancing the immune response

         It is clear that in the mouse most T-cells show predominant
    production of two different sets of cytokines with pronounced, often
    mutually exclusive, effects on different features of the immune
    response (Romagnani, 1992a,b; Bloom et al., 1992; Mosmann & Sad,
    1996). While some details of cytokine production are known to be
    different in the human, they are generally similar to that in the
    mouse. In brief, mouse Th1-cells produce IL-2, IFN-gamma and
    lymphotoxin (LT), whereas Th2-cells produce IL-4, 5,6,9,10,13, as
    shown in Table 1. Human Th1 and Th2 cells produce similar patterns,
    although the synthesis of IL-2,6,10,13 is not as tightly restricted to
    a single subset as in mouse T-cells. In the mouse Th1-cell (or Type I)
    responses result in delayed-type hypersensitivity (DTH) reactions,
    activation of macrophages to kill phagocytosed microorganisms, and in
    IgG2a, rather than IgG1 and IgE, synthesis. Th2 (Type 2) responses
    generate IgG1- and IgE-secreting cells, and eosinophilia. Notably,
    Th2-derived IL-4 is an important switch factor for B cells to produce
    the IgG1 and IgE immunoglobulin-isotypes. Th1- and Th2-cells arise
    from a common lineage since they use the same T-cell receptor
    repertoire, and naive precursor T-cells, not yet exhibiting either of
    these cytokine profiles (Th0), can differentiate into both directions
    (see also section 2.1.5). Although cytotoxic CD8+ T-cells often
    secrete a Th1-like cytokine pattern, there is evidence for the
    existence of Th2-like CD8+ T (Tc2) cells in humans and mice (Croft
    et al., 1994; Mosmann & Sad, 1996). Type 2 cytokines such as IL-4
    shift T cell differentiation away from the production of Type I
    cytokines, whereas the Type I cytokine IFN-gamma is very potent in
    preventing the development of Th2-cells.

         Cytokines are soluble mediators synthesized by cells of the 
    immune system that bind to specific receptors or target cells and 
    modulate cell function in immunological reactions (Fig. 1). When 
    starting clonal expansion after antigen stimulation, T-cells develop 
    major cytokine profiles depending on the site of primary contact. 
    Along mucosal surfaces predominant local IL-4 release, possibly by 
    mast cells, basophils or locally residing T-cells, favours the 
    development of Th2-cells (Scott, 1993; Weiner et al., 1994; Mosmann 
    et al., 1996). In some individuals over-prone to IgE-switching, this 
    response may be excessive, leading to mucosal allergies, such as 
    respiratory hypersensitivity (see also chapter 4). The induction of 
    Type 2 T-cell responses after antigen introduction along mucosal 
    surfaces is probably further promoted by high local densities of 
    B-cells as compared to the skin compartment. B-cells are excellent 
    IL-10 producers, and antigen-presentation by B-cells is known to 
    favour Th2 responses (Eynon & Parker, 1992). In addition to the 
    archetype Type 2 cytokines, TGF-beta has also been associated with Th2 
    functions, but preferential production by either a Th2 subset, or a 
    distinct Th3 subset (Chen et al., 1994), is more likely to occur. As 
    mentioned above, TGF-beta plays the key role in immune suppression 
    along mucosal surfaces, e.g., by controlling several different 
    IFN-gamma-associated effector T-cell and macrophage functions 
    (Karpus & Swanborg, 1991; Oswald et al., 1992; Khoury et al., 1992; 

        Table 1.  Cytokine production in the mouse
                                                                                                                           

    Cytokine
    production             T-cells                                                 Other cells
                   Th0       Th1       Th2       B-cell    Macrophage          NK-cell   Mast cell   Keratinocyte    LC
                                                                                                                           

    IL-1                                                                                             +alpha          +beta

    IL-2           +         +

    IFN-gamma      +         +                                                 +

    LT (TNF-beta)  +         +

    IL-3           +         +         +                                                 +

    GM-CSF         +         +         +                                                             +

    TNF-alpha      +         +         +         +                                       +           +

    IL-4           +                   +         +                                       +

    IL-5           +                   +

    IL-6           +                   +         +                                                   +               +

    IL-10          +                   +         +         +                             +           +

    IL-12                                        +         +                             +

    IL-13          +         +         +         +         +                             +
                                                                                                                           
    


    Meade et al., 1992) and by maintaining epithelial cell layer integrity 
    (Planchon et al., 1994). Moreover, TGF-beta serves as a switch 
    factor for IgA production. To what extent T-cells preferentially 
    releasing TGF-beta may also contribute to mucosal tolerance to 
    IgE-inducing atopic allergens is still unclear. In sharp contrast, 
    along the skin route local release of IL-12 from, for instance, 
    macrophages and NK-cells stimulates the production of IFN-gamma by 
    T cells and facilitates predominant development of Th1 cells. Exposure 
    of the skin to exogenous antigenic substances, including contact 
    allergens, therefore preferentially induces specific Type 1,
    pro-inflammatory T-cell responses.

    1.2.2  B-cells

         In contrast to T-lymphocyte maturation, the development of
    lymphocytes capable of synthesizing and secreting antibody
    (immunoglobulin) molecules in mammals is thought to occur in several
    sites, including the bone marrow, spleen and MALTs. Because these
    cells were first characterized in birds, which, unlike mammals,
    possess a unique lymphoid organ, the bursa of Fabricius, and because
    the precursors of these cells are formed in the bone marrow, these
    cells have been termed B-lymphocytes. B-cells tend to reside for long
    periods of time in the secondary lymphoid organs and form the lymphoid
    follicles and germinal centres. Following activation by antigen or
    antigen-activated T-helper cells (Noelle et al., 1990) and
    lymphokines, B-cells proliferate and terminally differentiate to
    antibody-producing plasma cells, which turn over rapidly and are
    replenished by newly differentiated cells.

         Like the T-cell antigen receptor (TCR)-CD3 complex, B-cells
    express surface antigen-combining receptor molecules which are of
    identical specificity to the immunoglobulins they synthesize and
    secrete. The diversity of the natural world has necessitated a complex
    series of molecular events in B-cell development designed to produce a
    spectrum of immunoglobulins capable of protecting the organism. B-cell
    maturation is marked by immunoglobulin gene rearrangements,
    recombinations and somatic mutations, so that a relatively small
    number of genes may efficiently produce a large number of antibody
    specificities.

         B-lymphocytes synthesize immunoglobulins of five different types:
    IgM, IgG, IgA, IgD, and IgE. These proteins are composed of two
    separate types of polypeptide chains joined by disulfide linkages,
    termed the heavy and light chains because of differences in their
    relative molecular masses (the heavy chains are about twice as large)
    (see Fig. 2). Light chains are derived from either kappa or lambda
    genes and combine with the five different heavy chains mu, gamma,
    alpha, delta and epsilon (i.e., for the five different types of
    immunoglobulin identified above). Enzymatic digestion of
    immunoglobulin molecules yields fragments which indicate arrangement
    in a Y-shaped structure, consisting of two arms containing the

    FIGURE 1


    FIGURE 1b


    FIGURE 1c


    antibody-combining sites for antigen, Fab fragments, and a tail region
    (Fc) which is important for effector functions and regulation of
    antibody responses. Surface immunoglobulin is predominantly of the IgM
    and IgD types on naive B cells and secreted immunoglobulin may be
    either IgM, IgG of four subclasses (1 to 4), IgA, or IgE. IgM is
    primarily secreted early, in what is termed the primary antibody
    response to antigen, with IgG constituting the later, secondary
    response. Lymphokines such as IL-4 and TGF-beta induce heavy chain 
    class switching in B-cell antibody responses, leading to the 
    production of either IgGl and IgE, or IgA, respectively (Coffman 
    et al., 1986; Coffman et al., 1989). The nature of the antigen 
    encountered portends these lymphokine-mediated events. IgA-secreting 
    B-cells are predominant in the MALTs, while IgE is of central 
    importance in allergic reactions.

         In addition to surface immunoglobulin, B-cells display receptors
    for Fc regions of immunoglobulin molecules, MHC Class II molecules,
    receptors for complement proteins, and the CD40 molecule which plays
    an essential role in the contact between B- and T-cells. B-cells
    appear to be comprised of two separate lineages, those that do and
    those that do not express the surface marker CD5 (E32). CD5+ B-cells
    comprise a small percentage of the splenic B-cell population, are more
    prevalent in the peritoneal cavity of mice, and appear to be
    long-lived, activated cells that differ from conventional B-cells in
    their activational characteristics and capacity for self-renewal.

    1.2.3  Macrophages

         Stem cells also give rise to mononuclear phagocytes of the
    myeloid series, of which the macrophage is the primary cell type.
    Immature macrophages leave the bone marrow and are found in the
    lymphoid organs, the liver, lungs, gastrointestinal tract, central
    nervous system, serous cavities, bone, synovium and skin, and
    differentiate within these sites. Macrophages are attracted to
    microbes by the gradient of foreign molecules emanating from them, a
    process called chemotaxis. Upon contact, the macrophage can engulf the
    microbe, process and present the derived antigen via its MHC molecules
    to T cells, and secrete cytokines (e.g., IL-1, TNF-alpha, IL-12),
    degradative enzymes, complement components, reactive oxygen
    intermediates and coagulation factors. Macrophages readily infiltrate
    tumours and provide one mechanism of host defence against
    malignancies.

    1.2.4  Antigen-presenting cells

         If an antigen penetrates the tissues it will be processed by
    antigen-presenting cells (APCs) and transported to the draining lymph
    nodes. Antigens that are encountered in the upper respiratory tract or
    intestine are trapped by local mucosal-associated lymphoid tissues,
    whereas antigens in the blood provoke a reaction in the spleen.

    FIGURE 2

    Macrophages in the liver will filter blood-borne antigens and degrade
    them without producing an immune response, since they are not
    strategically placed with respect to lymphoid tissue. Classically, it
    has always been recognized that antigens draining into lymphoid tissue
    are taken up by macrophages. They are then partially, if not
    completely, broken down in the lysosomes; some may escape from the
    cell in a soluble form to be taken up by other APCs and a fraction may
    reappear at the surface either as a large fragment or as a processed
    peptide associated with MHC Class II major histocompatibility
    molecules. Although resting resident macrophages do not express MHC
    Class II, antigens are usually encountered in the context of a
    microbial infectious agent which can induce the expression of MHC
    Class II by its adjuvant-like properties expressed through molecules
    such as bacterial lipopolysaccharide (LPS). There is general agreement
    that the APC must bear antigen on its surface for effective activation
    of lymphocytes and ample evidence that antigen-pulsed macrophages can
    stimulate specific T- and B-cells both  in vitro and when injected
    back  in vivo. Some antigens, such as polymeric carbohydrates like
    ficoll, cannot be degraded because the macrophages lack the enzymes
    required; in these instances, specialized macrophages in the marginal
    zone of the spleen or the lymph node subcapsular sinus, trap and
    present the antigen to B-cells directly, apparently without any
    processing or intervention from T-cells. Notwithstanding this
    impressive account of the macrophage in antigen presentation, there is
    one function where it is seemingly deficient, namely, the priming of
    naive lymphocytes. Animals that have been depleted of macrophages by
    selective uptake of liposomes containing the drug dichloromethylene
    diphosphonate are as good as control animals with intact macrophages
    in responding to T-dependent antigens. It must be concluded that cells
    other than macrophages prime T-helper cells and it is generally
    accepted that these belong to the group of dendritic cells.

         Dendritic cells are large, motile, weakly phagocytic,
    "professional" APCs that usually have several elongated pseudopodia.
    Dendritic cells comprise about 2% of the cells in the secondary
    lymphoid organs. They are localized strategically in the T-cell areas
    of the lymph node (interdigitating dendritic cells). Interdigitating
    cells express large amounts of MHC Class II molecules, and this
    expression plays a pivotal role in the presentation and induction of
    certain kinds of immune cells (such as Th 1) and the presentation of
    antigen to CD4+ T-cells. Active follicular dendritic cells, although
    not derived from haematopoietic stem cells, express high levels of
    CD23 (an IgE Fc receptor) and C3 receptors, which allows them to trap
    antigen-antibody complexes and present them to memory B-cells. Normal
    skin contains a population of dendritic cells called Langerhans cells
    that change their morphology to become interdigitating dendritic cells
    within the T-cell areas of lymph nodes. Langerhans cells give the
    immune system information regarding foreign substances that breach the
    skin. Langerhans cells pick up skin-sensitizing antigens (e.g.,
    antigens of the poison ivy plant) and migrate to the draining lymph
    nodes. Langerhans cells are important in the delayed-type
    hyper-sensitivity response known as contact dermatitis.

         The need for physical linkage of hapten and carrier strongly
    suggests that T-helper cells must recognize the carrier determinants
    on the responding B-cell in order to provide the relevant accessory
    stimulatory signals. However, since T-cells only recognize processed
    membrane-bound antigen in association with MHC molecules, the T-helper
    cells cannot recognize native antigen bound simply to the Ig-receptors
    of the B-cell. Primed B-cells can present antigen to T-helper cells;
    in fact, they work at much lower antigen concentrations than
    conventional presenting cells because they can focus antigen through
    their surface receptors. They must therefore be capable of processing
    the antigen and the current view is that antigen bound to surface Ig
    is internalized in endosomes, which then fuse with vesicles containing
    MHC Class II molecules with their invariant chain. Processing of the
    protein antigen then occurs and the resulting antigenic peptide is
    then recycled to the surface in association with the Class II
    molecules where it is available for recognition by specific T-helper
    cells.

    1.2.4.1  Co-stimulatory molecules in T-cell activation

         Binding of the antigen/MHC-complex to the T-cell receptor
    (Fig. 3) and co-receptors like CD4 and CD8 is not sufficient to
    stimulate naive T-lymphocytes to proliferate and differentiate into
    effector T-cells. For antigen-specific clonal expansion and
    differentiation, a second, co-stimulatory signal is required. The same
    cell that presents the specific antigen to the T-cell receptor must
    deliver this co-stimulatory signal. The best-characterized
    co-stimulatory moleculeson APCs are the so-called B7 molecules, B7.1
    (CD80) and B7.2 (CD 86). Their receptor on T-cells is CD28; all three
    molecules mentioned are members of the so-called immunoglobulin
    superfamily. B7.2 is present on resting APCs, whereas B7.1 is
    expressed predominantly on activated cells. It has been suggested that
    B7.2 is of particular importance in the allergic immune response and
    represents a potential therapeutic target (Robinson, 1998). However,
    clear functional differences between B7.1 and B7.2 have not been
    defined (Lenshow et al., 1996; Chambers & Allison, 1997).

         On naive T-cells, CD28 is the only receptor for B7 molecules.
    Activated T-cells, in contrast, also express another receptor for B7
    called CTLA-4, which closely resembles CD28 but delivers a negative
    signal to the T-cells (Chambers & Allison, 1997). Thus, binding of B7
    to CTLA-4 will contribute to limiting or down-regulating the
    proliferative response and T-cell production of IL-2.

         Because of the requirement for co-stimulatory signals to obtain
    productive antigenic stimulation of T-cells, only so-called
    professional APCs, that is cells that are able to deliver proper
    co-stimulation, can initiate a T-cell-dependent immune response. If
    antigen binds to the T-cell receptor in the absence of proper
    co-stimulation, the T-cell will not be activated but may instead
    become refractory to activation, a state called anergy. In addition to
    the co-stimulatory B7 molecules, a professional APC must also express

    FIGURE 3

    adhesion molecules like ICAM-1, ICAM-2 and LFA-3 and be able to
    process antigen. There is evidence that different types of APCs differ
    with regard to their co- stimulatory properties.

    1.2.5  Adhesion molecules

         Adhesive interactions of leukocytes with other immune cells or
    with non-immune cells are central to the successful functioning of the
    immune system. Such cell-cell interactions are mediated by different
    types of accessory molecules which stabilize attachment, for instance
    between T-cells and APCs, and which may provide (co-)stimulatory
    signals upon triggering of the antigen receptor. These molecules are
    also regularly used as identification markers for distinct leukocytes
    subclasses or for their activational state (Schleimer & Bochner,
    1998). Three families of such cell surface molecules have been
    categorized:

    (i)    The immunoglobulin-gene superfamily includes the
           antigen-specific receptors of B- and T-cells as well as the
           CD4 and CD8 molecules and their respective ligands MHC Class
           II and I; the adhesion molecules CD2, CD54, CD58 and CD102
           also belong to this group.

    (ii)   The integrin family accounts for antigen-independent adhesion
           between cells; their ligands are found on other leukocytes, on
           endothelial cells and in the extracellular matrix; some
           representative members of this family are CD11a/CD18,
           CD11b/CD18, CD11c/CD18 (referring to the alpha/beta chains,
           respectively) and the so-called very late activation (VLA-)
           molecules on T-lymphocytes, which facilitate the migration of
           these cells to peripheral inflammatory sites.

    (iii)  The third family, the selectins, can be expressed on
           leukocytes (L-selectin) and endothelium (E-selectin). These
           molecules play a role in the directed migration of lymphocytes
           (for instance naive lymphocytes bind preferentially to the
           high endothelial cells in the lymph nodes), neutrophils and
           macrophages.

         Table 2 shows the molecules facilitating the cellular contact
    between APC and T-cells, and adhesion molecules playing a role in the
    migration of leukocytes are shown in Table 3. Fig. 4 illustrates
    antigen presentation and cell-cell contact.

    1.2.6  Fc receptors

         Fc receptors (FcR) are cell surface glycoproteins interacting
    specifically with the Fc domains of different isotypes of
    immunoglobulins (Ravetch, 1994, 1997; Gergely & Sarmay, 1996; Deo et
    al., 1997; Vivier & Daeron 1997). FcRs are widely distributed on cells
    of the immune system and mediate different effector responses. In
    addition, they play an important role in the initiation of

    immunocomplex-triggered inflammation and regulate the antibody
    production of B-cells. Immunoglobulin-binding receptors, including the
    high affinity receptor for IgE (Fc-epsilon-RI) on mast cells and
    basophils, the high and low affinity receptors for IgG (Fc-gamma-RI,
    Fc-gamma-RII and Fc-gamma-RIII) and the high affinity receptor for
    IgA, belong to the immunoglobulin supergene family. The low affinity
    Fc-epsilon-RII (CD32) is a lectin-like molecule (Table 4).

         The ligand binding chains (alpha) of all Fc-gamma-Rs contain
    extracellular parts comprising Ig-domains (Fc-gamma-RI has three, the
    others two). The high affinity IgE-binding receptor (Fc-epsilon-RI) is
    a tetrameric molecule containing one alpha, one beta and two gamma
    chains. The IgE-binding site is located on the extracellular part of
    the alpha chain. The beta chain has four transmembrane loops while the
    dimeric gamma chains possess very long cytoplasmic tails.

         Fc-gamma-RI, Fc-gamma-RIII and Fc-epsilon-RI belong to the family
    of multisubunit immune recognition receptors (MIRRs), which are
    characterized by a complex hetero-oligomeric structure in which ligand
    binding and signal transducing functions are segregated into distinct
    receptor substructures (Table 5).

    1.2.7  Polymorphonuclear leukocytes

         Polymorphonuclear leukocytes (PMNs) are myeloid phagocytic cells
    important for the inflammatory responses of both specific and
    nonspecific immunity. Polymorphonuclear leukocytes are also called
    granulocytes because they contain granules composed of digestive
    enzymes and bactericidal substances. The granulocyte progenitor can
    develop into cells called either neutrophils, basophils/mast cells or
    eosinophils, names which refer to the variable dye staining patterns
    of their cytoplasm. These cells are also chemotactic and are attracted
    by lymphokines released from lymphocytes in areas of infection. Like
    macrophages, polymorphonuclear leukocytes participate in
    antibody-dependent cell-mediated cytotoxicity (ADCC) reactions, in
    which coating (opsonization) of microbial surfaces by specific
    antibody enhances their recognition by cytotoxic or phagocytic
    leukocytes.

    1.2.8  Cytotoxic lymphocytes

         Cytotoxic lymphocytes are defined by their capacity to recognize
    and kill target cells. These cells fall into at least two different
    populations, a) those that require recognition of MHC Class I
    molecules for their activation, namely CD8+ T-cells, and b) those
    that are silenced by recognition of these molecules, namely natural
    killer (NK) cells, previously named "null cells" or large granular
    lymphocytes (LGL). Cytotoxic CD8+ T-cells constitute the major
    population of cytotoxic T lymphocytes (CTL) and are crucial for the
    defence against intracellular, in particular viral, pathogens.
    Peptides derived from such pathogens are processed into the endogenous


        Table 2.  Adhesion and (co-)stimulatory molecules mediating antigen presentation
    to T-cells (modified from Janeway et al., 1997)
                                                                                                     

                                  Adhesion molecules expressed on         Ligand expressed on T-cell
                                  antigen-presenting cell (APC)
                                                                                                     

    Initial contact
    between APC and T-cell        CD58 (LFA-3)                            CD2
                                  CD54(ICAM-1)   }                        CD11a/CD18 (LFA-1)
                                  CD102 (ICAM-2) }
                                  CD11a/CD18 (LFA-1)                      CD50 (ICAM-3)

    Antigen presentation and
    T-cell activation             antigenic peptide in MHC context        TCR/CD3
                                  MHC-Class II                            CD4
                                  MHC-Class I                             CD8
                                  CD80 (B7.1)  }                          { CD28
                                  CD86 (B7.2)  }                          { CTLA-4
                                                                                                     

    Table 3.  Adhesion molecules mediating leukocyte migration (from Janeway et al., 1997)
                                                                                                     

                                  Adhesion molecules                      Ligand on endothelium or
                                  expressed on leukocyte                  extracellular matrix
                                                                                                     

    Migration of naive T-cells
    into lymphoid tissue          CD62L (L-selectin)                      { CD34
                                                                          { GlyCAM-1
                                                                          { MadCAM-1 (Mucosae)

    Migration of memory T-cells
    into peripheral tissue        CD11a/CD18(LFA-1)                       { CD54 (ICAM-1)
                                                                          { CD102 (ICAM-2)

                                  Cutaneous lymphocyte                    CD62E (E-selectin)
                                  antigen (CLA)

                                  CD49d/CD29 (VLA-4)                      CD106 (VCAM-1)
                                  CD49d/CD29 (VLA-5)                      fibronectin

    Migration of neutrophil
    and macrophages into
    peripheral tissue             sialyl-Lewis x moiety                   { CD62E (E-selectin)
                                                                          { CD62P (P-selectin)

                                  CD11a/CD18 (LFA-1)                      { CD54 (ICAM-1)
                                                                          { CD102 (ICAM-2)

                                  CD11b/CD18 (MAC-1)                      CD54 (ICAM-1)
                                                                                                     
    

    FIGURE 4

    pathway of antigen presentation and exposed on the outer cell membrane
    by Class I molecules. This complex is recognized by the T-cell
    receptor, after which CTL-target cell binding is further stabilized by
    CD8-Class I interaction. In contrast, NK cell-target cell recognition
    is largely non-specific, but involves receptors recognizing disturbed
    surface carbohydrates and an Fc receptor for IgG that can facilitate
    antibody-dependent cell-mediated cytotoxicity (ADCC). NK cells are
    unique in bearing distinct receptors which, when bound to MHC Class I
    molecules, deliver signals interfering with their cytolytic activity.

         For both types of cytotoxic lymphocytes the actual killing
    process involves two major mechanisms, i.e., release of a membrane
    pore-forming protein named perforin from granules, leading to osmotic
    lysis of target cells, and release of lymphotoxin which activates
    enzymes in the target cell to cleave DNA in the nucleus. The latter
    process is also known as apoptosis. Most cytotoxic lymphocytes also
    express a member of the tumour necrosis factor (TNF) superfamily,
    i.e., Fas-ligand, mediating a third lytic mechanism for target cells
    expressing the Fas antigen. The killing capacity of cytotoxic
    lymphocytes is greatly enhanced by distinct cytokines, in particular
    IL-2 and IL-12. Microscopically this is reflected by the appearance of
    more prominent granules, e.g., in the so-called lymphokine-activated
    killer (LAK) cells. Both major cytotoxic lymphocyte populations are
    crucial to various phases of viral attack, but are not prominent in
    causing allergic disorders. Nevertheless, contact allergens may
    directly associate with surface-bound Class I molecules or enter the
    cytoplasm of, for instance, Langerhans cells and associate with
    peptides presented along the endogenous route of antigen presentation.
    In this way, CD8+ T-cells may become involved in allergic contact
    dermatitis reactions.

    1.2.9  Mast cells

         Mast cells are derived from precursors in the bone marrow that
    migrate to specific tissue sites to mature. While they are found
    throughout the body, they are most prominent in the skin, the upper
    and lower respiratory tract, and the gastrointestinal tract (Tharp,
    1990). In most organs mast cells tend to be concentrated around the
    small blood vessels, the lymphatics, the nerves and the glandular
    tissue (Tharp, 1990). These cells contain numerous cytoplasmic
    granules that are enclosed by a bilayered membrane. There appear to be
    two different populations of mast cells in humans, based on the
    presence or absence of certain proteolytic enzymes, notably tryptase
    and chymase (Tharp, 1990). Mast cells found in the skin and connective
    tissue have both enzymes, while those in the alveoli, bronchial and
    bronchiolar regions, and mucosa of the small bowel contain only
    tryptase (Irani et al., 1986). However, both types of cells are
    triggered in the same manner.


        Table 4.  Cellular distribution and binding properties of Fc-gamma receptors
                                                                                                                        

    Class          CD     Relative molecular mass  Affinity (Ka)   Expressiona                             Ig-bindingb
                                                                                                                        

    Fc-gamma-RI    CD64   72 000                   108-109 M-1     Mo, M                                   hu, 3>1>4>>2

    Fc-gamma-RII   CD32   40 000                   <107 M-1        Mo, N, Ba, Eo, Langerhans cell, B-cell  hu, 3>1>>2,4
                                                                                                           mu, 2b>>2a

    Fc-gamma-RIII  CD16   50 000-80 000                            Thr, endothelial cells of the placenta

    Fc-gamma-IIIa                                  3×107 M-1       Mo, M, LGL/NK, T-cell                   hu, 1=3>>2,4
                                                                                                           mu, 1=3>>2,4

    Fc-gamma-IIIb                                  <107 M-1        N
                                                                                                                        

    a  Mo = monocyte, M = macrophage, N = neutrophil granulocyte, Ba = basophil granulocyte, Eo = eosinophil granulocyte,
       Thr = platelet, LGL = large granular lymphocyte, NK = natural killer cell
    b  hu = human, mu = murine

    Table 5.  Multisubunit immune recognition receptors (MIRRs) family
                                                                                                        

    Receptor            Ligand-binding subunit     Signal transducing subunit
                                                                                                        
    BCR
    (B-cell
    antigen receptor)   mIg                        Ig-alpha (CD79a)
                                                   Ig-beta (CD79b)

    TCR                 alpha-beta or gamma-delta  CD3-gamma, delta and epsilon zeta-zeta or zeta-eta
    (T-cell
    antigen receptor)

    Fc-epsilon-RI       alpha-chain                beta and gamma chain

    Fc-gamma-RIIIa      alpha-chain                Fc-epsilon-RI-gamma-chain or TCR zeta-chain

    Fc-gamma-RI         alpha-chain                Fc-epsilon-RI-gamma-chain
                                                                                                        
    

         Mast cells may be activated by antigen-specific IgE bound to high
    affinity receptors (Fc RI), antigen-specific IgE bound to low affinity
    IgG receptors (Fc-epsilon-RII/III), or through complement receptors.
    Following activation, most cells release preformed mediators such as
    histamines and generate newly formed mediators such as TNF-alpha and
    leukotriene C4 (LTC4) (Van Loveren et al., 1997). Both mast cells and
    basophils arise from CD34 pluripotent stem cells. At what point the
    cell lineages diverge is unknown, but mature mast cells depend on the
    local production of C-kit ligand (stem cell factor) for their
    survival. Basophils will not survive in the presence of stem cell
    factors but do respond to IL-3.

    1.2.10  Basophils

         Basophils represent approximately 1% of the white blood cells in
    peripheral blood. They have a half-life of about 3 days. They respond
    to chemotactic stimulation and tend to accumulate in inflammatory
    reactions. Basophils have high affinity IgE receptors as do mast
    cells. Cross-linking of surface-bound IgE by a multivalent specific
    allergen causes changes in the cell membrane and signal transduction
    that result in the release of mediators from the cytoplasmic granules.
    These preformed mediators include histamine, many other potent
    mediators, and proteolytic enzymes (Tharp, 1990; Goust, 1993; Janeway
    et al., 1997). Release of these substances from mast cells and
    basophils is responsible for the early phase symptoms seen in allergic
    reactions, which occur within 30 to 60 min after exposure to the
    allergen. IL-4 synthesis and release occurs hours later. Release of
    these basophil-derived mediators is believed to contribute to the late
    phase allergic response. The clinical manifestations due to release of
    both preformed and newly synthesized mediators from mast cells and
    basophils vary from a localized skin reaction to a systemic response
    known as anaphylaxis. Symptoms depend on variables such as route of
    exposure, dosage and frequency of exposure (Marsh & Norman, 1988).

    1.2.11  Eosinophils

         Eosinophils represent 2-5% of the leukocytes. Polymorphonuclear
    eosinophils resemble polymorphonuclear neutrophils, with the
    difference that they contain large red granulations (eosin staining)
    and refringent crystals, which may also be traced in the expectorates
    of asthmatic patients (Charcot-Leyden crystals). Eosinophil counts are
    increased, especially in allergic reactions, but they also act as a
    defence against certain parasites, in chronic inflammatory phenomena,
    and perhaps also in the defence against cancer. Like neutrophils, they
    do not return to the bone marrow from which they originate, but are
    eliminated via mucosal surfaces.

         In the biphasic pattern of certain asthma attacks (an acute phase
    followed, about 6 h later, by a late phase), eosinophils attracted to
    the inflammatory zone during the late phase cause extensive
    destruction of the bronchial mucosa. This is similar to the
    destruction by eosinophils of certain parasites like schistosomes,
    responsible for schistosomiasis.

    1.2.12  Complement components

         Protective immunity requires the interaction of the immune cell
    types described above with secreted proteins found in the blood and
    lymph. In addition to antibody and lymphokines, the complement
    proteins represent a series of important protective substances
    (Table 6). More than 20 of these proteins participate in reactions
    that mediate lysis of foreign cells. Complement-mediated lysis of
    bacterial cells, for example, can take place through two routes, the
    classical pathway, which is catalysed by complexes of antibody
    molecules, or the alternative pathway, which can be activated by the
    antigen alone and by some immunoglobulins (Fig. 5). This results in
    deposition of a membrane attack complex of complement proteins on the
    surface of the microbial cell, leading to lysis. This process occurs
    as a cascade of enzymatic cleavage reactions, yielding both the lytic
    structure and production of biologically active components that induce
    migration of lymphocytes and an inflammatory response.

    1.2.13  Immunoglobulins

         Table 7 summarizes the human immunoglobulin isotypes and their
    concentrations in serum.

    1.2.13.1  IgG

         IgG represents 75-80% of the total Ig in humans. IgG2 and IgG4
    cross the placental barrier. Thus, at birth, a baby temporarily
    carries IgG of its mother, which lasts for 4-6 months.

         IgG intervenes in infections by means of opsonization and it can
    neutralize toxins. IgG appears especially following a secondary immune
    response, i.e., after a second encounter with antigen. The secretion
    of IgG is modulated by collaboration between B- and T-lymphocytes. IgG
    is strongly opsonizing for macrophages and polymorphonuclear cells
    possessing receptors for the Fc portion of IgG.

         Antigenic analysis of IgG myelomas revealed further variation and
    showed that they could be grouped into four isotypic subclasses now
    termed IgG1, IgG2, IgG3 and IgG4. The differences all lie in the heavy
    chains, which have been labelled gamma1, gamma2, gamma3 and gamma4,
    respectively. These heavy chains show considerable homology and have
    certain structures in common with each other -- those which react with
    specific anti-antisera -- but each has one or more additional
    structures characteristic of its own subclass arising from differences
    in primary amino acid composition and in interchain disulfide
    bridging. These give rise to differences in biological behaviour
    (Table 8).


        Table 6.  Principal components of the complement system
                                                                                                                                    

    Protein                  Relative molecular           Concentration in           Characterization 
                             mass                         serum (µg/ml)              and function
                                                                                                                                    

    Early components
      Classical pathway

    C1q                      410 000                      70                         consists of a
                                                                                     collagen-like and
                                                                                     a globular part; binds to the Fc part of Ig

    C1r                       85 000                      50                         serine protease; activates C1s

    C1s                       85 000                      50                         serine protease; activates C4-C2

    C4                       210 000                      300                        C4b binds to C2b

    C2                       110 000                      25                         serine protease; catalytical part of C4bC2ba

      Lectin pathway

    MBL (Mannose-binding     410 000                      1                          consists of a collagen-like and a carbohydrate part
        lectin)

    MASP1 (Mannose-binding
    lectin associated
    serine protease)          85 000                      5                          serine protease; activates MASP2
    MASP2                     85 000                      5                          serine protease; activates C4

      Alternative pathway

    Factor-D                  25 000                      1                          serine protease; activates factor-B
    Factor-B                  93 000                      200                        serine protease; as the component of
                                                                                     C3bBba convertase activates C3
    Properdin                220 000                      25                         stabilizes the C3bBba convertase

    Table 6.  (continued)
                                                                                                                                    

    Protein                  Relative molecular           Concentration in           Characterization 
                             mass                         serum (µg/ml)              and function
                                                                                                                                    

    Common component of
    the various pathways
    C3                       190 000                      1300                       together with C3b, interacting with
                                                                                     C4b2ba and C3bBba forms C5-convertase;
                                                                                     fragment C3a is one of the anaphylatoxins

    Terminal components

    C5                       190 000                      70                         fragment C5b binds C6; fragment C5a
                                                                                     is one of the anaphylatoxins

    C6                       120 000                      60                         binds C7
    C7                       110 000                      55                         binds C8
    C8                       150 000                      55                         binds C9
    C9                        70 000                      60                         its polymerized form is the MAC
                                                                                     (membrane attack complex)
                                                                                                                                    

    a  The MBL-MASP complex (which is structurally similar to the C1 complex) activates the complement system.
       The carbohydrate-binding domain of MBL binds to the carbohydrate components of various microorganisms
       and the MASP cleaves C4.
    



    FIGURE 5



        Table 7.  Human immunoglobulin isotypes
                                                                                             

    Class       Subclass   H-chain    Relative molecular mass  Concentration in serum
                                                               (mg/ml)
                                                                                             

    IgA         IgA1       alpha-1    150 000,                 3.0
                                      300 000,
                                      400 000a

                IgA2       alpha-2    150 000,                 0.5
                                      300 000,
                                      400 000a

    IgD         -          delta      180 000                  trace
    IgE         -          epsilon    190 000                  trace
    IgG         IgG1       gamma-1    150 000                  9.0
                IgG2       gamma-2    150 000                  3.0
                IgG3       gamma-3    150 000                  1.0
                IgG4       gamma-4    150 000                  0.5
    IgM         -          mu         950 000b                 1.5
                                                                                             

    a  monomeric, dimeric, trimeric
    b  pentameric
    

    1.2.13.2  IgA

         IgA represents 15-20% of the human serum immunoglobulin pool,
    where it occurs as a monomer of the regular immunoglobulin four-chain
    unit, in contrast to secretory IgA, which mainly occurs in dimeric
    form. The J chain which joins 2 IgA monomers facilitates the transfer
    of the secretory component through cells. IgA is the predominant
    immunoglobulin in seromucous secretions such as saliva, colostrum,
    milk, and tracheobronchial and genitourinary secretions. Dimer
    secretory IgA (sIgA), which may be of either of two subclasses (IgA1
    or IgA2), but is mainly IgA2, is normally associated with yet another
    protein, known as the secretory component. The bound secretory
    component facilitates the transport of sIgA through the epithelial
    cell layer(s) into the secretions and protects the antibody-dimer
    against subsequent proteolytic attack. IgA2 predominates in secretions
    since many microorganisms in the respiratory and gastrointestinal
    tracts release proteases that cleave IgA1, but not IgA2. Next to IgA,
    varying levels of IgE may be produced by locally residing plasma
    cells, but the primary site of action of this antibody isotype is in
    the sub-epithelial mucosal layers, e.g., in sensitizing locally
    intruding protozoan parasites and worms for subsequent cytolytic
    attack, notably by eosinophils. The secretory IgA (IgA-s) does not
    opsonize. It fixes antigen via its variable part and forms unabsorbed
    complexes. By capturing antigens, it prevents bacteria and viruses
    from adhering to the mucous membrane, thereby preventing their
    penetration into the organism.

         The Fc fragment of IgA does not play any role, probably because
    it is obstructed by the secretory component.

         IgA deficiency is encountered in one of 700 individuals, causing
    in such patients more frequent respiratory or gastrointestinal
    infections. In case of IgA deficiency, IgM can take over. In severe
    cases, there may be a simultaneous deficiency of both IgA and IgM.

    1.2.13.3  IgM

         IgM represents about 10% of immunoglobulins. IgM antibodies are
    pentamers (5 units), the monomeric units being fixed by a J chain.
    They are also known as macroglobulins or heavy globulins. IgM is the
    first to appear in an immune response, and is the predominant antibody
    isotype in the early phase of humoral immunity. As it has a short life
    span, its presence points out to a recent infection (e.g., in
    toxoplasmosis). Owing to its polyvalent structure, IgM can easily
    produce agglutination and readily fixes complement. Because of its
    large volume, it remains localized principally in blood. It does not
    cross the placental barrier and is the first molecule to meet a viral
    or microbial intruder in a blood vessel.

         IgM antibodies tend to be of relatively low affinity as measured
    against single determinants (haptens) but, because of their high
    valency, they bind with considerable avidity to antigens with multiple


        Table 8.  The properties of human Ig isotypes
                                                                                        

                                 IgG1   IgG2  IgG3    IgG4   IgM    IgA1  IgA2  IgD    IgE
                                                                                        
    Complement activation,
    classical pathway            ++     +     +++     -      +++    -     -     -      -

    Complement activation,
    alternative pathway          -      -     -       -      -      +     -     -      -

    Placental transfer                  +             +      -      -     -     -      -

    Binding to macrophages
    and other phagocytic cells   +      -     +       -      -      -     -     -      +

    High affinity binding to
    mast cells and basophils     -      -     -       -      -      -     -     -      +++
                                                                                        
    

    epitopes. For the same reason, these antibodies are extremely
    efficient agglutinating and cytolytic agents and, since they appear
    early in the response to infection and are largely confined to the
    bloodstream, it is likely that they play a role of particular
    importance in cases of bacteraemia. The isohaemagglutinins (anti-A,
    anti-B) and many of the "natural" antibodies to microorganisms are
    usually IgM; antibodies to the typhoid O antigen (endotoxin) and the
    WR antibodies in syphilis are also found in this class. IgM appears to
    precede other isotypes in the phylogeny of the immune response in
    vertebrates.

         Monomeric IgM (i.e., a single four-peptide unit), with a
    hydrophobic sequence in the C-terminal end of the heavy chain to
    anchor the molecule in the cell membrane, is the major antibody
    receptor used by B-lymphocytes to recognize antigen.

    1.2.13.4  IgD

         This class was recognized through the discovery of a myeloma
    protein that did not have the antigenic specificity of A or M,
    although it reacted with antibodies to immunoglobulin light chains and
    had the basic four-peptide structure. The hinge region is particularly
    extended and, although protected to some degree by carbohydrate, it
    may be this feature that makes IgD, among the different immunoglobulin
    classes, uniquely susceptible to proteolytic degradation, and accounts
    for its short half-life in plasma (2.8 days). It has been demonstrated
    that nearly all the IgD is present, together with IgM, on the surface
    of a proportion of B-lymphocytes where it seems likely that they may
    operate as mutually interacting antigen receptors for the control of
    lymphocyte activation and suppression. The greater susceptibility of
    IgD to proteolysis on combination with antigen could well be
    implicated in such a function.

    1.2.13.5  IgE

         The plasma level of IgE in normal individuals is low (Table 7).
    The IgE level is commonly increased in patients suffering from Type I
    allergies. It is a cytophilic Ig, i.e., it fixes to the surface of
    certain cells, especially mast cells and basophils. It does not fix
    complement. IgE occurs predominantly in perivascular tissues where
    mast cells are localized. IgEs are responsible for Type I allergic
    reactions. The binding of IgE with an antigen specific to this IgE on
    the mast cell membrane provokes the release of mediators from mast
    cell granules (degranulation)(see section 1.2.9).

         IgE plays a major role in allergy, but it also appears to
    intervene in the defence against parasites and perhaps also against
    cancer cells. A high IgE level in apparently healthy babies has been
    suggested as an accurate indicator of later allergic disorders (see
    chapter 5).

         IgE levels are particularly elevated in atopic eczema and in
    intestinal parasitoses. Similarly elevated levels are also found in
    certain myelomas and in disorders involving a long- or short-term
    deficiency in T-lymphocytes, such as measles, infectious
    mononucleosis, Hodgkin's disease, and dysglobulinaemia. Specific IgE
    plays an important role in Type I allergies.

    1.3  Immunotoxicology

         Immunotoxicology may be defined as the scientific discipline
    concerned with the adverse effects resulting from the interaction of
    the immune system with xenobiotics. It includes the consequences of an
    action (i.e., either suppression or enhancement) by a substance (or
    its metabolite) on the immune system, as well as the immunological
    response to such a substance (IPCS, 1996). A major focus of
    immunotoxicology is the detection and evaluation of undesired effects
    of substances by means of appropriate experiments. The prime concern
    is to assess the importance of these interactions in regard to human
    health. Toxic responses may occur when the immune system is the target
    of chemical insults, resulting in altered immune function; this in
    turn can result in decreased resistance to infection, certain forms of
    neoplasia, or immune dysregulation or stimulation which exacerbates
    allergy or autoimmunity. Alternatively, toxicity may arise when the
    immune system responds to the antigenic specificity of the chemical as
    part of a specific immune response (i.e., allergy or autoimmunity).
    Certain drugs induce autoimmunity. The differentiation between direct
    toxicity and toxicity due to an immune response to a compound is, to a
    certain extent, artificial. Some compounds can exert a direct toxic
    action on the immune system as well as altering the immune response.
    Heavy metals like lead and mercury, for instance, manifest
    immunosuppressive activity, hypersensitivity and autoimmunity.

    1.4  Immunosuppression/immunodeficiency

    1.4.1  Biological basis of immunosuppression/immunodeficiency

         The occurrence of acquired immunodeficiency states was recognized
    sporadically in scattered individuals during the 1960s and 1970s. In
    the late 1970s and early 1980s, a new syndrome that spread rapidly
    through certain groups was identified as a generalized type of
    acquired immunodeficiency syndrome (AIDS). This disorder was found to
    be due to a specific retrovirus that infects and destroys T helper
    (Th) cells in humans (Fauci et al., 1991). These helper lymphocytes
    have been identified in experimental studies as the key cells in the
    recognition of antigen. Decrease in numbers of Th-cells leads to
    impaired immune responses to a variety of infectious agents as well as
    the occurrence of certain types of neoplasms. AIDS appears to result
    from declining numbers of Th-cells with persistence of residual
    populations of CD8+. Progression of AIDS is associated with
    progressive loss of the Th-cells and an increased frequency of
    infections by bacterial, fungal, viral and parasitic agents.

         Other types of acquired immunodeficiency conditions have been
    recognized and defined in the past two decades. Many have been related
    to specific immunosuppressive drugs, chemotherapeutic agents and
    certain chemicals (IPCS, 1996). The immunosuppressive effects of
    xenobiotics in humans due to environmental exposure, when compared to
    genetically determined immunodeficiency defects, do not reveal the
    same degree of severity and persistence in the xenobiotic-related
    immune defects as seen in the genetic disorders.

         The dynamic nature of the immune system renders it especially
    vulnerable to toxic influence. Reactions of lymphoid cells are
    associated with gene amplification, transcription and translation.
    Compounds that affect the processes of cell proliferation and
    differentiation are especially immunotoxic. This applies in particular
    to the rapidly dividing haematopoietic cells of the bone marrow and
    thymocytes. Thus, the disappearance of lymphoid cells from bone
    marrow, blood and tissue, and thymus weight may be the first and most
    obvious signs of toxicity. Thymocytes are very susceptible to the
    action of toxic compounds (Schuurman et al., 1992). It should be noted
    that thymocyte depletion, suggestive of toxicity towards this cell
    population, may actually be an indirect effect in cases where the cell
    microenvironment is damaged and unable to support thymocyte growth.
    The susceptibility of thymocytes to toxicity is related to the fragile
    composition of these cells, especially cortical thymocytes, and to the
    sensitive interactions between thymocytes and their microenvironment.
    For instance, thymocytes are programmed to enter apoptosis when
    activated during the physiological process of selection. The main
    function of the thymus is T-cell (repertoire) generation during fetal
    and early postnatal life. Its susceptibility to toxic compounds and
    the subsequent effects on the cell-mediated immune system are most
    prominent during this period of life. The skin, respiratory tract and
    gastrointestinal tract together form an enormous surface that is in
    close contact with the outside world, and they are potentially exposed
    to a vast magnitude of microbial agents and potential toxicants. For
    the respiratory tract, this is illustrated by human data on the
    immunopathogenesis of lung diseases including asthma, fibrosis and
    pulmonary infections. Examples of inhaled pollutants that may induce
    these diseases are oxidant gases and particulates such as silica,
    asbestos and coal dust.

         The skin is an important target in immunotoxicology, as, for
    instance, when there is contact with chemical allergens (Kimber &
    Cumberbatch, 1992a,b) and UV-B irradiation (Goettsch et al., 1993).
    The skin can respond to many xenobiotics by a specific immune response
    (contact hypersensitivity) or by a non-specific inflammatory response
    (contact irritancy); both responses are associated with the induction
    of pro-inflammatory cytokines.

         Drugs provide examples illustrating susceptibility to immunotoxic
    effects. A number of cytostatic drugs are immunosuppressant. In
    clinical medicine, cytostatic drugs used in cancer therapy often
    produce bone marrow depression as a major side effect with increased
    risk for infections as the result.

    1.4.2  Consequences of immunosuppression/immunodeficiency

         The major consequence of immunodeficiency or impaired immune
    responsiveness is failure of protection of the host by antibody or
    effector cells directed against specific target antigens. Antibody and
    effector cells are essential for a protective effect against
    infectious and toxic agents that can cause destructive tissue injury
    and disseminated infections (Buckley, 1992). An impaired immune
    response also limits the response to protective vaccines that normally
    build adequate levels of cellular and antibody protection against
    infectious agents. Selective impairment of immune responsiveness in
    some instances may also lead to hypersensitivity states due to
    dysregulation. This effect could also result in autoimmune disease by
    promoting recognition of self-antigens, and hyperresponsiveness with
    increased antibody and effector cell production (Bigazzi, 1988;
    Broughton & Thrasher, 1988; Chandor, 1988). Increased potential for
    the development of neoplasia and disseminated malignancies, especially
    those of the lymphocytic tissues, may occur with impaired immune
    surveillance (Radl et al., 1985; Byers et al., 1988).

         The duration of immunodeficiency states might be transient or
    long-lasting, depending on the severity and site of the specific
    xenobiotic effect (Bekesi et al., 1987; Broughton & Thrasher, 1988).
    The immune impairment that results from continued specific drug
    therapy with immunosuppressive agents or human immunodeficiency virus
    (HIV) infection are the only examples of long-lasting acquired
    immunodeficiency in humans (Jenkins et al., 1988; Fauci et al., 1991).
    Indeed, studies that have reported acquired deficiency of immune
    function as a result of xenobiotics or radiation have shown the marked
    capacity for self-restoring activity of the immune system, so that
    once an offending agent has been cleared from the body the various
    cellular components return to a normal state (Kishimoto & Hirano,
    1984).

    1.5  Immunological tolerance

         Immunological tolerance refers to a state of non-responsiveness
    that is specific for a particular antigen, and is induced by prior
    exposure to that antigen. Tolerance can be induced to non-self
    antigens, but the most important aspect of tolerance is
    self-tolerance, which prevents the body from mounting an immune attack
    against itself. The potential for attacking the body's own cells
    arises because the immune system randomly generates a great diversity
    of antigen-specific receptors, some of which will be self-reactive.
    Cells bearing these receptors must be eliminated, either functionally
    or physically.

    1.5.1  T-cell tolerance to self-antigens

         The thymus is central to the development of T-cells. Within the
    thymus, T-cells develop from precursors that have not undergone
    rearrangement of their T-cell antigen receptor (TCR) genes. In the
    thymus, T-cells acquire the "education" that ensures that they respond

    to antigens only in the context of molecules encoded by self major
    histocompatibility complex (MHC) molecules. It is likely that
    self-reactive T-cells are also dealt with and eliminated in the
    thymus.

         The high proliferative rate of thymocytes is paralleled by a
    massive rate of cell death: the vast majority of T-cells, at the
    double positive (CD4+ CD8+) stage, die within the thymus. Among the
    factors that account for this are aberrant T-cell antigen receptor
    (TCR) rearrangement, negative selection, and failure to be positively
    selected. Positive selection occurs when T-cells, with some degree of
    binding avidity for polymorphic regions of major histocompatibility
    complex (MHC) molecules, are selected for survival. The MHC molecules
    are encountered on thymic cortical epithelial cells, and binding is
    presumed to protect the cells from programmed cell death. This
    positive selection process ensures that the mature T-cell only
    recognizes antigen (peptides) when associated with self-MHC molecules,
    and so will be self-MHC restricted. Negative selection, on the other
    hand, eliminates self-reactive T-cells, discarding those clones of
    T-cells that are specifically reactive to self-antigens present
    intrathymically.

         The timing and precise localization of negative selection depends
    on a variety of factors, including the accessibility of developing
    T-cells to self-antigen, the combined avidity of the T-cell receptor
    and accessory molecules, CD8 or CD4, for the self-MHC-self-peptide
    complex, and the identity of the deleting cells. Elimination of
    self-reactive cells is clearly a function of the thymic dendritic
    cells or macrophages which are rich in MHC Class I and II molecules
    and situated predominantly at the corticomedullary junction. Some
    medullary or cortical epithelial cells may also impose negative
    selection. Other cells involved in deletion may be the thymocytes
    themselves. Specialized "veto" cells bearing self epitopes would
    impart a negative signal, killing the self-reactive clone. Under
    physiological conditions, veto signals occur when a T-cell with T-cell
    receptors for self antigens binds to a veto cell. The veto cell is a
    specialized T-cell expressing self epitopes. For the veto effect to
    occur, the T-cell antigen receptor (TCR) has to bind to self antigen
    in association with MHC Class I on the veto cell, while the CD8 of the
    veto cell binds to MHC Class I on the T-cell. Once binding has
    occurred, the T-cell is killed.

    1.5.2  B-cell tolerance to self antigens

         Production of high-affinity autoantibodies is T-cell dependent.
    For this reason, and since the threshold of tolerance for T-cells is
    lower than that for B-cells, the simplest explanation for
    non-self-reactivity by B-cells is a lack of T-cell help. Nevertheless,
    circumstances exist in which B-cells need to develop tolerance
    directly. For example, there may be cross-reactive antigens on
    microorganisms, which include both foreign T-cell-reactive epitopes

    and other epitopes resembling self epitopes and capable of stimulating
    B-cells (molecular mimicry). Such antigens could result in a vigorous
    antibody response to self antigens. Furthermore, in contrast to T-cell
    receptors, the immunoglobulin receptors on mature,
    antigenically-stimulated B-cells can undergo hypermutation and may
    acquire anti-self reactivities at this late stage. Tolerance must thus
    be imposed on B-cells, both during their development and after
    anti-genic stimulation in secondary lymphoid tissues.

         The fate of self-reactive B-cells has been determined using
    transgenic technology. The transgenic models showed that induction of
    tolerance by self-antigens could lead to one of several end results.
    The outcome depends on the affinity of the B-cell antigen receptor and
    on the nature of the antigen it encounters, whether an integral
    membrane protein, such as an MHC Class I molecule, or a soluble and
    largely monomeric protein present in the circulation.

         When B-cells encounter cell-membrane-associated self-antigens
    capable of cross-linking Ig receptors on the B-cells with high
    avidity, the B-cells are eliminated from lymphoid tissues. This type
    of tolerance occurs whether the self-antigens are expressed on cells
    in the bone marrow or elsewhere. In either case, the bone marrow
    contains residual self-reactive B-cells, suggesting that immature
    B-cells are less readily deleted than immature T-cells during the
    early stages of differentiation.

         If self-reactive B-cells are exposed to soluble antigen that is
    largely monomeric (not capable of cross-linking receptors), then the
    cells are not deleted from secondary lymphoid tissues, where they can
    be found in normal numbers, but are rendered anergic. This effect only
    occurs when the antigen is above a critical concentration threshold.
    Anergy is associated with down-regulation of the membrane IgM
    receptor. The maturation of the self-reactive B-cells is also arrested
    in the follicular mantle zone and there is a striking reduction in
    marginal zone B-cells with high levels of surface IgM. No evidence for
    the activity of T-cells or of anti-idiotypic B-cells was found in
    these transgenic models.

    1.5.3  Tolerance to non-self antigens

    1.5.3.1  Scope

         Exposure to environmental and occupational allergens mainly takes
    place along the skin and the mucosal surfaces lining the
    gastrointestinal tract and the airways. Since no nutrients have to
    pass the skin, skin barrier function simply focuses on exclusion of
    exogenous molecules. Any macromolecule bypassing the skin epithelial
    barrier is a potential health threat, and is subjected to
    pro-inflammatory responses aimed at the most rapid destruction and/or
    killing of the exogenous material. In sharp contrast, mucosal surfaces
    along the gastrointestinal tract and the airways face a liquid or
    moist environment which may contain valuable nutrient molecules, next

    to a plethora of potentially toxic substances, including
    microorganisms. Subtly balanced defence mechanisms have evolved,
    therefore, along these mucosal surfaces to exclude microorganisms, and
    to facilitate the entry of smaller nutrient molecules, such as
    oligopeptides.

         As a consequence, mucosal contacts with potential allergens may,
    depending on the conditions, lead to either tolerance or
    sensitization. The molecular and cell-biological characterization of
    cytokines and adhesion molecules has led to better understanding of
    the mechanisms involved in oral tolerance. There are primary,
    non-immunological factors determining mucosal defence against
    exogenous toxic pressures, including the roles of transmembrane
    transporter molecules and TGF-beta in epithelial barrier function, as 
    well as alveolar macrophages and secretory IgA. The dichotomy between
    Th1- and Th2-type immune responses in skin and mucosa, and the
    supplementary role of TGF-beta are important.

    1.5.3.2  Mucosal defence against exogenous toxic pressures

         Distinct molecular mechanisms provide primary protection of
    mucosal tissues against toxic pressure from exogenous toxic agents. If
    these mechanisms fail, exogenous compounds penetrate the mucosa, reach
    mucosal immunocytes, and induce undue immune reactivity. This leads to
    local release of immunopharmacological mediators, such as
    leukotrienes, further enhancing entry of xenobiotics by opening the
    tight junctions. Studies, primarily aiming at elucidating mechanisms
    of cytostatic drug-resistance in tumour cells, have shown the
    existence of different molecular pumps mediating transmembrane
    transport of potentially toxic molecules. Localization of these
    molecules on the outer plasmacellular membrane contributes to the
    efflux of exogenous toxic substrates from the cell interior to the
    extracellular space, and localization on vesicle membranes contributes
    to their loading into exocytotic vesicles, thus facilitating their
    removal. While over-expression of these molecules on tumour cells
    contributes to resistance to a vast array of cytostatic drugs
    (multidrug resistance: MDR), the presence of such molecular pumps on
    epithelial cells lining mucosal surfaces is thought to mediate a
    primary barrier function to exogenous toxic pressure.

         MDR-related proteins are abundantly present in various normal
    tissues (Flens et al., 1996; Izquierdo et al., 1996). There,
    MDR-related proteins represent physiological mechanisms of cellular
    resistance to potentially toxic compounds. In normal tissues high
    levels of these proteins can be observed on the luminal membranes of
    epithelial cells lining mucosal surfaces chronically exposed to
    xenobiotic agents, such as the respiratory epithelia in the trachea
    and bronchi within the lung, and colonic epithelial cells. In the gut
    they are thought to prevent too high intracellular concentrations of
    potentially toxic molecules showing some degree of lipophilicity (van
    der Valk et al., 1990; Weinstein et al., 1991). No regulatory

    mechanisms have yet been defined determining to what extent MDR
    molecules are expressed in mucosal lining cells. It is also still
    unknown whether chronic inflammatory processes in the gastrointestinal
    tract and airways might develop after failure of detoxifying
    mechanisms similar to those mediating drug-resistance in tumour cells.

         Mucosal epithelial barrier function is not only dependent on the
    capacity of individual cells to resist uptake and passage of
    potentially toxic molecules, but also on the integrity of the
    epithelial cell layer(s). Important roles in maintaining this
    integrity are played by two cytokines, IFN-gamma and TGF-beta
    (Planchon et al., 1996). Of substances released by lymphocytes,
    including those that reside in the mucosa, only IFN-gamma has been 
    reported to have a potent effect in reducing the barrier function of 
    epithelial monolayers  in vitro (Madara & Stafford, 1989; Adams et 
    al., 1993). TGF-beta was found to enhance the integrity of epithelium 
    for normal homoeostasis (Derynck et al., 1988; Graycar et al., 1989; 
    Planchon et al., 1994). TGF-beta stimulates the synthesis of 
    extracellular matrix proteins (collagen, fibronectin) by up-regulating 
    their gene expression (Ignotz & Massague, 1986) and alters the 
    expression of integrins that act as receptors for these proteins, 
    thereby enhancing the cell's ability to bind them (Heine et al., 
    1989). IFN-gamma and TGF-beta antagonism is most clearly revealed by 
    the striking ability of TGF-beta-1 to reduce the capacity of IFN-gamma 
    to disrupt epithelial barrier function (Planchon et al., 1996).

         Another critical factor in the prevention of the potential
    harmful entry of excessively large doses of antigens or microorganisms
    into the mucosal tissues is the presence of IgA in the mucosal
    secretions. IgA is highly efficient in complexing luminal antigenic
    molecules and particles, thus reducing their chance of sneaking
    through the epithelial barrier, and facilitates their uptake and
    degradation by luminal phagocytes, e.g., pulmonary alveolar
    macrophages (PAMs). The fact that TGF-beta is an important factor in
    switching B-cell immunoglobulin synthesis to IgA production supports
    the critical role of this cytokine in maintaining homoeostasis within
    the mucosal tissues.

         The maintenance of homoeostasis in the lungs requires particular
    protection against environmental antigens. Chronic inflammatory immune
    responses would be detrimental for these delicate tissues involved in
    gas exchange. Highly active macrophages are present within the
    alveolar spaces able to digest and eradicate exogenous antigens and
    microorganisms, thus preventing these from even reaching the
    epithelial barrier. Activation of PAMs is reflected by their
    production of nitric oxide synthetase, leading to the local release of
    nitric oxide, known as an effector molecule in macrophage-mediated
    antimicrobial responses (Nussler & Billiar, 1993). Since nitric oxide
    release is not a constitutive property of resident PAMs, effective
    scavenging function requires a milieu of activating cytokines, such as
    IFN-gamma, and the often synergistic cytokines IL-2 and TNF-alpha. On

    the other hand, under steady state conditions, pro-inflammatory
    processes are tightly controlled by lymphocytostatic signals generated
    by the same resident PAMs. The mechanism(s) by which PAMs mediate
    immune suppression, e.g., of T-cell proliferation, has been the
    subject of much debate, and proposed mediators include prostaglandins
    (Monick et al., 1987; Fireman et al., 1988), TGF-beta (Roth & Golub, 
    1993) and interleukin-1 receptor-antagonist (Moore et al., 1992). 
    TGF-beta has been identified as a most critical mediator in 
    suppressing local pro-inflammatory responses by its unique activity 
    in antagonizing IFN-gamma-induced macrophage activation (Bilyk & Holt,
    1995).

    1.5.3.3  Induction of oral tolerance

         Chase (1946) confirmed that oral feeding of antigen could result
    in a state of specific immunological unresponsiveness. Feeding contact
    allergens to guinea-pigs made the animals refractory to subsequent
    sensitization via the skin. Handling of antigen by the gut is
    important in terms of both general and secretory immunity. The
    induction of immunological unresponsiveness in humans by oral
    ingestion of potential allergens was supported by the observation that
    South American Indians ate poison ivy leaves in an attempt to prevent
    contact sensitivity reactions to the plant (Dakin, 1982).

         Systemic unresponsiveness after antigen feeding has been
    described for a large variety of T-cell-dependent antigens, of which
    the protein ovalbumin has been most extensively studied (reviewed in
    Mowat, 1987). In addition, proteins such as bovine serum albumin
    (Silverman et al., 1982; Domen et al., 1987), particulate
    (erythrocyte-bound) antigens (Kagnoff, 1982; MacDonald, 1983;
    Mattingly, 1984), inactivated viruses and bacteria (Stokes et al.,
    1979) and autoimmune-related antigens (Thompson & Staines, 1990), as
    well as contact allergens, have been shown to induce oral tolerance
    (Asherson et al., 1977; Newby et al., 1980; Gautam et al., 1985).
    Generally, T-cell-mediated delayed-type hypersensitivity responses and
    IgE production are the types of immune responses to which tolerance
    develops most readily. Persistent tolerance can be induced with
    relatively low antigen doses (proteins: Heppel & Kilshaw, 1982;
    Jarrett & Hall, 1984; contact allergens: Asherson et al., 1977; Polak,
    1980; van Hoogstraten et al., 1992; Hariya et al., 1994). In sharp
    contrast, local (secretory) IgA responses are generally unaffected
    (Challacombe, 1983; Fuller et al., 1990). The apparent ability of the
    intestinal immune system to prevent allergic hypersensitivity to
    soluble, non-replicating antigens seems to be an important factor in
    preventing enteropathies (Mowat, 1984, 1987; Mowat et al., 1986;
    Challacombe & Tomasi, 1987). In contrast to potentially harmful,
    pro-inflammatory DTH and IgE responses, the secretory IgA response
    seems favourable. This immunoglobulin does not fix complement, nor
    does it cause allergic reactions, whereas its release may rather
    prevent enteropathies by inhibition of the entry of potentially
    damaging molecules. Abrogation of oral tolerance to, for instance,
    ovalbumin was found to lead to hypersensitivity responses in the

    intestinal mucosa and gut-associated lymphoid tissues, resembling
    those observed in food-sensitive enteropathies, e.g., coeliac disease.
    Indeed, IgE and DTH responses are most frequently associated with
    clinical food hypersensitivity.

    1.5.3.4  Factors determining the development of oral tolerance

         Several factors can play a role in the development of mucosal
    tolerance, notably the nature of the antigens and the genetic
    background, age and immune status of the individual. With regard to
    the nature of the antigens, available experimental and clinical
    evidence indicates that the ability of antigens to sensitize along the
    skin route parallels the ability to induce tolerance upon mucosal
    exposure. Thus, feeding of chemicals such as dinitrochlorobenze (DNCB)
    and picryl chloride, which are strong sensitizers when first applied
    to the skin, rapidly induces tolerance. Also nickel, which is amongst
    the top ten of clinical contact sensitizing agents, is an effective
    tolerance inducer in both experimental animals and humans (van
    Hoogstraten, 1991, 1992, 1993). However, when the mucosal epithelial
    barrier fails to prevent antigen passage, in particular the entry of
    live viruses or bacteria, this may lead to priming for
    pro-inflammatory immune responses rather than to the induction of
    tolerance. The fact that such microorganisms are strong inducers of
    local IL-12 and IFN-gamma release suggests that these cytokines could
    play a role as antagonists for tolerance induction. Indeed, adequate
    vaccination via the oral route can be achieved with live, attenuated
    strains of microorganisms, e.g., with poliomyelitis vaccine (Stites &
    Terr, 1991).

         Essentially similar requirements for skin-sensitizing and
    mucosal-tolerizing capacities of chemical allergens are also evident
    from the apparent lack of major genetic influences on either of these
    phenomena in outbred animals or humans. No or minimal genetic
    restrictions have been found for the risk of developing contact
    allergies to, for instance, nickel, nor for the induction of oral
    tolerance to the same allergens. It would appear that the same
    T-cell-receptor repertoire is being addressed under both conditions
    but that, depending on the site of first encounter with the allergen,
    sensitization or tolerance may ensue. On the other hand, inbred mouse
    strains can show strong differences in their ability to develop
    tolerance after protein feeding (Stokes et al., 1983 a,b; Tomasi et
    al., 1983; Lamont et al., 1988). Noticeably, certain mouse strains
    that are prone to autoimmune diseases fail to develop oral tolerance
    to some proteins (Carr et al., 1985).

         With regard to age, it was demonstrated in mice that ovalbumin
    did not induce tolerance for either DTH or antibody responses during
    the early postnatal period (1-2 days old), suggesting an increased
    risk of allergic sensitization during infancy. The lack of tolerance
    development in neonatal mice may be due to immaturity of the
    intestinal immune system at birth in this species. The ability to
    develop tolerance starts around day 4, but a transient defect in

    tolerance induction occurs around the time of weaning (Strobel &
    Ferguson, 1984; Hananan, 1990). Interestingly, clinical food
    hypersensitivities in human infants often develop around the time of
    weaning. This may be directly related to the physiological and dietary
    changes associated with weaning, when large numbers of new antigens
    are introduced. At the other end of the time scale, in ageing
    individuals reduced abilities to develop new hypersensitivities and
    tolerance are observed.

    1.5.3.5  Orally induced flare-up reactions and desensitization

         Considering the immune status of individuals, strong and
    long-lasting oral tolerance can only be achieved in naive individuals,
    i.e., those who have not been previously exposed to the antigen via
    the skin. In mice, a single feed of ovalbumin was reported to suppress
    fully subsequent systemic immune responses, and this state of
    tolerance persisted for up to two years. In contrast, in primed
    animals tolerance is hard to induce but partial and transient
    unresponsiveness (desensitization) may eventually develop after
    prolonged feeding of the antigen. Similar results have been obtained
    in guinea-pig studies with various different chemical allergens,
    including dinitrochlorobenzene (Polak, 1980), nickel (van Hoogstraten,
    1994) and amlexanol (Hariya et al., 1994). Unfortunately, essentially
    similar results have been obtained in early clinical trials aiming at
    the treatment of autoimmune diseases, e.g., rheumatoid arthritis and
    multiple sclerosis, by oral administration of relevant auto-antigens
    (Weiner et al., 1994). Another problem with oral tolerance induction
    in previously sensitized individuals arises owing to the tendency of
    former inflammatory sites to re-inflame (flare-up reaction). Local
    flare-up reactions confirm a previous sensitization process, and are
    probably due to allergen-specific effector T-cells, which can persist
    for periods up to several months at former inflammatory sites (Scheper
    et al., 1983).

         Two distinct features of immunocyte maturation may explain the
    seemingly insurmountable differences between immunological responses
    in naive and primed individuals, involving changes in expression
    patterns of cellular adhesion/homing molecules, and lymphocyte
    maturation features. First, a qualitative distinction exists between
    naive (difficult to stimulate/afferently acting) cells and
    effector/memory cells (easy to stimulate/efferently acting). In
    contrast to naive lymphocytes, which only are activated by allergen
    (modified-self constituents) if presented by professional dendritic
    (e.g., Langerhans) cells, their progeny, known as effector/memory
    lymphocytes, can also be stimulated by other cell types presenting
    allergen-modified MHC Class II molecules, e.g., monocytes, endothelial
    cells and B-cells. Effector/memory cells display increased numbers of
    intercellular adhesion molecules (ICAMs), allowing for more
    promiscuous cellular interactions. Amongst these, the most prominent
    ICAMs are the CD28 and LFA-1 molecules, with B7-1/2 and ICAM-1 as
    their respective ligands on APCs. Also, priming of T-cells leads to
    the loss of homing receptors, such as L-selectin, which facilitate

    interactions with high endothelial venules in peripheral lymph nodes.
    Apparently, after sensitization T-cells are less capable of
    recirculating through the lymphoid organs, but gain in ability to
    migrate into the peripheral tissues. Indeed, interactions with
    endothelia within inflamed skin are facilitated by the enhanced
    expression of ICAMs like the cutaneous lymphocyte-associated antigen
    CLA. Thus, effector/memory T-cells largely distribute over the
    peripheral tissues where conditions may be insufficient to convey
    effective tolerogenic signals. The second problem in inducing
    tolerance in previously primed individuals relates directly to the
    actual mechanism(s) of oral tolerance.

    1.5.3.6  Mechanisms of tolerance

         As discussed above, a preliminary factor contributing to
    immunological non-responsiveness and/or lack of hypersensitivity
    reactions at mucosal surfaces is the epithelial barrier function,
    preventing entry of potentially harmful allergens. Obviously, from an
    immunological point of view, this is a null-event and does not have
    implications for subsequent encounters with the same allergen. Also as
    discussed above, TGF-beta, a cytokine locally produced by epithelial
    cells and immunocytes, plays a pivotal role in maintaining epithelial
    barrier integrity. Importantly, the same cytokine also has broad
    non-specific immunosuppressive functions, for example, antagonizing
    phagocytic effector cell functions of pulmonary alveolar macrophages.
    Similarly, other immunosuppressive cytokines may be locally released
    from epithelial cells and may act in concert with TGF-beta to
    down-regulate immune effector functions, such as epithelial
    cell-derived P15E-related factors which show sequence homology with
    retroviral envelope proteins (Oostendorp et al., 1993).

         In contrast, specific immunological tolerance depends on
    decreased responsiveness of specific B- or T-cells, or release of
    immunosuppressive mediators from these cells after specific challenge.
    Exposure to high doses of antigens may induce clonal deletion or
    anergy of specific B- or T-cells by induction of apoptosis or
    antigen-receptor down-regulation (Jones et al., 1990; Schönrich et
    al., 1991; Ohashi et al., 1991; Melamed & Friedman, 1993).

         Generally, ligation of the T-cell antigen receptor (TCR) in the
    absence of appropriate co-stimulatory signals results in T-cell
    non-responsiveness, not only in Th1- but also in Th2- cells. Human
    CD4+ Th2-clones specific for the house dust mite allergen  Der p I
    can be rendered non-responsive to subsequent  Der p I challenges by
    incubating them with  Der p I-derived peptides, representing the
    relevant minimal T-cell activation-inducing epitopes, in the absence
    of professional APC (Yssel et al., 1992). The anergized Th2-cells also
    failed to produce cytokines (including IL-4 and IL-13) and failed to
    provide help for B-cell IgE synthesis. The mechanisms underlying this
    T-cell unresponsiveness have not yet been determined. Although these
    cells cannot be activated through their T-cell antigen receptor (TCR),
    they proliferate well in response to IL-2, or following activation by

    Ca++ ionophore and the phorbol ester 12-O-tetradecanoylphorbol
    13-acetate (TPA), suggesting that TCR activation or signalling
    pathways immediately downstream of the TCR are disturbed.

         Interestingly, the anergized Th2 cells expressed normal levels of
    CD40 ligand, but their lack of help for B-cell IgE synthesis could not
    be restored by exogenous IL-4 or IL-13, suggesting that in addition to
    CD40L-CD40 interactions, other molecules are required for initiating
    productive T- and B-cell interactions resulting in Ig isotype
    production. It is likely that these molecules are down-regulated in
    anergic T-cells. Peptide-induced Th2 cell tolerance and inhibition of
    T-cell help for IgE synthesis may provide the basis for successful
    immunotherapy in allergy. This anergy-based type of tolerance is
    generally short-lived, since (functionally) deleted lymphocytes are
    gradually replenished by newly arising clones in the bone marrow and
    thymus and, in experimental animal models, cannot be transferred to
    naive recipients, since these still contain a fully functional
    repertoire, compensating for any missing clones. On the other hand,
    mucosal contacts of naive individuals with relatively low amounts of
    antigens, such as can be the case with environmental or occupational
    exposure to chemical sensitizers, frequently induces a long-lasting
    state of specific tolerance. Transfer of lymphoid cells, in particular
    T-cells, from orally tolerized animals to syngeneic naive recipients
    prevents their capacity to subsequently mount immune responses to the
    same allergen, revealing the existence of so-called regulatory or
    suppressor T-cells (Polak et al., 1980; van Hoogstraten et al., 1992,
    1994; Weiner et al., 1994).

         Although "professional" suppressor T-cells may not exist (Bloom
    et al., 1992; Arnon & Teitelbaum, 1993), available data support the
    possible development of specific regulatory T-cells that suppress
    distinct immune functions. Depending on the experimental models, such
    regulatory T-cells can belong to either or both the CD4+ or CD8+
    subsets (Bloom et al., 1992). Regulatory T-cells may exert their
    suppressive actions through different pathways, including the shedding
    of TCR-alpha chains or hapten-binding TCR, through anti-idiotypic
    reactivities, or through IL-2/cytokine consumption from the milieu
    (Bloom et al., 1992; Fairchild et al., 1993; Kuchroo et al., 1995).
    There is evidence that regulatory T-cells most often exert their role,
    after antigen-specific activation, by releasing distinct cytokines
    antagonizing specific effector T-cell functions (see section 1.2.1).

         When starting clonal expansion after antigen-stimulation, T-cells
    develop major cytokine profiles depending on the site of primary
    contact (see section 1.2.1.1). For potential mechanism(s) of oral
    tolerance T-cell subsets producing mutually suppressive cytokines can
    be regarded as suppressor, or, better, as regulatory cells, depending
    on the functions tested. Considering overt inflammatory reactions as
    being most harmful to the individual and the primary cause of mucosal
    hypersensitivities, Th2-cells and putative TGF-beta producing Th3-cells
    are the most obvious candidates to mediate oral tolerance to proteins
    and chemical allergens.

    1.5.3.7  Conclusions

         Although the phenomenon of oral tolerance has been known for over
    a century the research on cellular resistance molecules, T-cell
    cytokine patterns and cellular adhesion molecules has opened promising
    avenues for further research on mechanisms and therapeutic options.
    Clearly, the skin-versus-mucosa routing hypothesis discussed above
    leaves many questions unanswered, such as the question of why some
    chemicals may elicit strong Th2 responses and IgE antibody production
    even when applied to the skin, without apparent reduction of delayed
    allergic reactivity (Dearman et al., 1991). The preliminary
    understanding of regulatory mechanisms in allergic contact dermatitis
    has not yet led to further therapeutic progress. So far, no methods of
    permanent desensitization have been devised. Nevertheless, the way in
    which T-cells specifically recognize distinct allergens, as well as
    how these and other inflammatory cells interact to generate
    inflammation, is beginning to be understood. Defined cellular
    interaction molecules and mediators provide promising targets for
    anti-inflammatory drugs. Obviously, drugs found to be effective in
    preventing severe T-cell-mediated conditions, e.g., rejection of a
    vital organ graft, should be carefully evaluated before their use in
    allergic skin disease is considered.
    

    2.  HYPERSENSITIVITY AND AUTOIMMUNITY -  OVERVIEW OF MECHANISMS

         Numerous environmental chemicals have the ability to produce a
    hypersensitivity response. Although hypersensitivity diseases are
    common, affecting millions of people, the incidence associated with
    environmental pollutants or occupational exposure is largely unknown.
    The characteristic that distinguishes allergic responses from immune
    mechanisms involved in host defence is the nature of the reaction,
    which often leads to tissue damage. Chemically induced
    hypersensitivities usually fall into two responses distinguished not
    only mechanistically but temporally: (1) immediate hypersensitivity,
    which is mediated by immunoglobulin, most commonly IgE, and is
    manifested within minutes of exposure to an allergen, and (2)
    delayed-type hypersensitivity (DTH), a cell-mediated response that
    occurs within 24-48 h. The type of immediate hypersensitivity response
    elicited (anaphylactic, cytotoxic, Arthus or immune complex) depends
    on the interaction of a sensitizing antigen or structurally related
    compound with antibody. Delayed-type hypersensitivity responses are
    characterized by T-lymphocytes bearing antigen-specific receptors
    which, on contact with macrophage-associated antigen, respond by
    secreting cytokines that mediate the delayed-type hypersensitivity
    response. Almost any organ can be targeted by hypersensitivity
    reactions, including the gastrointestinal tract, blood elements and
    vessels, joints, kidneys, central nervous system and thyroid, although
    the skin and lung, respectively, are the most common targets.

         Various risk factors are involved in producing allergic
    sensitization and influencing its severity. For instance, in the case
    of aeroallergens, exposure can play a role in the primary
    sensitization, in the development of symptomatic allergic disease, and
    in the frequency and severity of acute symptomatic episodes. Other
    risk factors include genetic predisposition, and age at the time of
    the primary exposure.

         Exposure to enzymes (mainly proteases) used in detergents have
    also been associated with respiratory sensitization and symptoms.
    Though sensitization is due to more than one factor, magnitude of
    exposure has been demonstrated as a critical factor in the control of
    primary sensitization to enzyme-containing detergents (Sarlo et al.,
    1997).

         Environmental factors have been suggested to contribute to the
    prevalence of allergic diseases by modulating the allergen load
    required for the sensitization as well as for the exacerbation and
    intensity of allergic symptoms (Ollier & Davies, 1994).

    2.1  Classification of immune reactions

         Gell & Coombs (1963) classified immune reactions into four basic
    types. Since then knowledge of immune reactions has increased and the
    frequent overlaps between the different types must be stressed. This
    classification is still very useful but the physiopathological reality
    is frequently more complex.

         The four major types of hypersensitivity according to Gell &
    Coombs (1963) are:

         Type I anaphylactic, immediate reaction 
         Type II cytotoxic reaction 
         Type III immune complex reaction
         Type IV delayed or cell-mediated reaction

         Sometimes a fifth type of hypersensitivity is added, i.e., Type V
    stimulatory hypersensitivity (Roitt et al., 1998). In addition,
    certain allergic diseases can be expressions of two or more types of
    hypersensitivity.

         The sections below review the mechanistic basis for phenomena and
    diseases associated with each type of hypersensitivity.

    2.1.1  Type I hypersensitivity

         The distinguishing feature of Type I hypersensitivity is the
    short time lag, usually seconds to minutes, between exposure to
    antigen and the onset of clinical symptoms. The key reactant in Type I
    or immediate sensitivity reactions is IgE (see Fig. 6). Antigens that
    trigger formation of IgE are called atopic antigens, or allergens
    (Marsh & Norman, 1988). Atopy refers to an inherited tendency to
    respond to naturally occurring inhaled and ingested allergens with
    continual production of IgE (Terr, 1994a). Patients who exhibit
    allergic or immediate hypersensitivity reactions typically produce
    antigen-specific IgE in response to a small concentration of antigen
    (Atkinson & Platts-Mills, 1988). IgE levels appear to depend on the
    interaction of both genetic and environmental factors.

         Prausnitz & Kustner (1921) showed that a serum factor was
    responsible for Type I reactions. This type of reaction is known as
    passive cutaneous anaphylaxis. It occurs when serum is transferred
    from an allergic individual to a non-allergic individual, and then the
    second individual is challenged with specific antigen. This experiment
    was conducted in 1921 but it was not until 1966 that the serum factor
    responsible, namely IgE, was identified (Ishizaka & Ishizaka, 1966).
    IgE is primarily synthesized in the lymphoid tissue of the respiratory
    and gastrointestinal tracts. The regulation of IgE production appears
    to be a function of T-cells. Th2 cytokines, in particular IL-4 and
    IL-13 are essential for IgE synthesis, i.e., for the final
    differentiation and isotype switch of the IgE-producing B-cells,
    committing particular B-cells to IgE production (Goust, 1993). IL-2,
    IL-5 and IL-6 also play a role, probably as sequential growth and
    differentiation factors that select for IgE synthesis (Tharp, 1990).

         Once an individual has become sensitized, the IgE produced
    spreads throughout the body and binds in the peripheral tissues to
    mast cells and basophils via the high affinity receptor for 

    FIGURE 6


    IgE (Fc-epsilon-RI). Upon contact with the allergen, the IgE molecules
    will be cross-linked and the cells will release their granules
    supplying the tissue with histamine, proteolytic enzymes, heparin and
    chemotactic factors for eosinophils, neutrophils and monocytes. These
    mediators induce vasodilatation, increased vascular permeability and
    smooth muscle contraction and lead to an "immediate reaction", which
    becomes clinically manifest within 20 min as a typical "wheal and
    flare" in the skin or as bronchoconstriction in the respiratory tract.

         At the same time, from the cell membrane new mediators, such as
    prostaglandin-D2, thromboxanes and leukotrienes are being generated.
    Together with the now attracted and activated eosinophils (which
    produce platelet activating factor and major basic protein), these
    mediators cause further infiltration, smooth muscle contraction,
    mucosal oedema and damage of the epithelial cells, resulting in the so
    called "late phase" reaction (12-24 h after challenge). Like the
    immediate reaction the late phase responses can be observed both in
    the skin and in the respiratory tract.

         While actual antibody synthesis is regulated by the action of
    cytokines, the tendency to respond to specific allergens appears to be
    linked to inheritance of certain MHC genes. Various HLA class II
    antigens seem to be associated with a high response to individual
    allergens (Goust, 1993). As an example, individuals who possess the
    HLA antigens B7 and DR2 are more likely to respond to a specific
    ragweed antigen (Goust, 1993). The nature of this association is
    unclear at this time.

    2.1.1.1  Anaphylaxis

         Anaphylaxis is the most severe type of allergic response, as it
    involves multiple organs and may be fatal. Anaphylactic reactions are
    typically triggered by glycoproteins or large polypeptides. Smaller
    molecules, such as penicillin, are haptens that may become immunogenic
    by combining with host cells or proteins. Typical agents that induce
    anaphylaxis include venom from insects in the  Hymenoptera family,
    drugs such as penicillin, and foods such as seafood or egg albumin
    (Widmann, 1989).

         Allergic reaction to allergens (e.g., in food, venom) that result
    in systemic anaphylaxis are, in the vast majority of instances,
    believed to be mediated by allergen-specific IgE bound to high
    affinity IgE receptors (Fc-epsilon-RI) on the surfaces of basophils
    and mast cells. As described earlier, the subsequent activation of
    basophils/mast cells results in the release (e.g., histamines) and
    generation (e.g., leukotrienes) of potent chemical mediators of
    anaphylaxis.

    2.1.2  Type II hypersensitivity

         Type II hypersensitivity reactions are caused by IgG and IgM
    antibodies directed towards cell surface antigens. These antigens may
    be altered self-antigens or heteroantigens. Such antibodies, bound to

    the cell membrane, can activate inflammatory phagocytes by Fc receptor
    triggering. These phagocytes will then try to kill or to inactivate
    their target as they would kill a microorganism. If they are unable to
    phagocytose the whole cell, they will cause cell damage by secreting
    oxygen radicals and by generating inflammatory mediators such as
    arachidonic acid metabolites (prostaglandins and leukotrienes) from
    their cell membrane.

         Moreover, cell-bound antibodies activate the complement system.
    The presence of C3b on the cell membrane, in addition to the
    immunoglobulin, facilitates phagocytosis, whereas the further
    complement cascade will induce membrane perforation and cell lysis.
    Together, these reactions result in destruction of antibody-coated
    cells and thus in cytopenia or in considerable tissue damage.

         Not only granulocytes and macrophages are able to kill
    antibody-coated cells. Specialized large granular non-B,
    non-T-lymphoid cells, called natural killer (NK) cells, also bear Fc
    receptors (CD16) and are capable of killing antibody-coated target
    cells. NK cell-mediated killing is achieved by the release of
    cytoplasmic granules containing perforin and granzymes. This process
    is called antibody-dependent cell-mediated cytotoxicity (ADCC) and,
    although not yet recognized at the time of Gell & Coombs (1963), it
    should strictly be considered as a Type II effector mechanism. ADCC
    reactions have been well established  in vitro to tumour antigens and
    viral proteins, but their precise role in host defence and
    hypersensitivity reactions is still not completely understood.

    2.1.3  Type III hypersensitivity -- immune complex reaction

         Type III hypersensitivity reactions are similar to Type II
    reactions in that IgG or IgM is involved and that destruction is
    complement-mediated. However, in the case of Type III diseases, the
    antigen is soluble. When soluble antigen combines with antibody,
    complexes are formed that precipitate out of the serum. These
    complexes deposit in the tissues and bind complement, causing damage
    to the particular tissue. Deposition of antigen-antibody complexes is
    influenced by the relative concentration of both components. If a
    large excess of antigen is present, sites on antibody molecules become
    filled before cross-links can be formed. In antibody excess, a lattice
    cannot be formed due to the relative sparsity of antigenic determinant
    sites. The small complexes that result in either of the above cases
    remain suspended or may pass directly into the urine. Precipitating
    complexes, on the other hand, occur in mild antigen excess, and these
    are the ones most likely to deposit in the tissues. Sites where this
    typically occurs include the glomerular basement membrane, vascular
    endothelium, joint linings, and pulmonary alveolar membranes (Roitt et
    al., 1998).

         Complement binds to these complexes in the tissues, causing the
    release of mediators that increase vasodilation and vasopermeability,
    attract macrophages and neutrophils, and enhance binding of phagocytic

    cells by means of C3b deposited in the tissues. If the target cells
    are large and cannot be engulfed for phagocytosis to take place,
    granule and lysosomal contents are released by a process known as
    exocytosis (Roitt et al., 1998). This results in the damage to host
    tissue that is typified by Type III reactions.

    2.1.3.1  Arthus reaction

         The classic example of a Type III reaction is the Arthus
    reaction, a local necrotic lesion resulting from a local
    antigen-antibody reaction produced by intradermal injection of an
    antigen into a previously sensitized animal. This reaction is
    characterized by erythema and oedema, peaks within 3 to 8 h, and is
    followed by a haemorrhagic necrotic lesion that ulcerates. The
    inflammatory response is due to antigen-antibody combination and
    subsequent formation of immune complexes that deposit in small dermal
    blood vessels. Complement is fixed, attracting neutrophils and causing
    aggregation of platelets. Activation of complement is, in fact,
    essential for the Arthus reaction, as the C3a and C5a generated
    activated mast cells to release permeability factors, with the
    consequent localization of immune complexes along the endothelial cell
    basement membrane (Terr, 1994b). The Arthus reaction is rare in
    humans.

    2.1.4  Type IV -- delayed-type hypersensitivity

         Type IV reactions were originally described by Gell & Coombs
    (1963) as those skin reactions which take more than 12 h to develop
    after antigen application. The classical Type IV reaction is the
    tuberculin reaction, which reaches its maximum 24-72 h after the
    intradermal injection of mycobacterial extracts. This delayed type
    skin reaction to intradermally injected protein is characterized by a
    pronounced induration reflecting a dense mononuclear cell infiltrate.

         Since it became clear that antigen-specific T-cells are
    responsible for these reactions, the term Type IV reactivity has been
    used not only in relation to delayed-type hypersensitivity (DTH)
    reactions in the skin, but also to T-cell-mediated inflammatory
    reactions in other tissues. In addition, other T-cell-mediated
    reactions, such as those to infectious agents or tumour antigens,
    which are rather protective than hypersensitive, are regularly
    described as Type IV reactions.

         Although CD8+ T-cells have been shown in some experimental
    animal models to transfer DTH, generally CD4+ T-cells are held
    responsible for DTH responsiveness. The majority of antigen specific
    T-cells cloned from DTH reaction sites are of the CD4+ subset.
    Increased frequencies of antigen-specific CD4+ T-cells can also be
    detected in the circulation of sensitized individuals. Such memory
    T-cells show enhanced expression of adhesion molecules, which

    facilitates their recirculation through the peripheral tissues. So,
    whereas priming of naive T-cells takes place in the lymph nodes
    draining the area of antigen contact, the secondary DTH response of
    memory T-cells rather takes place in the peripheral tissues at the
    site of antigen contact.

         Here they may encounter the antigens for which they were
    originally sensitized. The T-cells do not recognize the whole antigen
    or conformational epitopes as antibodies do, but they recognize small
    peptides derived from these antigens after processing by
    antigen-presenting cells (APC). MHC class II molecules bind these
    peptides already within the intracellular vesicles and present them
    subsequently on the APC membrane to helper T-cells (Fig. 5). If the
    memory T-cells recognize the peptide in its MHC class II context, the
    cells become activated and produce a characteristic set of cytokines.

         In the DTH reaction that now develops, predominantly mononuclear
    cells are attracted from the circulation and contribute to the local
    inflammatory reaction. An essential chemokine found to play a role in
    the early accumulation of leukocytes at the DTH reaction site is IL-8
    (Larsen et al., 1995), whereas RANTES (Regulated in Activation Normal
    T-cells Expressed and Secreted), produced by endothelial cells, was
    shown to attract preferentially macrophages and CD4+ T-cells to the
    DTH reaction (Marfaing-Koka et al., 1995). In addition to a number of
    different chemokines, IFN-gamma, TNF-alpha and LT (lymphotoxin) are 
    produced in the DTH reaction (Tsicopoulos et al., 1992). These are 
    typical Th1 effector cytokines, which are either directly cytotoxic 
    for pathogens or indirectly by activating the macrophage bactericidal 
    mechanism. Together, the cytokine cascade during this secondary 
    response shows an extreme amplification power, as illustrated by 
    experimental studies in which measurable oedema could be triggered by 
    only one specific T-cell (Marchal et al., 1982). Therefore, DTH 
    reactions are mediated by Th1 cells, the most prominent cytokines 
    being IL-2, LT and IFN-gamma. Indeed, at the site of DTH reactions 
    these cytokines can be detected (Tsicopoulos et al., 1992). It should 
    be realized, however, that the immune response is always the resultant 
    of a Th1-Th2 balance and that this delicate balance can be influenced 
    by several external factors, such as drugs, hormones, infections and 
    altered antigen exposure. Chronic antigen stimulation, for instance, 
    may induce a shift away from Th1, DTH-associated immunity towards a 
    Th2 response (Kitagaki et al., 1995; Mosmann & Sad, 1996). Th2 
    cytokines, such as IL-4, IL-5 and IL-10, rather help to induce 
    antibody responses, particularly IgE. In chronic infectious disease 
    indeed high levels of antibodies can be detected while DTH reactivity 
    is waning ( Mycobacteria,  Trichophyton). In the human system, a 
    Th1 to Th2 shift correlating with a clinical conversion from disease 
    resistance to susceptibility and disease progression has been shown 
    in  Leishmania,  Candida,  Mycobacteria and HIV infection
    (Mosmann & Sad, 1996).

    2.1.4.1  Mechanisms of allergic contact dermatitis

     a)  Sensitization

         In allergic contact dermatitis, Type IV reactivity is raised
    against small, chemically reactive environmental agents that enter the
    body via the skin. In the skin, epidermal dendritic Langerhans cells
    (LC), bearing large numbers of class II molecules (HLA-DR, -DP and
    -DQ) on their cell membrane, are the primary allergen-presenting
    cells. They form a contiguous network in which agents penetrating the
    skin are efficiently trapped. Langerhans cells stem from the bone
    marrow, but their continuous presence in the epidermis is at least
    partly maintained by local proliferation (Czernielewski & Demarchez,
    1987; Breathnach, 1988).

         Upon penetration through the epidermis, contact allergens readily
    bind to a plethora of skin constituents. Whereas most allergens bind
    spontaneously, some need metabolic conversion (Anderson et al., 1995)
    or photoinduced activation before they bind. The latter allergens are
    called contact photoallergens (White, 1992).

         Only those allergens that modify the Langerhans cell MHC Class II
    molecules can eventually sensitize T-cells; this occurs either by
    direct binding to the MHC Class II molecules and to peptides within
    their grooves or by uptake and processing of haptenized proteins
    followed by presentation of the derived peptides in the MHC Class II
    molecules of the antigen-presenting cells. It has been shown that in
    individuals allergic to nickel some nickel-specific T-cell clones
    recognize unprocessed nickel, bound to the MHC Class II molecules of
    fixed antigen-presenting cells, whereas other nickel-specific T-cell
    clones are dependent on viable antigen-presenting cells for
    processing, most likely of preformed nickel-protein conjugates (Moulon
    et al., 1995).

         In a similar way, MHC Class I peptides may become modified by the
    allergen, triggering class I restricted, CD8-positive T-cell clones.
    Notably the size of the allergic metal ions is much smaller than the
    peptides to which they bind in the groove. In some instances different
    metallic allergens modify the MHC Class II peptides in a very similar
    way. T-cell clones then show complete cross reactivity between the
    metals, for instance between nickel and palladium, or between nickel
    and copper (Pistoor et al., 1995). Clinical signs of cross reactivity
    of nickel with other related metals, such as cobalt, however, most
    probably result from concomitant sensitization by exposure to metal
    alloys.

         The majority of chemically reactive allergen, however, binds
    covalently to distinct amino acids, thus forming haptenized proteins.
    Usually, haptens, like picryl- or penicilloyl- are much larger than
    metal allergens, and hapten-specific T-cell responses, i.e.,
    independent of the carrier protein, can be observed.

         Upon exposure of the skin to chemical contact sensitizing agents,
    cytokine production in the epidermis by both keratinocytes and
    Langerhans cells is immediately up-regulated, thereby initiating the
    process of Langerhans cell maturation and migration.

         The first cytokine to be up-regulated, within 15 min after
    allergen application, is IL1-beta, produced by Langerhans cells. This
    up-regulation was found to be allergen-specific, just like the
    subsequent production of IL-1-alpha, and of the chemokines IP-10
    (IFN-gamma-inducible protein 10) and MIP-2 (macrophage inflammatory
    protein-2) by keratinocytes, and could not be detected upon irritant
    application. TNF-alpha up-regulation in keratinocytes, on the other
    hand, appeared to be a less allergen-specific event. The most relevant
    cytokines for Langerhans cell maturation and egress are GM-CSF, IL-1
    and TNF-alpha, while IL-10, which is also produced by keratinocytes,
    but at a later stage, may serve as a down-regulatory molecule for
    Langerhans cell maturation. (Heufler et al., 1988; Kimber &
    Cumberbatch, 1992a,b; Enk & Katz, 1995). Interestingly, IL-1 and
    TNF-alpha were found to down-regulate the membrane expression on
    Langerhans cells of E-cadherin, a molecule that mediates
    Langerhans-cell-keratinocyte adhesion. Thus, Langerhans cells with
    allergen-modified MHC Class II molecules leave the epidermis and
    migrate via the dermis and lymphatics to the draining lymph nodes,
    where they settle within the paracortical areas. Indeed increased
    numbers of dendritic cells appear in the regional lymph nodes around
    24 h after hapten application (Kimber et al., 1990). Whereas resident
    epidermal Langerhans cells are still relatively inefficient
    antigen-presenting cells, once they have arrived in the lymph nodes
    they have matured into fully active antigen-presenting dendritic cells
    and are capable of stimulating even naive unprimed T-cells. Naive
    cells express, in contrast to memory cells, low levels of cellular
    adhesion molecules (CAM) and therefore require optimally functioning
    antigen-presenting cells for stimulation. Matured Langerhans cells or
    dendritic cells (DCs) have an increased expression of MHC Class II,
    ICAM-1 and B7 molecules, allowing for optimal T-cell triggering
    (Steinman et al., 1995); in addition the intricate structure of the
    paracortical area offers an appropriate environment for this
    sensitization process to take place. Naive T-cells, again in contrast
    to memory cells, recirculate preferentially through the peripheral
    lymphoid organs, rather than through the tissues, due to the
    expression of distinct adhesion molecules (L-selectin) that recognize
    the high endothelial venules in the lymph nodes. The probability of
    hitting unprimed specific T-cells is thus increased.

         When successful triggering and subsequent proliferation of
    allergen-specific T-cells have taken place, the lymphocyte progeny
    will leave the lymph nodes to join the recirculating pool of
    lymphocytes. The frequency of specific cells in the circulation can
    thus be increased from around 1:100 000 to 1:1000-10 000 and the
    individual has now become "sensitized".

     b)  Elicitation of allergic contact dermatitis

         Upon re-exposure to contact sensitizing agents, specific
    recirculating memory T-cells present in the skin immediately recognize
    the allergen modified MHC Class II molecules on the Langerhans cell
    membranes. The probability that the allergen is indeed found by
    specific memory T-cells is largely increased by the expression of
    organ-specific interaction molecules on the T-cell surface. The
    cutaneous lymphocyte-associated antigen (CLA), recognized by the
    monoclonal antibody HECA-452, is present on a small subpopulation
    (approximately 16%) of peripheral blood T-cells which preferentially
    recirculates via the skin. Here the endothelial adhesion molecule
    E-selectin acts as a vascular addressin for the skin-homing memory
    T-cells (Picker et al., 1993; Bos & Kapsenberg, 1993). As described in
    the section on Type IV - delayed-type hypersensitivity (section
    2.1.4), mainly CD4-positive allergen-specific cells thus enter the
    skin. Since memory cells have relatively low stimulation thresholds,
    they can be triggered by less efficient antigen-presenting cells, like
    the local resident Langerhans cells. The T-cells will now initiate a
    Th1-type cytokine cascade, which eventually leads after 24-72 h to the
    typical delayed-type contact allergic reaction. Because the reaction
    takes place in superficial layers of the skin, erythema and blistering
    are characteristic features, in contrast to the tuberculin DTH where
    induration is most pronounced.

         The challenge reaction in allergic contact dermatitis resolves
    spontaneously within one week. It is therefore commonly used as a
    primary diagnostic test in allergic contact dermatitis. To this end,
    low non-toxic dosages of allergen are generally applied onto the skin
    under an occlusive patch to allow for maximal skin penetration. The
    main drawbacks of such an  in vivo skin test procedure are the
    potential sensitization and boosting by such an intense allergen
    contact. Indeed it was shown experimentally in guinea-pigs that even
    one epicutaneous application of allergen could direct the immune
    response towards (still subclinical) sensitization, as shown by a
    failure of subsequent tolerance induction (Van Hoogstraten et al.,
    1994). Also clinically, occasional sensitization by epicutaneous skin
    tests can be observed. For this reason much effort has been put in the
    development of  in vitro diagnostic procedures in allergic contact
    dermatitis (Von Blomberg et al., 1990).

         Up to now,  in vitro assays in allergic contact dermatitis have
    been successful for relatively non-toxic water-soluble allergens, such
    as metal salts. For other allergens, occasionally positive results are
    obtained by pre-pulsing antigen-presenting cells or proteins or by
    using special solvents. So, despite the fact that most of our
    knowledge of the pathogenesis of human allergic contact dermatitis is
    due to  in vitro experiments with blood from allergic patients, for
    routine assessment of allergic contact dermatitis these assays are
    still too complicated.

         Repeated contact with low dosages of allergen, as typically
    occurs for most contact allergens, may lead to continuous triggering
    of Type IV reactivity in the skin and thus to an allergic contact
    dermatitis (Scheper & Von Blomberg, 1992). The dermatitis only
    disappears when the allergen is entirely eliminated from the
    environment.

         Even if the reaction is clinically healed, allergen-specific
    T-cells may persist in the skin for up to several months. Thus,
    locally increased allergen-specific hyperreactivity, either detectable
    through accelerated "retest" reactivity (peaking at 6-8 h) or flare-up
    reactivity after allergen entry from the circulation, may be observed
    for several months at former allergic contact dermatitis reaction
    sites (Scheper et al., 1983; Yamashita et al., 1989). The presence of
    specific T-cells at former eczematous sites can thus be maintained by
    low dosages of inhaled or ingested allergen, in the absence of
    allergenic skin contacts.

         Repeated contact with relatively high dosages of allergen, on the
    other hand, may result in a local desensitization. The initial
    erythematous reaction gradually decreases. Such a local
    hypo-responsiveness of the skin, which is known as "hardening" in
    occupational contact dermatitis, is largely reversed after a period of
    allergen restrain. However, also systemically, DTH reactivity
    decreases upon repeated allergen application. This decrease in DTH is
    associated with increased antibody responses and a shift towards
    immediate-type hypersensitivity, reflecting a shift from Th1 to Th2
    reactivity (Boerrigter & Scheper, 1987; Kitagaki et al., 1995).

         It appears, therefore, that although exposure of the skin to
    exogenous antigens generally results in Th1 responses, the
    micro-environment in chronically inflamed tissues, rather than the
    site of allergen exposure or the nature of the allergen, determines
    the type of immune reaction.

    2.1.4.2  T-cell responses in chemically induced pulmonary diseases

         Asthma is a chronic pulmonary inflammatory disease associated
    with bronchial hyperreactivity. In the majority of asthma cases a
    clear association exists with atopic IgE-mediated hypersensitivity,
    involving relatively large protein allergens. Here T-cells dominate in
    the late phase and the chronic reaction. The pivotal role of T-cells
    in chronic asthma is stressed by the finding of activated T-cells in
    the bronchial mucosa and the effectiveness of T-cell immunosuppressive
    drugs. In particular, the number of activated CD4-positive cells was
    found to correlate with the numbers of eosinophils in the
    bronchoalveolar lavage (BAL) and with disease severity (Walker et al.,
    1991). The T-cells, present in the bronchial mucosae and in the lavage
    fluid, were shown to produce predominantly Th2 cytokines, in
    particular IL-5, a cytokine known to activate eosinophils (Corrigan &
    Kay, 1992). Therefore, although T-cell-mediated immunity is clearly
    playing a role, Type IV reactivity, mediated by Th1 cells, does not
    seem to be involved in this type of asthma.

         Of particular interest is the hypersensitivity pneumonitis
    induced by environmental small chemical allergens. Such allergens are
    known to cause DTH when applied to the skin. Occasionally these
    allergens induce, in addition or as first manifestation, asthmatic
    disease upon inhalation. It could be questioned whether these
    allergens, in contrast to the atopic protein allergens, would induce
    Type IV reactivity.

         Experimentally it has been shown in mice that Type IV
    hypersensitivity to small chemical allergens, such as picryl chloride,
    can indeed induce lung disease upon intranasal application (Garssen et
    al., 1991, 1994).

         Contact allergens that have been reported to induce asthma
    include formaldehyde, platinum salts, nickel, cobalt and chromium
    (Nordman et al., 1985; Estlander et al., 1993; Cirla, 1994; 
    Park et al., 1994; Merget et al., 1996). In a number of cases this 
    asthmatic disease could be associated with the presence of circulatory 
    IgG to the causative allergen and positive bronchial provocation
    tests.

         Trimellitic anhydride, phthalic anhydride and toluene
    diisocyanate are reactive chemicals behaving primarily as respiratory
    allergens, causing asthmatic disease and pulmonary irritation. The
    immune reactions leading to asthmatic disease are quite variable; the
    role of clear-cut Type IV reactivity is uncertain.

    2.1.5  Type V stimulatory hypersensitivity

         Stimulatory hypersensitivity occurs when antibodies binding to a
    cell surface molecule cause inappropriate stimulation of the cell.
    Normal feedback inhibition will then fail. An example is Graves'
    disease (exophthalmic goitre), in which autoantibodies to the
    thyroid-stimulating hormone receptor on thyroid cells stimulate the
    production of excessive amounts of thyroid hormone, resulting in
    disease.

    2.2  Regulation of hypersensitivity

         In 1986, the existence of two CD4+ Th-cell subsets was
    discovered in mice, and they were designated Th1 and Th2. Their
    identification has greatly improved understanding of the regulation of
    immune effector functions, not least on Type I and Type IV
    hypersensitivity responses. These Th subsets are defined by the
    patterns of cytokines that they produce, which leads to strikingly
    different T-cell functions (Table 9). Broadly speaking, Th2-cells are
    more efficient B-cell helpers, especially in the production of IgE
    antibody, whereas Th1-cells mediate DTH reactions. In addition, they
    cross-regulate by producing mutually antagonistic cytokines. Their
    specific function and characteristics in rodents and humans have not
    yet been clearly established (Muraille & Leo, 1998).


        Table 9.  Characteristics of Th1- and Th2-associated immunitya in vivo
    (modified from Röcken et al., 1996)
                                                                                             

    Characteristics            Th1                                   Th2
                                                                                             

    IFN-gamma                  high                                  variable, frequently low
    IL-2                       high                                  variable, frequently low
    IL-4                       low/negative                          high

    major mode of action       DTH reactions                         eosinophil-associated
                               (cellular immunity)                   cytotoxicity
                               complement-binding                    non-complement-binding
                               antibodies and IgE                    antibodies and IgE

    protective effects         against intracellular                 against
                               microorganisms and                    extracellular parasites
                               tumours

    harmful effects            contact hypersensitivity              atopic diseases
                               tissue-specific autoimmunity          immunoglobulin-mediated
                               allergic encephalitis                 autoimmunity
                               juvenile diabetes                     bullous autoimmune
                               rheumatoid arthritis                  diseases
                               thyroiditis                           sclerosing diseases ?
                               uveitis
                                                                                             

    a  Th1 and Th2 immunity characterizes T-cell populations, not single T-cells
    

         In addition to Th1- and Th2-cells, additional cytokine production
    phenotypes of CD4+ cells exist. They are, however, characterized
    less thoroughly. Most resting T-cells mainly produce IL-2 on first
    contact with antigen, and differentiate within a few days into cells
    producing multiple cytokines, such as IL-4 and IFN-gamma. In addition
    to Th1- and Th2-cells, the existence of undefined precursor cells has
    been suggested. These precursor cells (IL-2 producing) are the virgin
    Th cells, producing only or predominantly IL-2, and Th0 cells are in
    the process of differentiation, producing cytokines of both Th1 type
    (such as IL-2 and IFN-gamma) and Th2 type (such as IL-4, IL-5 and
    IL-10). The pathways of differentiation from the precursor cells are,
    however, unclear. In addition, it is unknown whether there is a single
    common precursor cell or whether precursor cells are already committed
    to a particular cytokine pattern before exposure to antigen (the
    cytokine production profiles of Th-cell subsets in the mouse are shown
    in Table 1). In conclusion, it is believed that Th1- and Th2-cells
    represent the most differentiated populations of the CD4+ phenotype
    that develop following prolonged exposure to antigen or following
    stimulation by potent immunogens.

         At least two mechanisms can influence the selective
    differentiation of Th-cell subsets. Firstly, the cytokines that are
    present during differentiation, in particular IFN-gamma, IL-4 and
    IL-12, may greatly influence the type of Th that will be generated.
    IF-gamma augments development of Th-type responses and IL-4 promotes
    differentiation of Th-cells (Romagnani, 1992a). Secondly, the type of
    APC is thought to influence the characteristics of immune responses.
    Upon activation, Th2 cells express p39 on their surface, which
    interacts with CD40 on the surface of B-cells. The interactions of p39
    with CD40 and of T-cell antigen receptor (TCR) with antigen and MHC
    Class II together lead to production of IL-4, IL-5 and IL-6 by Th2
    cells, stimulating B-cells to antibody production. Th1 cells, on the
    other hand, may interact with macrophages. A pair of cell surface
    molecules analogous to p39/CD40 have not as yet been identified.
    However, the interaction of Th1 cells with macrophages leads to
    IFN-gamma production by Th1 cells, stimulating macrophages to produce
    monokines.

         The difference in APCs, macrophage versus B-cell, that
    preferentially activates Th1 or Th2 suggests differences in antigen
    requirements for activation, e.g., large particulate antigens
    requiring phagocytosis for Th1 and low antigen concentration for Th2.
    Whereas moderate concentrations of antigen preferentially activate
    Th1, extremely high concentrations are believed to inhibit Th1 and
    select for Th2 responses (Pfeiffer et al., 1991).

         If Th1 and Th2 clones are stimulated by immobilized anti-CD3 (in
    the absence of APC), both types produce their respective cytokine
    pattern. The proliferative responses are, however, very different.
    Whereas Th2 clones exhibit good proliferative responses, Th1 not only
    fail to do so, but are even rendered incapable of proliferating in

    response to exogenously added IL-2 (Williams & Unanue, 1990; Williams
    et al., 1990). These Th1 clones are in a state of anergy or tolerance
    (Schwartz & Weiss, 1990).

         IFN-gamma inhibits the proliferation of Th2 responding to either
    IL-2 or IL-4, but does not inhibit Th1. IL-10 inhibits the synthesis
    of cytokines by Th1 cells, and, although growth factor requirement is
    not affected, the reduction in IL-2 synthesis can lead to decreased
    proliferation. It has been shown in  in vitro human systems that
    IL-10 can suppress the antigen-presenting capacity of monocytes and
    dendritic cells by down-regulation of MHC Class II. IL-10 had no
    effect on the antigen-presenting capacity of B-cells or
    down-regulation of their MHC Class II. These results suggest a
    mechanism for the general observation that macrophages/dendritic cells
    preferentially stimulate Th1, whereas B-cells preferentially stimulate
    Th2.

         IL-2 is a T-cell growth factor (TCGF) that mediates autocrine
    proliferation of Th1, whereas the TCGF IL-4 mediates autocrine
    proliferation of Th2. Interestingly, it has been shown that IL-4 is
    the major TCGF produced by T-cells from lymphoid organs that drain
    mucosal tissues, whereas IL-2 is the major TCGF produced by T-cells
    from other lymphoid organs (Daynes et al., 1990b). Involvement of
    dehydroepiandosterone in this site/tissue-specific control on
    lymphokine production was suggested (Daynes et al., 1990a).
    Dihydrotestosterone and 1,2,5-dihydroxyvitamin D3 also change the
    cytokine production pattern of T-cells.

         In humans, a predominant fraction of CD4+ T-cell clones was
    found to produce IL-2, IL-4 and IFN-gamma, although the quantities
    varied considerably. Bearing in mind the findings in mice (see above),
    it was thought that unrestricted profiles are mainly a property of 
    T-cells that are not yet committed to a certain differentiation 
    pathway. Consequently, functional heterogeneity of CD4+ cells 
    should most likely be found in chronically stimulated responders. 
    Kapsenberg et al. (1991) studied two categories of patients, those 
    with nickel hypersensitivity, an example of Type IV hypersensitivity, 
    and those with house dust mite ( Dermatophagoides pteronysinnus 
    (Dp))hypersensitivity, an example of Type I hypersensitivity.

         Most house dust mite-specific T-cell clones from peripheral blood
    (Wierenga et al., 1990) and lesional skin biopsies of house dust
    mite-allergic patients show a Th2-like production profile. House dust
    mite-specific clones from atopic patients induce IgE production (see
    also below). It was shown that this production is dependent on a high
    IL-4/IFN-gamma ratio, and is not dependent on the origin of B-cells.
    Only IgE specific to house dust mite (and not, for instance, IgE
    specific to tetanus toxoid or  Candida albicans) was elevated in
    atopic house dust mite-allergic patients.

         The majority of allergen-specific human T-cell clones produce
    IL-4 and IL-5, but not IFN-gamma. Virtually all T-cell clones specific
    for bacterial components, which were derived from the same patients,
    was found to produce large amounts of IL-2 and IFN-gamma, and few
    produced IL-4 and/or IL-5 (Wierenga et al., 1990; Parronchi et al.,
    1991). In a subsequent study, antigen-specific T-cell clones were
    derived for the bacterial antigen purified protein derivate (PPD) from
     Mycobacterium tuberculosis and for the helminth antigen  Toxicara
     canis excretory-secretory (TES). Most PPD-specific clones produced
    IL-2 and IFN-gamma, but not IL-4 and IL-5, whereas most TES-specific 
    clones produced IL-4 and IL-5, but not IL-2 and IFN-gamma. This study 
    shows that in the course of natural immunization certain infectious agents
    preferentially expand T-cell subsets. PPD expands Th1, parallelling
    the (Th1-mediated) tuberculin DTH, whereas TES expands Th2,
    parallelling the (Th2-mediated) parasite infection.

         In a large series of human T-cell clones, all Th1 clones were
    found to lyse EBV-transformed autologous B-cells pulsed with the
    specific antigen, and the decrease of Ig production correlated with
    the lytic activity of Th1 clones against autologous antigen-presenting
    B-cell targets (Romagnani, 1991). This suggests an important mechanism
    for down-regulation of antibody responses  in vivo.

         Almost all nickel-specific T-cell clones produce TNF-alpha,
    GM-CSF, IL-2 and high levels of IFN-gamma, but low or undetectable
    levels of IL-4 and IL-5, thus resembling Th1 cells. Nickel induces DTH
    in the skin of allergic patients. Since IFN-gamma is an important
    mediator for DTH, IFN-gamma may be essential to DTH. However, no clear
    difference in cytokine production profile between allergic patients
    and control individuals was found.

    2.2.1  Regulation of IgE synthesis by IL-4 and IFN-gamma

         Atopy is associated with enhanced serum titres of
    allergen-specific IgE. The production of IgE is heightened and
    sustained by B-cells in atopic patients. IL-2 secreted by Th cells is
    necessary for the production of all isotypes of immunoglobulins
    (Kapsenberg et al., 1991). Activated B-cells are induced by IL-4 to
    undergo immunoglobulin heavy-chain rearrangements to the
    epsilon-constant region, resulting in synthesis of IgE (Coffman et
    al., 1986). So far, IL-4 can mediate this isotype switch, which is
    blocked very efficiently by IFN-gamma (Romagnani, 1991). IFN-gamma
    induces switching to gamma-2a (Coffman et al., 1986). IL-4 and
    IFN-gamma are produced by Th2 and Th1 cells, respectively; a response
    that involves mainly Th2 cells should produce a large amount of IgE,
    whereas responses involving mainly Th1 cells, such as DTH reactions,
    should be non-permissive for IgE production.  In vivo experiments
    have confirmed these predictions. IL-4-deficient mice lack IgE and
    IgG1 responses (Kuhn et al., 1991), whereas transgenic mice
    constitutively producing IL-4 show elevated serum IgE levels.
    Injection of mice with anti-IgD antibodies results in a strong
    stimulation of both B- and T-cell populations, leading to polyclonal

    antibody production and very high IgE levels. Anti-IL-4 antibodies
    dramatically reduce IgE levels after anti-IgD immunization, whereas
    anti-IFN-gamma antibodies elevate IgE levels even further. Similarly,
    administration of IFN-gamma results in considerable inhibition of the
    IgE response. Because the anti-IgD immunization leads to a response
    that involves high levels of Th2 cytokines, all of these results are
    consistent with the effects of IL-4 and IFN-gamma on IgE synthesis as
    defined by  in vitro model systems. Similar correlations between 
    Th2-like responses and high IgE production are seen during several
    parasite infections.

    2.2.2  Eosinophilia and IL-5

         Many parasitic infections induce high levels of circulating
    eosinophils. Because IL-5 has been implicated as a major growth and
    differentiation factor for eosinophils the association of IgE and
    eosinophilia may be explained by the association of IL-4 and IL-5 as
    products of Th2-cells (Gulbenkian et al., 1992). Supporting evidence
    has been provided by experiments  in vivo, in which administration of
    anti-IL-5 during a strong anti-parasitic immune response completely
    abrogated eosinophilia (Coffman et al., 1989), and from studies of
    transgenic mice that express high levels of IL-5. The major
    abnormality in these animals is the presence of extremely high levels
    of eosinophils in the blood and various lymphoid organs. Patients with
    filaria-induced eosinophilia exhibit a significantly greater frequency
    of IL-5-producing T-cells than uninfected individuals.

    2.2.3  The relationship between Th2 cells and type I hypersensitivity

         In mice, in addition to enhancing IgE production via IL-4, Th2
    cells also influence other features of allergic reactions. Firstly,
    IL-3, IL-4 and IL-10 are mast cell growth factors that act in synergy,
    at least  in vitro, and secondly, IL-5 induces the proliferation and
    differentiation of eosinophils  in vitro and  in vivo (Coffman,
    1989; Sanderson, 1990). In addition, IL-3 and IL-4 have been shown to
    enhance the secretory function of murine mast cells. So, Th2-cell
    activation not only increases the level of IgE synthesized, but also
    potentially increases the number of IgE-binding cells that will
    degranulate in response to allergen challenge.

         Mast cells and basophils produce IL-4. It has been hypothesized
    that IL-4 produced by these cells induces the development of Th2
    cells, and that these cells in turn produce IL-4. In addition, mast
    cells are an important source of IL-5.

    2.2.4  IL-12 drives the immune response towards Th1

         The pivotal role of the cytokine IL-12 in the differentiation of
    Th-cells towards Th1 is evident from both  in vitro and  in vivo
    studies (Scott, 1993). IL-12 is produced by T-cells, B-cells,
    macrophages and dendritic cells and stimulates the production of
    IFN-gamma from T-cells and NK-cells. IL-12 enhances Th1-cell expansion

    in cell lines from atopic patients (Manetti et al., 1993). The
    presence of IL-12 during primary stimulation of naive CD4+ cells skews
    the response in the direction of Th1 differentiation. These data
    suggest that IL-12 may be the IL-4 equivalent for the differentiation
    of Th1-cells. IL-10 has been shown to inhibit lymphocyte IFN-gamma
    production by suppressing IL-12 synthesis in accessory cells. A
    variety of pathogens that are associated with Th1 development have
    been shown to induce IL-12 production (Scott, 1993).

    2.2.5  IL-13, an interleukin-4-like cytokine

         Information on cytokine IL-13 is based on limited information
    about its activities  in vitro. As it shares biological activities
    with IL-4, these activities will, however, be briefly discussed. IL-13
    is produced by activated T-cells. The activities  in vitro of IL-13
    are similar to those of IL-4, with two major exceptions. Firstly,
    IL-13 does not act on T-cells and secondly, IL-13 does not act on
    murine B-cells (Zurawski & de Vries, 1994). Similarly to IL-4 and
    IL-10, IL-13 inhibits the production by LPS-stimulated monocytes of
    proinflammatory cytokines, chemokines and haematopoietic growth
    factors. In contrast to IL-10, however, IL-13 up-regulates the
    antigen-presenting capacity of monocytes. Similarly to IL-4, IL-13
    inhibits transcription of IFN-gamma and both alpha- and beta-chains of
    IL-12. Thus, IL-13 may (like IL-4) suppress the development of
    Th1- cells through down-regulation of IFN-gamma and IL-12 production
    by monocytes, favouring the generation of Th2 cells. Also in the
    mouse, IL-13 inhibits production of proinflammatory cytokines and
    expression of IL-12 alpha- and beta-chain mRNA. Murine IL-13 does not
    affect macrophage antigen-presenting capacity. Similarly to IL-4,
    IL-13 acts on human B-cells in inducing class switch to production of
    IgG4 and IgE and inducing CD23 surface expression (Punnonen et al.,
    1993; Punnonen & de Vries, 1994). Following activation of T-cells,
    IL-13 is produced earlier and for much longer periods than IL-4 (Yssel
    et al., 1994). Thus, IL-13 may play an important role in the
    regulation of enhanced IgE synthesis in allergic patients. In contrast
    to IL-4, murine and human IL-13 do not induce IgE synthesis in murine
    B-cells. Importantly, this may restrict the use of mice as an animal
    model for allergy.

         In summary, IL-13 may favour development of Th2-cells, consistent
    with the induction of IgG4 and IgE synthesis. Determination of the
    actual role of IL-13 requires more information on the biological
    effects  in vivo.

    2.3  Autoimmune reactions

         A wide spectrum of human and animal diseases appears to be wholly
    or partially attributable to autoimmune reactions. Despite the
    extensive growth of information relating to the mechanisms of
    self-tolerance (see section 1.5), the understanding of the mechanisms
    leading to pathogenic autoimmunity is still fragmentary and incomplete
    (Theofilopoulos, 1995a).

         Important issues that need to be resolved in this context
    concern: (i) the nature of the inciting antigens (self, neo-self,
    foreign); (ii) the definition of the criteria by which a disease can
    be termed autoimmune; (iii) the principles that govern the spectrum
    and extent of an autoimmune response; (iv) the mechanisms by which
    spontaneous remissions and exacerbations of autoimmune diseases occur;
    (v) the nature of environmental factors that initiate/precipitate
    autoimmune reactions; (vi) the structural and other characteristics
    that differentiate pathogenic from non-pathogenic autoantibodies and
    T-cells; and (vii) the identity of the genes that predispose or
    accelerate autoimmunity, as well as their mechanism of action
    (Theofilopoulos, 1995a).

         The most urgent of these questions concerns the nature of the
    inciting antigen. Although autoimmune disorders are often defined and
    diagnosed by the presence of autoantibodies (Osterland, 1994), it
    should be noted that (a) autoantibodies may indeed be the actual
    pathogenic agents of disease (e.g., autoimmune haemolytic anaemia,
    pemphigus, and myasthenia gravis; see sections 2.6.3, 2.6.5 and
    2.6.6), (b) they may arise as a consequence of another disease process
    (e.g., organ-specific autoantibodies due to tissue damage to those
    organs), or (c) they may merely mark, like footprints, the presence of
    the etiological agent while not themselves causing disease (Naparstek
    & Plotz, 1993; Theofilopoulos, 1995a). The latter possibility is
    complicated by the fact that determinants recognized by the
    autoantibody and the prerequisite Th-cell may reside on different
    molecules within a supramolecular complex (Theofilopoulos, 1995a). For
    example, for many years, it was believed that native nDNA itself was
    the immunogen for anti-nDNA antibodies, but efforts to induce such
    autoantibodies by immunization with nDNA have generally been
    unsuccessful. It has been suggested that for anti-nDNA antibody
    induction, the scenario may involve intermolecular help, via the
    binding of nucleosomes or other protein-DNA complexes to anti-DNA
    idiotype-displaying B-cells, followed by processing of the protein and
    presentation to the corresponding Th-cells (Theofilopoulos, 1995a). In
    this connection it is of interest that in systemic autoimmune diseases
    autoantibodies frequently appear to be directed against the entire set
    of polypeptides associated with discrete supramolecular cellular
    entities, such as the nucleosome particle or the nucleocytoplasmic
    ribonucleoprotein particles (see Table 10).

         It has become clear that T-cells are primary players in the
    initiation and perpetuation of spontaneous (Theofilopoulos & Dixon,
    1985; Singer & Theofilopoulos, 1990) as well as induced systemic
    autoimmune disorders (Druet, 1989; Goldman et al., 1991). Many immune
    responses seem to be functionally dominated either by Th1 or Th2
    cytokines. Therefore, the Th1-Th2 balance during immune reactions  in
     vivo significantly determines the outcome of immunopathological
    processes (Röcken et al., 1996). Whereas organ-specific autoimmune
    disease are predominantly mediated by IFN-gamma-producing Th1-cells,
    IL-4-producing Th2-cells are involved in immunoglobulin-mediated
    autoimmune diseases such as systemic lupus erythematosus (SLE)
    (Goldman et al., 1991; Röcken et al., 1996) (Table 9).


        Table 10.  Examples of autoantigens in organ-specific and systemic autoimmune diseasesa
                                                                                                               

    Organ/cell/nucleus                   Target antigens                      Diagnosis
                                                                                                               

    Organ-specific autoimmune diseases

    Pancreatic islet cells               glutamic acid decarboxylase 65       insulin-dependent diabetes mellitus
                                         glutamic acid decarboxylase 67
                                         tyrosine phosphatase IA-2
                                         tyrosine phosphatase IA-2b

    Adrenal cortex                       21-hydroxylase                       Addison's disease

    Leydig cells, testes, granulosa      cytochrome side-chain cleavage       hypogonadism
    theca                                enzyme

    Ovary                                17a-hydroxylase                      hypogonadism

    Gastric parietal cell                H+/K+-ATPase                         pernicious anaemia
                                         intrinsic factor

    Thyroid epithelium                   thyroid peroxidase                   autoimmune thyroid diseases
                                         thyroglobulin
                                         thyroid-stimulating hormone (TSH)
                                         TSH-receptor
                                         triiodothyronine
                                         thyroxine

    Hepatocyte                           CYP 2D6 (LKM-1)                      chronic active hepatitis
                                         halothane-induced hepatitis

    Melanocyte                           tyrosinase                           vitiligo

    Parathyroid                          calcium-sensing receptor             autoimmune parathyroidism

    Table 10.  (continued)
                                                                                                               

    Organ/cell/nucleus                   Target antigens                      Diagnosis
                                                                                                               

    Systemic autoimmune diseases

    Native DNA                           DNA backbone                         systemic lupus erythematosus (SLE)-renal

    ss-DNA                               nucleotides                          SLE and other connective tissue diseases

    Nucleoprotein                        DNA histone                          SLE - central nervous system, 
                                         histone 1 H1, 2A, 2B, 3, 4           renal - drug-induced SLE
                                         histone 2 H3                         connective tissue disease

    Sm                                   SnRNP                                SLE

    Nuclear RNP                          non Sm SnRNP                         mixed connective tissues disease, SLE

    Ribosomal RNP                        phosphoproteins                      SLE

    Scl-70                               topoisomerase 1                      scleroderma

    Centromere                           kinetochore                          CRESTb, Raynaud's syndrome

    SS-A (Ro)                            RNP                                  SLE-cutaneous, photosensitivity

    SS-B (La)                            RNA-pol protein                      Sicca syndrome, SLE, neonatal lupus

    Cardiolipin                          phospholipid                         SLE - thrombosis, cytopenia

    PM-1                                 protein complex                      myositis, scleroderma

    Jo-1                                 histidyl tRNA synthesis              myositis

    Mi-2                                                                      dermatomyositis

    Table 10.  (continued)
                                                                                                               

    Organ/cell/nucleus                   Target antigens                      Diagnosis
                                                                                                               

    PCNA                                 cyclin                               SLE

    Ku                                   protein on terminal chromosome       SLE
                                         nucleolar                            RNA-pol 1, RNA
                                         fibrillaren                          scleroderma, drug-induced connective
                                                                              tissue disease

    Nuclear membrane                     laminins                             scleroderma, SLE
                                                                                                               

    a  modified from Osterland (1994) and Song et al. (1996a); responses encompass both Th1 and Th2 responses
       and involve both Th1 (IFN-alpha) and Th2 (IL-4)
    b  CREST (calcinosis, Raynaud's phenomenon, oesophageal involvement, sclerodactyly and telangiectasia)
    
         The major non-mutually exclusive etiological concepts of
    autoimmune disorders have been reviewed (Theofilopoulos, 1995a,b) and
    are summarized in Table 11.

    Table 11.  Possible mechanisms of autoimmune reactions
    (modified from Theofilopoulos, 1995a,b)
                                                                

                   Release of anatomically sequestered antigens

                   The "cryptic self" hypothesis

                   The self-ignorance hypothesis

                   The molecular mimicry hypothesis

                   The "modified self" hypothesis

                   Immunoregulatory disturbances

                   Errors in central or peripheral tolerance

                   Polyclonal activators
                                                                


    2.4  Possible mechanisms of autoimmune reactions

    2.4.1  Release of anatomically sequestered antigens

         In general, antigens associated with peripheral tissues,
    especially those sequestered behind anatomic barriers, may not come
    into contact with the developing T-cell repertoire, and, therefore,
    tolerance may be unnecessary for such antigens. Induction of
    organ-specific autoimmune disease following contact with antigens of
    such so-called "immunologically privileged" sites has been well
    documented, as exemplified by the development of ophthalmia following
    eye injury and orchitis following vasectomy.

         Data have also clearly established that antigens associated with
    peripheral tissues can cause tolerance, and therefore loss of
    susceptibility to tissue-specific autoimmune diseases, when
    experimentally introduced into the thymus. Intrathymic injection of
    pancreatic islet cells can prevent autoimmune diabetes in the
    BioBreeding (BB) rat (Posselt et al., 1992) and the non-obese diabetic
    (NOD) mouse (Gerling et al., 1992). Tissue trauma alone may not be
    sufficient to elicit a conventional self-directed immunological
    response. Tissue-trophic pathogens, such as viruses, may be important
    in inducing the initial damage that results not only in availability
    of previously sequestered antigens but also in the production of
    co-stimulatory factors necessary for the immune response.

    2.4.2  The "cryptic self" hypothesis

         A corollary hypothesis for the mechanism of induction of
    pathogenic autoimmune responses addresses molecular, rather than
    anatomic, sequestration and relates to the presence of cryptic
    self-determinants. Each self-protein presents only a small minority of
    dominant determinants, which are involved in negative selection during
    thymic maturation and development of tolerance of the organism to
    them. Because of many constraints to peptide presentation, only a few
    peptide stretches of a given protein antigen are presented to the
    T-cell repertoire, namely those that have the highest affinity to the
    MHC-binding site and are present at a sufficient concentration. These
    peptides are the so-called dominant antigenic determinants. It is
    important to realize that, because antigen-presenting cells cannot
    distinguish "self" and "non-self" proteins, foreign and "self"
    peptides are presented indiscriminately (Bloksma et al., 1995). The
    subsequent immune responses, however, are diametrically opposed to
    each other. Whereas foreign peptide sequences, in general, induce
    "stimulatory" T-cell responses, the dominantly presented "self"
    sequences induce "inhibitory" T-cell responses through
    peptide-specific thymic cell deletion during development of the T-cell
    repertoire and/or induction of specific tolerance or anergy in the
    established peripheral T-cell repertoire. The poorly displayed
    majority of subdominant/cryptic determinants, constituting the
    "cryptic self", do not induce tolerance and, therefore, a large cohort
    of potentially "self"-reactive T-cells exists. The presentation of
    cryptic "self" peptides, however, can be up-regulated under certain
    conditions (Lehmann et al., 1993). Evidence for the role of cryptic
    determinants in the pathogenesis of autoimmunity has been provided in
    the non-obese diabetic mouse (NOD) model (Kaufman et al., 1993; Tisch
    et al., 1993), but the exact mechanisms of these immune responses are
    not fully known. One suggestion is that pathogens such as viruses may
    provide the initial stimulus through increased presentation of the
    subdominant determinant, either by molecular mimicry (see below)
    and/or by interferon-induced up-regulation of gene-expression,
    including genes for antigen-presenting MHC molecules (Theofilopoulos,
    1995a).

         Processing of chemically altered "self" proteins may result in
    the presentation of cryptic, thus potentially T-cell-activating,
    self-peptides by creation of new binding sites with high affinity to
    MHC molecules or modification/preventing of the physiological
    intracellular protein degradation (Bloksma et al., 1995). Expression
    of "cryptic self" peptides of nucleolar proteins appears to be a
    decisive step in the pathogenesis of HgCl2-induced formation of
    anti-nucleolar autoantibodies in mice (Kubicka-Muranyi et al., 1996).

    2.4.3  The self-ignorance hypothesis

         Evidence suggests that mature resting T-cells specific for
    extrathymic antigens presented by non-professional antigen-presenting
    cells (other than dendritic cells and macrophages) are induced to

    undergo anergy because of the absence of appropriate "second signals"
    or "co-stimulatory" factors (Theofilopoulos, 1995a). An alternative
    possibility is that there is no induction of anergy, but that the
    mature T-cells are unable to receive appropriate signals and/or help.
    This would result in T-cells simply ignoring such antigens and
    remaining quiescent. It follows that, if adequate antigen presentation
    and co-stimulation occurs through professional antigen-presenting
    cells, then these self-reactive but quiescent cells may be activated
    and cause tissue damage (Theofilopoulos, 1995a).

    2.4.4  The molecular mimicry hypothesis

         Molecular mimicry is defined by homology in a linear amino acid
    sequence between "self" molecules and foreign molecules. The above
    theories of cryptic or ignored "self" are compatible with the
    molecular mimicry hypothesis of autoimmunity, particularly as it
    pertains to infectious agents. Closely related or identical peptides
    are often found in unrelated proteins. Thus, many peptide fragments of
    infectious agents are homologous with host proteins. Among microbial
    antigens implicated in autoimmunity induced by molecular mimicry, heat
    shock proteins (hsp), found in virtually all life forms, have received
    prime attention (Minowada & Welch, 1995). Comparisons of the amino
    acid sequence of hsp60 with the entire database of known human
    sequences revealed that 86 human peptides have similar regions to
    hsp60 and, of these, 19 are known disease-associated autoantigens
    (Jones et al., 1993). However, the importance of mimicry to the
    pathogenesis of spontaneous autoimmune disease is uncertain, as it is
    unclear why immunological responses to hsp, which are expressed in
    every cell, could lead to organ-specific autoimmune diseases.

    2.4.5  The "modified self" hypothesis

         This theory suggests that autoimmunity may arise as a result of
    an immune response against modified "self" determinants ("neo-self"
    determinants), which may be particularly relevant for chemical-induced
    autoimmune responses. Drugs, their metabolites or other haptenic
    chemicals may bind to "self" determinants. A number of possibilities
    should be considered.

    2.4.5.1  Hapten-induced antibody responses to "modified self"

         In such reactions the hapten conjugates to "self" and forms an
    integral component of the determinant that is recognized by the
    antibody. In this mechanism hapten-specific T-cells provide cognate
    help to the B-cell that is then induced to synthesize antibodies which
    recognize the hapten-modified but not the native form of the "self"
    protein. Therefore, these reactions against a particular hapten are
    not truly autoimmune in nature. Penicillin, quinidine, halothane (Gut
    et al., 1995), and tienilic acid are good examples of compounds that
    can induce antibody responses to hapten-modified "self".

    2.4.5.2  Hapten-induced autoantibodies that recognize "self" proteins

         In their native form these can be considered true autoimmune
    responses, since the determinant that is recognized by antibody does
    not incorporate a drug-derived determinant. However, the determinant
    that is recognized by the Th-cells that promote the B-cell response
    may be drug-derived. The following theories have been put forward:

    a)   Drugs might break tolerance by binding to "self" macromolecules, 
         thereby creating new determinants that could be recognized 
         by T-cells. T-cells recognizing this new determinant would 
         clonally expand and go on to provide help for B cells that
         recognize adjacent autoantigens on the same drug "self"
         conjugate. These in turn would clonally expand and differentiate
         into autoantibody-producing plasma cells (Fig. 7). In this way
         the normal process of suppression that operates through either
         clonal or functional deletion of Th-cells is effectively bypassed
         (Allison, 1989). A considerable body of experimental evidence,
         largely from work with mice, supports this concept.
         Administration of arsenilic acid-conjugated autologous
         thyroglobulin or dinitrophenylated autologous immunoglobulin to
         mice has been found to lead to breakdown of tolerance and
         elicitation of autoantibodies to these self-proteins (Weigle,
         1965; Iverson, 1970). Furthermore, mice sensitized to
          p-aminobenzoic acid (PAB) and then administered PAB-conjugated
         isologous red blood cells developed a T-cell-dependent antibody
         response to their own red blood cells, with consequent haemolytic
         anaemia (Yamashita et al., 1976).

    b)   It has been postulated that a drug or metabolite might
         interact chemically with "self"-MHC molecules on
         antigen-presenting cells (macrophages or B-cells) in such a way
         that they appear as "non-self" to T-cells. These T-cells,
         following clonal expansion, would then provide help
         indiscriminately to all B-cells carrying the drug-modified
         "self"-MHC molecule. Assuming that the drug modifies MHC
         molecules without regard for the antigen specificity of the
         B-cell, the resulting cognate T-/B-cell interaction would lead to
         polyclonal B-cell activation and induction of synthesis of
         antibodies of multiple, including "anti-self", specificities
         (Gleichmann et al., 1984, 1989). This mechanism would be
         analogous to a graft-versus-host (GVH) reaction. Indeed,
         experiments with mercuric chloride (Pelletier et al., 1994),
         D-penicillamine (Tournade et al., 1990) and gold salts (Schuhmann
         et al., 1990) in Brown Norway rats or particular strains of mice
         led to immune disregulatory changes (elevated immunoglobulin
         levels, particularly IgE, induction of autoantibodies to the
         glomerular basement membrane, DNA, IgG, collagen and nuclear and
         nucleolar proteins) resembling those seen in graft-versus-host
         (GVH) disease (Gleichmann et al., 1984, 1989; Goldman et al.,
         1991; Bloksma et al., 1995). The elevations in IgE, IgG1 and IL-4
         in mercury chloride-treated susceptible mice and rats implicate
         the Th2 subset in this response (Goldman et al., 1991).

    FIGURE 7

              In order to become antigenic to T-cells, haptens need to
         bind to carrier proteins and it has been discussed whether 
         or not T-cells may require covalent modification of MHC 
         molecules for hapten recognition. Several studies investigating
         trinitrophenol- and gold-hapten formation have pointed to a major
         role of hapten-modified MHC-associated peptides as 
         T-cell-antigenic structures (Martin & Weltzien, 1994; Sinigaglia,
         1994; Weltzien et al., 1996).

    c)   Another theory of drug-induced autoimmunity suggests that
         certain drugs or chemicals might induce, or protect from
         suppression, populations of T-cells that recognize unmodified
         "self" MHC. This would be analogous to graft-versus-host
         reactions, but the difference with the aforementioned mechanism
         would be that the chemical's effect is targeted at the T-cell
         rather than the B-cell. In the Brown Norway (BN) rat model of
         autoimmunity induced by D-penicillamine, gold and mercuric
         chloride, autoreactive T-cells that recognize unmodified "self"
         MHC Class II molecules on normal B-cells have been reported,
         rather than T-cells that recognize chemically modified "self"
         (Pelletier et al., 1994). This supports the concept that several
         compounds might induce autoreactivity by modifying T-cells rather
         than B-cells.

    2.4.6  Immunoregulatory disturbances

    2.4.6.1  Errors in central or peripheral tolerance

         Errors in central or peripheral tolerance at the T- or B-cell
    level have also been suggested as causes for autoimmunity.

         The association between development of immunodeficiency, benign
    or neoplastic lymphoproliferation and autoimmune diseases,
    particularly in the context of thymic abnormalities, is well known
    (Fudenberg, 1966). It has been observed upon immunosuppressive
    treatment, among others with cyclophosphamide and cyclosporin A. The
    reversibility of lymphoproliferative lesions upon withdrawal of the
    immunosuppressive drug therapy suggests a causal relationship (Starzl
    et al., 1984). Studies in rodents have provided more solid evidence of
    the relationship between the development of autoimmune disease and
    induced disturbance of thymic function (Sakaguchi & Sakaguchi, 1989,
    1990; Barrett et al., 1995). Notably, cyclosporin A, which is
    successfully used in the prevention of transplant rejection and
    treatment of various autoimmune diseases in humans, has been shown to
    interfere with the deletion of T-cells recognizing autoantigens in the
    thymic medulla and to cause organ-specific and systemic autoimmune
    disease under specific conditions. This occurs when cyclosporin A is
    given to neonates (Sakaguchi & Sakaguchi, 1989), but not to older
    animals (Hess & Fischer, 1989), and to bone marrow transplant
    recipients that received a high dose of irradiation prior to
    transplantation (Glazier et al., 1983; Hess & Fischer, 1989). The

    development of autoimmune disease under these conditions has been
    attributed to the absence of an established regulatory peripheral
    T-cell repertoire. Because cyclosporin A may interfere at different
    levels of immunological tolerance, autoreactive T-cells leaving the
    thymus as a consequence of cyclosporin A treatment may not be
    functionally inactivated in the periphery (Prud'Homme et al., 1991).
    However, a study using the bone marrow transplant model in different
    mouse strains suggested the involvement of other mechanisms, because
    effects of cyclosporin-A on T-cell deletion did not correlate with
    development of autoimmune effects (Bryson et al., 1991). The study
    suggested a polyfactorial etiology of cyclosporin-A-induced autoimmune
    disease and may explain why autoimmune side-effects have been observed
    only rarely in cyclosporin A-treated human bone marrow transplant
    patients (Jones et al., 1989).

         Patients with primary immunodefiency, especially various B-cell
    deficiencies, are known to have a high incidence of autoimmune disease
    (Rosen, 1987). For example, selective IgA deficiency is associated
    with systemic autoimmune diseases, such as systemic lupus
    erythematosus (SLE) (Cleland & Bell, 1978; Rosen, 1987). Moreover,
    drugs with a documented ability to cause systemic autoimmune
    disorders, i.e., diphenylhydantoin (Seager et al., 1975) and
    D-penicillamine, have been shown to reduce secretory and/or serum IgA
    levels. However, the relationship between IgA deficiency and
    susceptibility to autoimmune disease is not known. It is most likely
    influenced by other factors as well, since the prevalence of selective
    IgA deficiency in a normal population is much higher (1 in 700) than
    the prevalence of systemic autoimmune disease.

         Both cyclosporin A and diphenylhydantoin have immunosuppressive
    activities and affect the thymus. Although neonatal exposure
    experiments with diphenylhydantoin have been performed (Chapman &
    Roberts, 1984; Kohler et al., 1987), autoimmune side effects have not
    been reported. This may be related to the different intrathymic
    targets of both compounds. Cyclosporin A is thought to disturb
    thymocyte differentiation by affecting interdigitating and epithelial
    cells (Schuurman et al., 1992), while diphenylhydantoin affects the
    more immature cortical thymocytes probably by a
    glucocorticoid-mediated effect. As pointed out by Schuurman et al.
    (1992), such differences in intrathymic targets may have different
    consequences for immune function ranging from immunodeficiency to
    autoimmune disorders. It illustrates the complex relationship between
    immunodeficiency, lymphoproliferation and autoimmune effects and the
    difficulty of immunotoxicological hazard identification (chapter 6)
    and risk assessment (chapter 7).

    2.4.6.2  Polyclonal activators

         Polyclonal B- and/or T-cell activation has been considered a
    contributing or initiating mechanism of autoimmunity, particularly in
    systemic diseases. Although exogenous polyclonal B-cell activators

    (i.e., lipopolysaccharide) may exacerbate or precipitate SLE, they
    appear to be insufficient in themselves (Hang et al., 1985;
    Theofilopoulos, 1995a).

         Polyclonal T-cell activation in autoimmune disease is exemplified
    in graft-versus-host (GVH)-induced autoimmunity, where alloreactive
    donor T-cells initiate recipient B-cell differentiation into
    antibody-secreting cells, particularly those recognizing polymeric
    "self" antigens (Gleichmann et al., 1989; Goldman et al., 1991;
    Bloksma et al., 1995). It has been suggested that in this model, as
    well as in some models of chemically induced systemic autoimmunity,
    there is a predominant engagement of Th2-cells that promote the
    humoral response (IL-4 hyperproduction) (Goldman et al., 1991).
    Polyclonal stimulation of a large set of T-cells by bacterial/viral
    superantigens is another possible scenario. T-cells that react with
    MHC Class II-bound superantigens on B-cells may mutually stimulate
    superantigen-displaying B-cells, thereby leading to production of
    polyclonal immunoglobulins and, in some instances, autoantibodies
    (Friedman et al., 1991).

         The development of autoimmune reactions as outlined above is only
    the first step in the production of autoimmune disease. Multiple
    mechanisms can lead to the same overall clinical manifestations both
    in organ-specific and in systemic autoimmune syndromes, and therefore,
    expectations for a single etiological explanation appears unrealistic.
    For organ-specific autoimmune diseases, the most straightforward
    explanation to emerge is the concept that these diseases are caused by
    otherwise conventional immunological responses against self-antigens
    for which T-cell tolerance is normally not established (i.e., anatomic
    sequestration, inadequate presentation due to the cryptic nature of
    the self-determinant, and/or lack of co-stimulatory factors). With
    regard to systemic autoimmune diseases such as SLE, the situation is
    less clear, but neither exogenous polyclonal B- or T-cell activators
    nor immunoregulatory disturbances appear to provide satisfactory
    explanations. Physical, chemical and infectious assaults may
    precipitate heterogenous syndromes such as SLE, characterized by an
    almost all-encompassing autoimmune response against a vast array of
    mostly dissimilar self-antigens, possibly mediated by the engagement
    of a large set of non-tolerant T-cells that recognize diverse
    self-peptides displayed on MHC molecules.

    2.5  Type I hypersensitivity diseases and allied disorders

         Allergy and atopy have become synonymous for the same set of
    hypersensitivity disorders, several of which commonly occur in the
    same individual. They comprise predisposition to develop IgE-mediated
    immediate (Type I) hypersensitivity responses to common environmental
    antigens, in part genetically mediated and manifested as eczema,
    rhinitis, conjunctivitis and asthma.

         Allergic diseases which are considered to result from Type I
    (immediate) hypersensitivity reactions are shown in Table 12.

        Table 12.  Examples of Type I hypersensitivity and reaction sites
                                                                      

    Disease                                         Reaction site
                                                                      

    Urticaria                              Skin
    Atopic eczema                          Skin
    Angioedema                             Skin or mucous membranes
    Asthma                                 Respiratory tract
    Rhinitis                               Respiratory tract
    Conjunctivitis                         Conjunctiva

    Anaphylaxis            }               Variable, including skin,
    Insect venom allergy   }               gastrointestinal tract,
    Food allergy           }               respiratory system,
    Drug allergy           }               cardiovascular system or
                           }               generalized
                                                                      
    
         The ability of protein antigens encountered in the environment or
    workplace to cause IgE antibody-mediated rhinitis and asthma is now
    well established. Thus, for example, a variety of pollens is known to
    cause seasonal hay fever in susceptible individuals. It is now
    apparent that certain chemicals are able to induce similar symptoms in
    a proportion of exposed individuals (Butcher & Salvaggio, 1986; Karol,
    1992). Among chemicals of small relative molecular mass known to cause
    respiratory allergy in humans are: acid anhydrides such as phthalic
    anhydride, tetrachlorophthalic anhydride, hexahydrophthalic anhydride
    and trimellitic anhydride (Bernstein et al., 1982a, 1984; Moller et
    al., 1985); certain isocyanates including toluene diisocyanate,
    diphenylmethane-4,4'diisocyanate and hexamethylene diisocyanate
    (Tanser et al., 1973; Zamit-Tabona et al., 1983; Keskinen et al.,
    1988); some reactive dyes (Alanko et al., 1978); and platinum salts
    (Biagini et al., 1985). A number of chemicals induce hypersensitivity
    disorders that have features similar to Type I hypersensitivity
    reactions but do not easily fall within the classification of Gell &
    Coombs (1963).

         The characteristics of respiratory allergic hypersensitivity to
    chemicals are of specific pulmonary reactions usually induced only in
    a minority of the exposed population and which are provoked by
    atmospheric concentrations of the allergen that were previously
    tolerable and that fail to cause symptoms in non-sensitized
    individuals. Thus, it has been found that with toluene diisocyanate an
    asthmatic response can be caused by atmospheric concentrations of the
    chemical far below those that are necessary to induce irritant effects
    (Newman Taylor, 1988).

         Allergic respiratory hypersensitivity induced by chemicals may be
    of immediate- and/or late-onset. An obligatory universal role for IgE
    antibody in the pathogenesis of chemical respiratory allergen is
    uncertain, not least because many symptomatic individuals lack
    detectable IgE for the relevant allergen. In some cases it may be that
    insufficiently sensitive or inappropriate methods have been employed
    for detection of IgE antibody. Nonetheless, it is possible that
    T-lymphocytes and cell-mediated immune responses may also effect
    respiratory hypersensitivity reactions to chemicals. There is
    generally a latent period between the onset of exposure and the
    appearance of respiratory symptoms. In the case of certain
    diisocyanates, asthma has been found to develop within a few months.
    In other instances, however, there may be a latent period of several
    years. While this is almost certainly the case for protein respiratory
    allergens, there is no  a priori reason to suppose that provocation
    of the immune responses necessary for respiratory sensitization to
    chemical allergens will result only from exposure via the respiratory
    tract. Indeed, there is evidence that occupational respiratory
    sensitization may be caused by dermal exposure to chemical allergens
    following industrial spillage or splashing (Karol, 1986).

         Allergic respiratory hypersensitivity, by definition, results
    from the induction of a specific immunological response. While there
    is no doubt that the acute onset of respiratory symptoms associated
    with hypersensitivity to protein aeroallergens is due to
    homocytotropic (primarily IgE) antibody, the nature of the immune
    responses responsible for chemical respiratory allergy is still
    controversial. Although IgE specific for all recognized chemical
    respiratory allergens has been found, and despite a clear association
    for some chemical allergens between the presence of specific IgE
    antibody and the development of respiratory symptoms, a clear link
    between allergic responses and serum IgE antibody has, in some
    instances (notably with some diisocyanates), failed to emerge. It is
    nevertheless the case that the induction of acute-onset
    hypersensitivity reactions in the respiratory tract is usually
    considered as being dependent upon IgE antibody and the elicitation of
    classical immediate-type hypersensitivity responses.

         In the light of present uncertainties, perhaps the most realistic
    conclusion that can be drawn is that in many, but perhaps not all,
    cases the development of chemically induced respiratory allergy is
    dependent upon IgE antibody and the elicitation of immediate-type
    hypersensitivity reactions in the respiratory tract. It is possible,
    however, that in some instances respiratory hypersensitivity to
    chemical allergens results from the action of T-lymphocytes operating
    independently of IgE antibody. Irrespective of a putative
    IgE-independent cell-mediated immune mechanism for the induction of
    chemical respiratory hypersensitivity, it now appears likely that
    T-lymphocytes play an important role in late phase reactions and in
    the pathogenesis of chronic bronchial inflammation.

    2.5.1  Asthma

    2.5.1.1  Definition

         Asthma is a respiratory disease that eludes easy definition. It
    is characterized by variable airflow limitation due to bronchial
    responsiveness and often by inflammatory changes in the airways.
    Asthma has been classified as intrinsic or extrinsic; extrinsic asthma
    is provoked by sensitivity to a foreign substance, including
    idiosyncratic drug rections, while intrinsic asthma is characterized
    by reactivity to non-allergic factors, such as infection and physical
    and/or psychological stimuli (Barbee, 1987). However, this
    classification is considered artificial because the clinical signs of
    both types of asthma are similar.

         The US National Institutes of Health (NIH, 1991) published a
    consensus definition that included the following characteristics:
    airway obstruction that is reversible (but not completely so in some
    patients) either spontaneously or with treatment; airway inflammation,
    and increased airway responsiveness to a variety of stimuli. In
    practice, and especially in epidemiological surveys, it has been
    diagnosed from the replies to questionnaires that have focused on such
    symptoms as episodic wheezing and shortness of breath (see section
    5.2.1.2b). Asthma is distinguished from chronic obstructive pulmonary
    disease (COPD), i.e., chronic bronchitis and emphysema, by the
    prominent reversibility of the airways obstruction.

         The term reactive airways dysfunction syndrome (RADS) was coined
    to refer to persistent asthma after high-level irritant exposure
    (Brooks et al., 1985), but the term irritant-induced asthma is just as
    suitable. To prevent unnecessary confusion, the use of terms other
    than asthma should be avoided.

         The prevalence of asthma has been increasing in a number of
    countries in recent years (Buist & Vollmer, 1990; Strachan, 1995;
    ISAAC, 1998). Although some of the increase may be the result of a
    change in diagnostic classification and increased reporting, a true
    increase in disease prevalence is likely. The causes of this increase
    are currently unknown, but environmental pollution is one potential
    contributory factor.

         Allergy is associated with asthma. Up to 80% of patients with
    asthma have positive immediate reactions to skin-prick testing with a
    battery of common aeroallergens (Nelson, 1985), although this
    percentage probably over-represents the importance of allergy in
    asthma. Whereas allergy clearly plays a primary role in childhood
    asthma, many adults with asthma do not appear to be sensitized to
    specific aeroallergens. This observation provided the basis for the
    traditional characterization of the disease into two major types:
    i) extrinsic asthma (with sensitization to specific aeroallergens) and
    ii) intrinsic asthma (without specific sensitization).

         There is a genetic component to the risk of developing asthma.
    Children with one asthmatic parent have an increased risk of
    developing the disease themselves, and when both parents are
    asthmatic, the risk is even higher. A parental history of atopy also
    increases the risk. Up to 40% of the population is atopic: however,
    many sensitized people do not develop asthma or asymptomatic airway
    hyperresponsiveness (Witt et al., 1986). Thus allergy alone does not
    explain the development of persistent asthma, although continuous or
    recurrent exposure to allergen may serve to sustain asthma in a
    genetically susceptible subpopulation.

         Infections aggravate asthma and elicit exacerbations of the
    disease (Johnston et al., 1995). When it comes to the role of viral
    infections in the induction of the disease, evidence is conflicting
    (Martinez, 1995). There is evidence that some infections, in
    particular respiratory syncytial virus (RSV), may predispose for the
    development of asthma (Sigurs et al., 1995). On the other hand, there
    is increasing evidence that childhood infections may protect against
    the development of allergy and allergic diseases, including asthma
    (Holt, 1996; Shaheen et al., 1996; Shirakawa et al., 1997; Matricardi
    et al., 1997). Some anecdotal evidence and small studies suggested
    that childhood vaccination may increase the prevalence of asthma and
    allergy (Kemp et al., 1997). 

    2.5.1.2  Airways inflammation and asthma

         Over the past decade airway inflammation has emerged as an
    important feature of clinical asthma. It has long been known from
    autopsy studies of patients that die from status asthmaticus that
    airway inflammation is present in such patients. The use of
    fibre-optic bronchoscopy to obtain bronchoalveolar lavage and
    bronchial-mucosal biopsy specimens has allowed the study of patients
    with less severe asthma. Airway inflammation is clearly present in
    these patients as well. Asthmatic airways are characterized by: (a)
    infiltration with inflammatory cells, especially eosinophils; (b)
    oedema; and (c) loss of epithelial integrity. Airflow obstruction in
    asthma is believed to be the result of changes associated with airway
    inflammation, mucus production and bronchoconstriction. Airway
    inflammation is believed to play an important role in the genesis of
    airway hyperresponsiveness in asthma (Holgate et al., 1987).

         Much of the research on mechanisms that mediate airway
    inflammation in asthma has focused on allergen-induced responses.
    Inhalation of allergen in a specifically sensitized patient with
    asthma will trigger rapid-onset but self-limited bronchoconstriction,
    called the early response. In 30 to 50% of extrinsic asthmatic
    subjects undergoing an allergen inhalation challenge, a delayed
    reaction will occur 4 to 8 h later, called the late response (O'Byrne
    et al., 1987). The late response is characterized by persistent
    airflow obstruction, airway inflammation and airway
    hyperresponsiveness (Cartier et al., 1982). Mast cell degranulation
    and release of mediators such as histamine are believed to be

    responsible for the immediate response (Liu et al., 1990). The role of
    the mast cell in the genesis of the late response is more
    controversial, but the release of chemoattractant substances such as
    leukotrienes and cytokines (i.e., interleukins: IL-3, IL-4 and IL-5)
    may be involved in the influx of neutrophils and eosinophils into the
    airway epithelium, which is a key feature of this response. The
    infiltration of the airway wall with eosinophils is also a key feature
    of the late response (Metzger et al., 1987; Djukanovic et al., 1990).
    The number of Th2-cells in the airway epithelium appears to be higher
    in patients with allergy-related asthma and may be responsible for the
    maintenance of chronic airway inflammation (Ollerenshaw & Woolcock,
    1992). The Th2-cells are involved in the release of cytokines that may
    activate both mast cells (IL-3 and IL-4) and eosinophils (IL-5). The
    eosinophil can release proteins (e.g., major basic protein,
    eosinophilic cationic protein, eosinophilic peroxidase or
    eosinophil-derived neurotoxin), lipid mediators, oxygen radicals and
    enzymes that can cause epithelial injury.

    2.5.2  Occupational asthma

         Occupational asthma induced by protein allergens is invariably
    associated with atopy and with the presence of specific IgE antibody.
    In contrast, occupational asthma induced by chemical allergens is not
    restricted to atopic individuals and is not always associated with the
    presence of demonstrable IgE antibody. For both forms of asthma the
    inflammatory response in the respiratory tract is similar and
    characterized by T-lymphocyte and eosinophil infiltration.

         The immunopathology of occupational asthma has the characteristic
    features of airway smooth muscle contraction, oedema, and fluid
    accumulation, resulting presumably from the local release by mast
    cells of inflammatory mediators such as histamine and leukotrienes.
    Alternatively, it has been hypothesized that, in some instances of
    chemically induced respiratory allergic hypersensitivity, the initial
    inflammatory response results from a chronic cell-mediated immune
    mechanism operating independently, or in the absence, of IgE antibody
    (Corrigan & Kay, 1992). Chronic inflammation is recognized as playing
    an important role in asthma and is associated with infiltration of the
    bronchial mucosa with inflammatory cells, mucus production, the
    destruction and sloughing of airway epithelial cells, and
    subepithelial fibrosis secondary to collagen deposition (Roche et al.,
    1989; Beasley et al., 1989). Of particular importance in the
    development of bronchial mucosal inflammation and injury is the
    eosinophil, acting in concert with infiltrating T-lymphocytes (Beasley
    et al., 1989; Gleich, 1990). While the exact role of eosinophils in
    the development of bronchial hyperreactivity has yet to be
    established, there is no doubt that the eosinophilia associated with
    allergen-induced respiratory reactions is influenced markedly by
    cytokines and, in particular, by IL-5 (Chand et al., 1992; Gulbenkian
    et al., 1992; Iwami et al., 1993). A role for T- lymphocytes in asthma
    begs questions regarding the nature of allergen handling in the

    respiratory tract and the characteristics of local antigen-presenting
    cells. In the context of primary sensitization following inhalation
    exposure to the inducing allergen, it is likely that the network of
    dendritic cells found within the airway epithelium is of vital
    importance (Holt et al., 1990; Schon-Hegrad et al., 1991).

    2.5.2.1  Occupational asthma and allergy

         Hypersensitivity-induced occupational asthma (see also section
    4.3.3) fulfils the criteria for an acquired specific hypersensitivity
    response:

    a)   It occurs in only a proportion -- usually a minority -- of
         those exposed to the allergen.

    b)   It develops only after an initial symptom-free period of
         exposure ranging from days even up to several years.

    c)   In those who develop asthma, airway responses (both
         reduction in calibre and induction of hyperresponsiveness to
         non-specific stimuli) are provoked by inhalation of the specific
         agent in concentrations that were previously tolerable and that
         do not provoke similar responses in others equally exposed.

         These characteristics have stimulated a search for evidence of a
    specific immunological response to the causes of occupational asthma,
    both proteins and chemicals of low relative molecular mass. Attention
    has been directed towards the identification of specific IgE and IgG
    antibodies. In general, IgE and IgG4 have been found in exposed
    populations to be associated with disease and total specific IgG with
    exposure. For example, specific IgE and IgG4 were associated with
    asthma and IgG with exposure to acid anhydride workers (Forster et
    al., 1988).

         Studies have suggested a central role for the T-lymphocyte and in
    particular the Th2-lymphocyte in the development of the eosinophilic
    bronchitis characteristic of asthma. Evidence for the involvement of
    T-lymphocytes in occupational asthma was found in nine patients with
    isocyanate-induced asthma who had activated T-lymphocytes and
    eosinophils in bronchial biopsy specimens (Bentley et al., 1991).
    Nonetheless, the IgE antibody-mast cell interaction is probably an
    important associated response dependent upon Th2-lymphocyte
    stimulation, and specific IgE remains a valuable marker of the
    immunological response associated with asthma caused by several agents
    inhaled at work.

         Specific IgE has been identified in the sera of patients with
    asthma caused by some low relative molecular mass chemicals,
    particularly acid anhydrides (Newman Taylor et al., 1987) and reactive
    dyes (Luczynska & Topping, 1986), but not others, notably isocyanates.
    In a study to examine the determinants of allergenicity of low

    relative molecular mass chemicals, the properties of two beta lactam
    antibiotics were compared: clavulanic acid, which is not allergenic;
    and a carbapenam MM2283, which can cause asthma and stimulate IgE
    antibody production in man. The characteristics identified as relevant
    to allergenicity were (a) reactivity with body proteins; (b) hapten of
    single chemical structure and (c) stability of the conjugate formed
    (Davies et al., 1977).

         Specific IgE antibody has been identified in only some 15% of
    cases of isocyanate-induced asthma. This may reflect the difficulties
    of working with reactive chemicals in  in vitro systems or failure to
    prepare the relevant  in vivo chemical-protein conjugate for the 
     in vitro test.

         Duration and intensity of exposure are the major factors
    contributing to the development of occupational asthma in populations
    exposed to its causes. Additional factors such as atopy and tobacco
    smoking may also contribute. The importance of these factors varies
    for different causes of the disease. However, for no cause do they
    adequately explain the development of the disease in the minority who
    develop it. In part this may reflect the limited knowledge of
    exposures experienced but probably also suggests other important,
    including genetic (such as HLA haplotype), determinants.

    2.5.3  Atmospheric pollutants and asthma

         There is evidence that air pollutants are involved in
    exacerbating asthma (Vos et al., 1996). Evidence from laboratory
    studies suggests that certain air pollutants have the potential to
    stimulate bronchoconstriction or airways inflammation (see also
    chapter 5.) Exposure to SO2 is associated with chest tightness and
    bronchoconstriction, with the concentration required to induce a
    response being dependent upon the degree of hyperresponsiveness. It
    may be that the effects of SO2 will be augmented in the presence of
    other pollutants. It has been reported that the sensitivity of mild
    asthmatics to SO2 is increased by prior exposure to ozone (O3).
    Ozone is a prototype oxidant pollutant that reacts rapidly with tissue
    components. It is formed by photochemical reactions involving oxides
    of nitrogen and organic molecules and occurs with other photochemical
    oxidants and fine particles in the complex mixture called "smog".

         Bates & Sizto (1987) studied hospital admissions in Southern
    Ontario, Canada, an area with a population of seven million people,
    and observed an association between rates of admissions for asthmatic
    subjects during the summer season and ambient air levels of both O3
    and suspended sulfates. However, the study design could not separate
    the 03 effects from concomitant effects of acidic aerosol and SO2.
    Thurston et al. (1992a,b) found strong associations between elevated
    summer "haze" pollution (H+, sulfate, O3) and increased asthma (and
    total respiratory) admissions to hospitals in Buffalo and New York
    City, USA, especially in 1988 when air pollution was severe. However,
    the specific role of O3 as opposed to H+ was less clear.

         Controlled (environmental chamber) human exposure studies have
    clearly demonstrated that some healthy young adults and children
    respond to O3 exposure (at levels occurring in ambient air) with
    irritative cough and substernal chest pain on inspiration and
    decrements in FVC and FEV1 (Koren et al., 1989; Folinsbee, 1992).
    When exercising outdoors in summer such individuals show decrements in
    FEV1 that are consistent with the observed ambient air O3 levels.
    Controlled exposure to similar levels of ozone has also been shown to
    cause an inflammatory response of the respiratory tract in all species
    that have been studied including humans (Lippmann, 1989). The use of
    bronchoalveolar lavage (BAL) as a research tool has afforded the
    opportunity to sample lung and lower airways after exposure to O3 and
    to ascertain the extent and course of inflammation and its
    constitutive elements. The BAL studies (Devlin et al., 1991) have
    clearly demonstrated that O3, even at very low concentration, causes
    increases in numbers of neutrophils, and a variety of other
    constituents of BAL fluid, some with potential inflammatory properties
    such as prostaglandin E2, fibronectin, elastase and IL-6.
    Inflammation was also detected in the upper airways of O3-exposed
    subjects as shown by an increase in neutrophils and other inflammatory
    indicators in the nasal lavage (NAL) fluid (Koren et al., 1990).
    Interestingly, both NAL fluid and BAL fluid from non-asthmatic
    subjects exposed to O3 have been shown to contain the mast cell
    marker tryptase. This and another study (Bascom et al., 1990)
    suggested that O3-induced inflammation may share certain features of
    the response observed in subjects with allergic rhinitis challenged
    with allergen.

         A study demonstrated that asthmatic subjects exposed to low
    levels of O3 (0.16 ppm) for 7.6 h while performing moderate exercise
    showed more respiratory symptoms and greater decrements in FEV1 than
    did similarly exposed non-asthmatics (Ball et al., 1993).

         The concept of influencing the asthmatic response by combining
    exposure to O3 with specific allergen challenge has created interest
    in the potential "indirect" effects of O3 exposure. In one study,
    individuals with allergic rhinitis were initially exposed to clean air
    or 0.5 ppm O3 for 4 h (Bascom et al., 1990). The high level of
    exposure to O3 did not enhance the subsequent acute response to
    antigen in the nose under these experimental conditions. A study by
    Molfino et al. (1991) examined the effect of pre-exposure to O3 (0.12
    ppm for one hour at rest) on the subsequent airway response to inhaled
    ragweed or grass pollen antigen in seven subjects with allergic
    asthma. They reported O3-induced increases in bronchial
    responsiveness to specific allergen challenge. Preliminary data from
    studies currently conducted examining the effects of pre-exposure to
    O3 (0.4 ppm for 2 h at rest) followed by a specific allergen nasal
    challenge in asthmatics sensitive to house dust mite suggest that the
    O3 pre-exposure caused a significant decrease in the dose of allergen
    needed to induce symptoms (Peden et al., 1994). Eosinophil influx and
    increase in eosinophil cationic protein were observed 4 h after nasal

    allergen challenge following both O3 and clean air pre-exposure.
    These changes were more dramatic following O3 pre-exposure although
    the mean allergen dose was smaller.

         The health relevance of oxides of nitrogen, and in particulate
    NO2, has attracted some interest since the gas is present not only
    outdoors but also indoors. A number of studies suggest mild effects of
    NO2 in asthmatics at concentrations less than 1 ppm but others have
    not found responses at levels up to 4 ppm.

         Particulate air pollutants, especially fine particles derived
    from combustion sources, are also of interest although there have been
    few controlled exposure studies outside those involving acid aerosols.
    Bioaerosols, to which an asthmatic is sensitized, are well known to
    exacerbate asthma. Epidemiological studies describing the increase in
    mortality associated with particulate matter (PM) provide provocative
    evidence for adverse pulmonary health effects associated with
    particulate pollution (Dockery et al., 1993, 1994; Brunekreef et al.,
    1995; Pope et al., 1995). The association between PM and acute
    mortality and morbidity has been demonstrated most strongly with
    elderly people who have chronic cardiopulmonary disease (Pope et al.,
    1992; Burnett et al., 1995; Schwartz & Morris, 1995). Experimental
    studies with diesel exhaust particles show that they increase IL-4 and
    specific IgE production, and exacerbate the response to allergen in
    allergic individuals (Diaz-Sanchez et al., 1997). Studies in mice have
    demonstrated that diesel exhaust particles facilitate the induction of
    allergy (Takafuji et al., 1987; Lovik et al., 1997). Chemicals
    adsorbed to the diesel exhaust particles, as well as carbon particles
    with very little chemicals on them appear to enhance the allergic
    immune response (Diaz-Sanchez et al., 1997, 1999; Lovik et al., 1997).

         Environmental air pollutants including tobacco smoke may affect
    the prevalence and/or severity of asthma in several different ways. In
    hyperresponsive airways, pollutants may act as triggers of asthmatic
    reactions without the presence of the specific allergen.
    Alternatively, a pollutant could induce or increase airway
    inflammation and, as a result, cause airway hyperreactivity that
    persists after exposure has ceased. Some pollutants may have a direct
    toxic effect on the respiratory epithelium leading to inflammation,
    airway hyperreactivity and the appearance of asthma-like symptoms in
    previously non-asthmatic individuals. Lastly, there are certain
    pollutants that may have the ability to augment or modify immune
    responses to inhaled antigens or to enhance the severity of reactions
    elicited in the respiratory tract following inhalation exposure of the
    sensitized individual to the inducing allergen.

    2.5.4  Rhinitis

         Rhinitis frequently, but not invariably, occurs in atopic
    diseases. Similarities and differences between rhinitis and asthma are
    considered below.

         Allergic responses of the nasal mucosa cause an orchestrated set
    of responses. The acute allergic reaction occurs within minutes and is
    manifested as rhinorrhoea, pruritus and sneezing, and congestion, due
    (respectively) to increased vascular permeability, sensory nerve
    stimulation, and vasodilation with sinusoidal pooling plus oedema
    formation. These responses are due to mediators released from the
    mucosal mast cells, and histamine is a major participant.

         Following this acute response is the slower development of the
    late phase allergic reaction which is manifested by congestion and
    hyperirritability and is due to cellular infiltration with
    eosinophils, neutrophils and some basophils. There is interest in
    whether lymphocytes also participate in this reaction, but the data
    are not clear as yet.

         Of the cells that participate in rhinitis, mast cells,
    neutrophils, eosinophils and lymphocytes may all be important. Mast
    cells initiate the response through the release of the mediators of
    anaphylaxis. Work also indicates that mast cells generate a number of
    cytokines (generally thought of as lymphocyte products, but clearly
    generated by activated mast cells as well). These products include
    IL-3, IL-4, IL-5, IL-6 and TNF. Neutrophils are the first cells to
    infiltrate areas undergoing allergic reactions. The role of the
    neutrophil in allergy is not clear. However, neutrophils appear to be
    necessary for the development of increased airway hyperactivity in
    animal models of asthma. Neutrophils also release factors that
    activate mast cells (neutrophil-derived histamine releasing factor),
    and the influx of neutrophils occurs simultaneously with recrudescent
    histamine release in the late phase reaction. Eosinophils have
    received a lot of attention, as they are the hallmark of allergic
    inflammation. Eosinophils infiltrate areas more slowly than do
    neutrophils, but persist much longer. The eosinophil can cause
    epithelial denudation, mucus secretion and histamine release. Both
    eosinophil and neutrophil infiltrates are inhibited by
    corticosteroids.

         Interest has focused on the possible contribution by lymphocytes
    to the late-phase reaction. After mast cell activation, about 10% of
    the superficial lymphocytes express the IL-2 receptor, indicating
    their activation. There are suggestions that some cytokines are
    released during this time period, either from mast cells or
    lymphocytes.

    2.5.5  Atopic eczema

         In atopic eczema, the patient is much troubled by itching skin;
    there is a history of chronic or chronically relapsing dermatitis,
    worst on the flexures, which are excoriated and lichenified, and there
    is a family or personal history of atopy. This is the typical picture
    of atopic eczema, though some of the features may be absent (Hanifin &
    Rajka, 1980). In any discussion of pathogenesis, family history is
    important because atopic eczema is part of the atopic syndrome that
    includes genetically determined phenotypes such as extrinsic bronchial

    asthma, allergic rhinitis, allergic conjunctivitis and
    gastrointestinal allergy. Important laboratory indices are blood and
    tissue eosinophilia and antigen-specific IgE bound to mast cells in
    skin (intracutaneous challenge) or peripheral blood
    (radioallergosorbence assays). The Wiscott-Aldrich syndrome and
    hyper-IgE syndrome, which can closely resemble atopic eczema, are
    usually distinguishable by the associated life-threatening infections.

         The clinical course of atopic eczema is unpredictable. Sometimes
    it remits in childhood, but occasional patients have recurrences
    throughout life. Some patients (or their parents) are convinced that
    exacerbations are related to stress and/or exposure to environmental
    antigens such as food or animals. Secondary skin infection by
     Staphylococcus aureus, herpes simplex virus, varicella virus and,
    possibly, fungal infections can lead to severe exacerbations. Finally,
    autonomic nervous system disturbances and changes in fatty acid
    metabolism and phosphodiesterase activity have been implicated.

         Despite the development of numerous theories, the pathophysiology
    of eczema is still remarkably little understood. Researchers are
    currently focusing on Langerhans cells, which are thought to be
    involved in eczema, because these cells possess abundant receptors for
    IgE. Once in contact with allergen distributed after ingestion or
    following direct skin contact, Langerhans cells present the allergen
    to T-lymphocytes. They may also be directly stimulated to produce
    inflammatory cytokines, which are responsible for eczematous lesions.
    Atopic eczema is often accompanied by very high IgE levels. In babies,
    an elevated IgE level is taken as a reliable predictive sign for the
    development of asthma and/or hay fever in later life.

         The relation between cell-mediated immunity and IgE in atopic
    eczema was first established by Bruijnzeel-Koomen et al. (1986) who
    identified the presence of IgE on Langerhans cells in atopic eczema.
    It is now evident that this binding of IgE is the result of the
    presence of the high-affinity receptor for IgE on these Langerhans
    cells (Bieber & Ring, 1992). Langerhans cells and other
    antigen-presenting cells in skin also express low-affinity Fc
    receptors that efficiently bind allergen-precomplexed IgE. The
    functional consequence of the expression of these Fc receptors for IgE
    on antigen-presenting cells in skin is that the local response to
    minute quantities of allergens in the skin is amplified. By
    facilitated antigen-processing, only minute quantities of allergens
    are needed to be presented to T-cells, because the
    IgE-receptor-allergen complex aids processing and subsequent
    presentation up to a 1000-fold (Van der Heijden et al., 1993).
    Therefore, the onset of atopic eczema as an expression of atopic
    allergy may result from an interplay between the degrees of expression
    of one or more Fc receptor types, the serum concentration of
    allergen-specific IgE, and the number of skin-infiltrating T-cells
    specific for that allergen and, of course, exposure to the allergen.

         Atopic syndrome is genetically determined. When both parents have
    atopic disease of the same sort, their child has a risk of around 70%
    of developing a similar phenotype. If parents have different atopic
    diseases, the incidence of atopic disease in a child is 30% (Björksten
    & Kjellmann, 1987). With asthmatics as probands in molecular genetic
    studies, a gene predisposing to atopy has been found on chromosome I
    Iql3 (Cookson et al., 1989), possibly coding for the beta subunit of
    high-affinity IgE Type I Fc receptor (Sandford et al., 1993). However,
    the genetic mapping of atopy is far from simple. For example, the
    increasing prevalence of atopic eczema in the past three decades
    (Williams, 1992) is difficult to explain on the basis of genetics
    alone. Furthermore, a maternal pattern of inheritance has been found
    (Cookson et al., 1992), which might be due to paternal genomic
    imprinting or to maternal modification of developing immune responses
     in utero or via breast milk. Linkage of atopy with a gene on I Iql3
    could not be shown when patients with atopic eczema were taken as
    probands. Thus more than one gene seems to be involved.

         Environmental factors, such as exposure to allergens, are thought
    to be involved in the phenotypic expression of atopic eczema. For
    example, the presence of a strong atopic background has been
    associated with enhanced protective responses to helminthic infections
    (Lynch et al., 1998). However, a precise understanding of the
    environmental factors that determine whether or not the atopic
    genotype is expressed as an atopic phenotype is lacking.

    2.5.6  Urticaria

         Urticaria (hives, nettle rash) may be defined as an eruption of
    short-lived red oedematous swellings of the skin, associated with
    itching. The relative incidence of the different types of urticaria
    and angioedema in the general population is unknown.

         Urticaria usually involves degranulation of mast cells and
    release of histamine. Many different elicitors have to be considered.
    Allergy due to a reaction between a specific antigen and a mast
    cell-fixed IgE antibody is only one mechanism. Pseudo-allergic
    reactions, toxic effects and viral infections play a major role.

         Acute urticaria resolves within a period of six weeks. If it
    persists, it is called chronic urticaria. Wheals may be circular,
    polycyclic or figured. If subcutaneous extension occurs, angioedema is
    present. Although, like urticaria, angioedema may occur anywhere, the
    genitalia, eyelids, lips and mucous membranes are especially common
    sites. Itching is almost always present in patients with urticaria but
    is inconsistent in angioedema. The duration of urticarial wheals is
    usually 3 to 4 h, but angioedema lesions may last much longer.

         Skin previously involved by wheals or angioedema looks entirely
    normal apart from occasional purpura or other signs of trauma due to
    scratching. The mucous membranes are frequently involved including the
    tongue, soft palate and pharynx. Although discomfort and breathing

    difficulty may occur, fatalities are almost exclusively associated
    with hereditary angioedema. Acute urticaria may be associated with
    systemic anaphylactic symptoms (wheezing, dyspnoea, syncope, abdominal
    pain, vomiting). Occasionally acute urticaria may merge into serum
    sickness, arthritis, fever, proteinuria). Common causes of allergic
    acute urticaria include ingestion of penicillin, shellfish, soft fruit
    and nuts.

         Urticaria of immunological origin may arise rapidly (often less
    than 60 min) at the site of contact of the skin or mucous membranes
    with a specific substance.

         Contact urticaria may also be of non-immunological origin, and
    there are frequent instances in which the mechanism is uncertain. When
    an immune mechanism is involved, the final common pathway is probably
    the same. Contact urticaria of immunological origin involves
    IgE-mediated hypersensitivity as indicated by a positive
    radioallergosorbent test (RAST). In non-immunological examples, the
    offending substance may evoke histamine release directly from
    cutaneous mast cells. Such substances include ammonium persulfate
    (Mahzoon et al., 1977), dimethyl sulfoxide (Odom & Maibach, 1976) and
    cinnamaldehyde (Kirton, 1978); however, several other mechanisms are
    also involved.

         Immunological contact urticaria is more frequent in atopic
    subjects. These patients often give a history of acute oedema of the
    lips or buccal mucous membrane after ingestion of food items such as
    fish, egg or nuts. In common with other types of allergy, healthy
    control subjects are negative on skin testing. The offending allergen
    is usually a high relative molecular mass substance and skin testing
    is rarely positive in completely normal skin. Open and closed patch
    tests and closed patch tests on lightly abraded skin (scratch-patch
    tests) should be performed. The diagnosis is confirmed by a positive
    radioallergosorbent test (RAST).

         Non-immunological contact urticaria may be elicited in healthy
    asymptomatic individuals, with the triggering substance frequently
    being of low relative molecular mass, and contact reactions may be
    elicitable in clinically normal skin. The danger of such generalized
    reactions should be borne in mind before skin testing is performed.

    2.5.7  Gastrointestinal tract diseases: mechanisms of food-induced
           symptoms

    2.5.7.1  Non IgE-mediated food-sensitive enteropathy

         Slow onset gastrointestinal symptoms are described in children,
    especially in relation to ingestion of cow's milk. The clinical
    features are chronic diarrhoea and failure to thrive. The pathological
    lesion found in the small intestine is crypt hyperplastic villous
    atrophy of variable severity. The lesions are often patchy. There is
    an increased expression of the markers of T-cell activation on the
    T-cells of the lamina propria, and it is likely that a cell-mediated

    reaction in the lamina propria is the basis of the abnormality,
    although IgE involvement has also been described (Walker-Smith, 1992).
    Nagata et al. (1995) suggested that activated CD4 cells in the lamina
    propria of the small intestinal mucosa may contribute to the mucosal
    damage, probably by releasing cytokines.

    2.5.7.2  IgE-mediated food allergy

         Food allergic patients often describe itching and tingling of the
    mouth and throat as the first immediate symptoms of an allergic food
    reactions. In addition papules/blisters on the mucosa and swelling of
    the lips can be seen. These symptoms occur as a result of direct
    contact between the allergen and the mucosa of the mouth and throat.
    The concentration of mast cells is very high in the oropharyngeal
    mucosa and the symptoms are probably caused by degranulation of
    mucosal mast cells bearing specific IgE towards the offending allergen
    (Pastorello et al., 1995).

         Symptoms like nausea, vomiting, abdominal pains, loose stools and
    gas production are described in connection with immediate allergic
    reactions. In a direct challenge of the gastric mucosa using a
    gastrofibrescope, Romanski (1987, 1989) found gastric changes within
    5-20 min of contact with the introduced food. The macroscopic changes
    were: pale mucosa, oedema, punctate haemorrhage, hyperperistalsis,
    hypersecretion, erythema. Microscopic examination revealed oedema,
    hyperaemia, capillary haemorrhage, eosinophilic infiltration and
    inflammation.

         The underlying mechanisms of IgE-mediated gastrointestinal
    symptoms are a result of degranulation of intestinal mast cells with
    release of mediators that act directly on the epithelium, endothelium
    or muscle indirectly through nerves and mesenchymal cells. The result
    is altered gastric acid secretion, ion transport, mucus production,
    gut barrier function, and motility (Crowe & Perdue, 1992).

    2.5.7.3  Role of gastrointestinal tract physiology in food allergy

         Many elements of the gastrointestinal tract physiology influence
    the ultimate allergenicity of food proteins. These include the pH,
    digestive enzymes, bile, peristalsis, transit time, bacterial
    fermentation, and the intestinal barrier function, permeability, and
    absorption. Several food allergens or allergenic determinants were
    reported to be relatively resistant to acid denaturation and
    proteolytic digestion (Elsayed & Apold, 1977; Schwartz et al., 1980;
    Kurisaki et al., 1981; Metcalfe, 1985; Taylor, 1986; Taylor, 1992;
    Kortekangas-Savolainen et al., 1993). Unfortunately, insufficient
    information is available on possible differences in susceptibility to
    acid denaturation and gastrointestinal digestion between strongly
    allergenic food proteins and proteins that possess weak or virtually
    no allergenic potential. Attempts have also been made to correlate the
    susceptibility to enzymatic breakdown of cow's milk proteins, their
    intestinal permeability and allergenic properties (Taylor, 1986;

    Marcon-Genty et al., 1989; Savilahti & Kuitunen, 1992). The important
    role of digestion with respect to food protein allergenicity was
    clearly demonstrated in mice showing that pre-feeding of an
    endopeptidase inhibitor (aprotinin) to mice resulted in an inhibition
    of normally expected oral tolerance induction by protein feeding
    (Hanson et al., 1993). An abnormal digestive breakdown of proteins may
    also be of importance, since intragastric administration more easily
    results in anaphylactic sensitization as compared to  ad libitum 
    feedings, generally resulting in tolerance induction, as has been
    shown in rodents (Knippels et al., 1997). However, as digestion of
    food proteins is part of the normal sequence of events following
    consumption of food, it is likely that food allergic patients become
    sensitized to digested allergens rather than to the native proteins.
    Enzymatically digested food allergens may show the same, more, or less
    binding to specific IgE from patients (Haddad et al., 1979; Schwartz
    et al., 1980).

         The intestinal barrier function, permeability, and absorption are
    also hardly, or not, taken into account in the evaluation of the
    allergenicity of food proteins. Knowledge of the intestinal uptake of
    specific protein antigens and their fragments may provide some
    additional information in the evaluation of the potential
    allergenicity of protein products. There is evidence of limited
    macromolecular exclusion by the epithelial barrier (Seifert et al.,
    1974, 1977; Gardner, 1988; Teichberg, 1990).

    2.6  Type II hypersensitivity diseases

         Pathogenic Type II reactions may occur towards autoantigens,
    alloantigens (in blood transfusions), infective agents and drugs or
    chemicals, as described above. As shown in Table 13, these immune
    reactions may cause corresponding disorders, i.e., autoimmune
    diseases, transplantation/transfusion reactions or drug-induced
    haemolytic reactions. As an illustration of Type II reaction-induced
    diseases, three autoimmune disorders that are also inducible by drugs,
    i.e., haemolytic reactions, pemphigus and myasthenia gravis, will be
    dealt with in more detail.

    2.6.1  Drug-induced Type II reactivity

         Some drugs or their metabolites are chemically reactive agents
    that readily bind to cells and tissues. Such drugs, present on the
    cell membrane of blood cells, are obvious targets for pathogenic Type
    II reactivity.

         The most frequent allergic reaction occurs with penicillin and
    its relatives. Benzylpenicillin is a small molecule with a relative
    molecular mass of 372.47 and with a highly reactive beta-lactam ring,
    which may bind to amino groups on proteins (carrier), forming covalent
    conjugates. The thus formed penicilloyl hapten is considered as the
    major determinant in penicillin allergy. Although penicillin is able


        Table 13.  Clinical disease due to Type II hypersensitivity reactions
                                                                                                                             

                        Antigen                               Disease                        Symptoms
                                                                                                                             

    Autoantigens        glomerular basement membrane          Goodpasture's syndrome         vasculitis, renal failure

                        epidermal desmosomes (desmoglein-3)   pemphigus vulgaris             skin blistering (intra-epidermal)

                        epidermal hemidesmosomes on           bullous pemphigoid             skin blistering (subepidermal)
                        basal keratinocytes

                        acetylcholine receptor                myasthenia gravis              striated muscle weakness

                        Rhesus antigen                        autoimmune haemolytic          destruction of red cells, anaemia
                                                              anaemia

                        platelet integrin gpIIb:IIIa          autoimmune thrombocytopenia    abnormal bleeding
                                                              purpura

    Alloantigens        donor red cell antigens               delayed haemolytic             destruction of transfused red cells
                                                              transfusion reaction

    Infective agents    Streptococcal cell wall antigens      acute rheumatic fever          arthritis, myocarditis
                        cross-reacting with cardiac muscle

                        Klebsiella antigens cross-reacting    ankylosing spondylitis (?)     arthritis involving the spine
                        with HLA-B27

    Drugs, chemicals    penicillins, cephalosporins           drug-specific haemolytic       lysis of hapten-coated red cells
                        trimellitic anhydride                 anaemia
                                                                                                                             
    

    to induce all types of hypersensitivity reactions (IgE-, immune
    complex- or T-cell-mediated), haemolytic anaemia with
    penicillin-specific IgG antibodies reacting with penicillin-coated
    erythrocytes is a typical example of Type II reactivity.

         Interestingly, the specificity of drug-induced antibodies is
    often much broader than would be expected from the penicillin example.
    Ultimately, drugs trigger Type II reactivity without being involved in
    the final destructive reaction. In addition to hapten-specific
    antibodies, drugs can induce antibodies to metabolites, to
    drug-carrier combinations or to the carrier alone, resulting in
    clear-cut autoimmune reactivity. D-penicillamine is a classical
    example of a drug inducing autoimmunity, but chemicals such as mercury
    and gold are also able to induce autoimmunity.

         The mechanism by which drugs can induce autoantibodies is shown
    in detail in Fig. 6. By presenting the hapten in or on their MHC-class
    II molecules, autoreactive B-cells, which are normally present at very
    low frequencies without being activated, can trigger hapten-specific
    T-cells to help them (the B-cells) differentiate into
    antibody-producing plasma cells. Although the induction of the disease
    is drug-dependent, the Type II effector reaction towards autologous
    targets may be drug-independent. Hence, in this case the induced
    autoimmune disease would continue after the exposure to the drug had
    ceased.

    2.6.2  Transfusion reactions

         Transfusion reactions are examples of the cellular destruction
    that results from antibody combining with heteroantigens. There are at
    least 21 blood group systems, with more than 600 antigens within these
    systems. Some antigens are stronger than others and are more likely to
    stimulate antibody production. Certain antibodies are produced
    naturally with no prior exposure to red blood cells, while other
    antibodies are only produced after contact with cells carrying that
    antigen.

         The ABO blood groups are of primary importance in considering
    transfusions. Anti-A and anti-B antibodies are so-called naturally
    occurring antibodies. Individuals do not form such antibodies to their
    own red blood cells. Thus, an individual who has Type A blood would
    have anti-B in the serum, and a person with Type B blood has anti-A
    antibodies. An individual with Type O blood has both anti-A and anti-B
    in the serum, as O cells have neither of these two antigens.

         If a patient is given blood for which antibodies are already
    present, a transfusion reaction occurs. This can range from acute
    massive intravascular haemolysis to a small decrease in red blood cell
    survival. Acute haemolytic transfusion reactions may occur within
    minutes or hours after transfusion of incompatible blood.

         Delayed haemolytic reactions occur 4 to 10 days following a
    transfusion and are due to a secondary response to the antigen.
    Antibody-coated red blood cells are removed extravascularly, in the
    spleen or in the liver, and the patient may experience a mild fever
    and anaemia.

         Haemolytic disease of the newborn appears in infants whose
    mothers have been sensitized by exposure to fetal blood cells carrying
    antigens, commonly of the Rhesus family, that differ from their own.
    The mother makes IgG antibodies in response, and these cross the
    placenta to cause destruction of fetal red cells. A common antigen
    involved is the Rhesus D antigen

    2.6.3  Autoimmune haemolytic anaemia

         Drugs account for about 12% of the autoimmune haemolytic anaemia.
    They can cause haemolysis by three different mechanisms: by acting as
    a hapten, by inducing a classical autoimmune haemolytic anaemia, or by
    forming immune complexes with antibodies that can be adsorbed by the
    patient's red cells, the "innocent bystanders" (Fig. 8).

         Antigen-presenting cells phagocytose and process haptenized
    cells, such as erythrocytes. In addition, free drug molecules may bind
    to MHC-class II or to peptides within the groove. The hapten is thus
    presented by MHC-class II molecules to the T-cell receptor (TCR)
    of T helper cells. Hapten-specific T-cells now proliferate and
    differentiate, so that they can either attack haptenized cells (The
    cells causing Type IV reactivity, not shown) or can help nearby
    B-cells to produce antibodies (in particular The cells).

         B-cells ingest and process the antigens to which their
    immunoglobulin receptors bind and present peptides, including
    haptenized peptides, derived from these antigens in their MHC-class II
    molecules. Thus B-cells may trigger hapten-specific T-cells by
    presenting haptenized peptides. Alternatively, external drug molecules
    can bind to peptides in the MHC-class II groove. Differentiation of
    B-cells is dependent on adjacent T-cells providing membrane signals
    (to CD40, not shown) and the growth factors IL-4 and IL-6.

         In drug-specific haemolytic anaemia (on the left of Fig. 8),
    drug-specific B-cells present the drug to drug-specific T-cells.
    Mutual activation of T- and B-cells now induces the B-cell to become a
    plasma cell, producing drug-specific antibodies. Eventually these
    antibodies lead to destruction of haptenized erythrocytes, while
    normal cells remain intact.

         In drug-induced autoimmune haemolytic anaemia (on the right of
    Fig. 8), an autoreactive B-cell ingests and processes erythrocyte
    membranes, including the haptenized parts. Normally, autoreactive
    B cells exist but do not become activated by lack of appropriate
    stimulating T-cells. If drug-specific T-cells are present, however,

    FIGURE 8

    these B-cells, presenting haptenized peptides in at least some of
    their class II molecules, may become activated and differentiate into
    autoantibody-producing plasma cells. These antibodies may induce
    haemolysis of all erythrocytes in a drug-independent manner.

         The hapten type of autoimmune haemolytic anaemia is caused by the
    presence of drug-specific antibodies. These antibodies may be partly
    considered as autoimmune since the combination of drug and autologous
    carrier forms the actual target of the antibodies. When drugs, like
    penicillin, bind covalently to red blood cells, these drug-specific
    antibodies bind to the cells and induce their elimination by
    phagocytosis in the spleen. The induction of high titres of penicillin
    IgG antibodies typically occurs upon intramuscular administration,
    rather than upon intravenous penicillin therapy. On the other hand,
    relatively high intravenous doses of penicillin are required to make
    the erythrocytes susceptible to immune-mediated haemolysis. Thus, most
    patients with penicillin-induced haemolytic anaemia have received
    large doses of drug over a protracted period. After discontinuation of
    therapy the haemolysis quickly resolves and the antiglobulin test
    becomes negative within weeks.

         Some drugs appear to be able to induce true autoimmune haemolytic
    anaemia, with Rhesus antigens as the most common targets of the red
    cell antibodies. It is conceivable that the same auto-specificities
    are found in drug-induced and idiopathic autoimmune haemolytic
    anaemia, because the "normally" present (but silent) autoreactive
    B-cells become activated upon haptenization of the autoantigens, as
    shown in Fig. 8. As in drug-induced pemphigus and myasthenia, the drug
    itself does not seem to be involved in the destructive autoimmune
    reaction. While several drugs (Table 14) have been reported to provoke
    red cell autoantibodies, alpha-methyldopa is the best studied example.
    Only after prolonged therapy anti-red cell autoantibodies (IgG
    anti-Rh) are formed. Upon withdrawal of the drug the antibody titres
    usually decline and haemolysis ceases. Alpha-Methyldopa not only
    induces red cell antibodies, but also antinuclear factors, rheumatoid
    factors and gastric mucosa antibodies.

        Table 14.  Summary of drugs causing the different types of
    immune or autoimmune haemolytic anaemia (from: Foerster, 1993)
                                                                                    
    Type of drug-induced (A)IHA        Drugs
                                                                                    

    Drug- or hapten-specific IHA       penicillin, cephalosporins, tetracycline

    Autoimmune IHA                     alpha-methyldopa, levodopa, mefanamic acid,
                                       procainamide

    Immune complex mediated IHA        stibophen, p-aminosalicylic acid, chlorambucil,
    ("innocent bystander" type)        quinidine, quinine, phenacetin, sulfonamides,
                                       isoniazid, rifampicin, etc.
                                                                                    
    
         If soluble drug-specific antibodies are present, they may form
    immune complexes with administered drugs and fix complement. The
    complexes are then adsorbed by erythrocytes and thrombocytes resulting
    in lysis or clearance of these "innocent bystanders". Strictly
    speaking, this haemolysis is caused by Type III reactivity. The
    mechanism of adsorption, however, is not completely understood. It is
    clear that it does not simply involve Fc receptors, since the F(ab)2
    domain of the antibodies, in particular, adheres to the target cells.
    Low-affinity attachment of drugs to the cells seem to make them more
    susceptible for complex binding, and specific red cell antigens also
    seem to be involved. Perhaps the bystanders are less innocent than
    initially thought, and Type II reactivity combines here effectively
    with Type III reactivity. Many different drugs, usually of low
    relative molecular mass, are able to induce this type of haemolysis
    (Table 14) (Foerster, 1993).

    2.6.4  Autoimmune thrombocytopenic purpura

         Autoimmune or idiopathic thrombocytopenic purpura (ITP) is
    another example of a Type II reaction involving destruction of
    self-antigens. This disease is characterized by shortened platelet
    survival and the presence of antibody bound to platelets. It can be
    classified as acute, intermittent or chronic, depending on the
    severity and frequency of the symptoms. Acute ITP occurs mainly in
    children following an upper respiratory viral illness (Karpatkin,
    1988). The disease lasts an average of 1 to 2 months. Intermittent ITP
    may occur in a child or an adult. It is characterized by episodes
    where the platelet count drops, followed by periods where the count is
    normal. Chronic ITP is seen in adults, and it may last for years, or
    indefinitely (Karpatkin, 1988).

         ITP can be drug-induced by the following: quinidine, quinine,
    sulfonamides,  p-aminosalicylic acid, phenytoin and sedormid (no
    longer used). The drug acts as a hapten and adheres to the surface of
    the platelets. This type of ITP is reversed when the drug is
    withdrawn.

    2.6.5  Pemphigus and pemphigoid

         Type II reactions in the skin may cause different types of
    blistering diseases depending on the antigen (location) to which the
    autoantibodies are directed.

         Antibodies towards desmosomal antigens induce intra-epidermal
    blistering (acantholysis), leading to pemphigus, which is a
    potentially fatal disease. The presence of these intra-epidermal
    antibodies can be shown by direct immunofluorescence of perilesional
    skin and provides the main diagnostic parameter. Most patients also
    have circulating antibodies with titres reflecting disease activity.
    Since removal of these antibodies by plasmapheresis reduces disease

    activity and transfer of positive sera to mouse and monkeys can induce
    pemphigus-like lesions, it is believed that the anti-desmosomal
    antibodies are the causative agent of the clinical lesions in
    pemphigus.

         Investigations have unravelled the different desmosomal molecules
    serving as autoantigens in pemphigus. The most important antigens in
    pemphigus are the desmogleins; these are transmembrane glycoproteins,
    which are members of the cadherin gene superfamily. Desmogleins bear
    the same calcium-binding motifs as other cadherins do, and calcium
    appears to be essential for the formation of the conformational
    epitopes that are recognized by pemphigus sera (Amagi et al., 1995).
    Interestingly, the two clinical variants of the disease, pemphigus
    vulgaris and pemphigus foliaceus, develop antibodies to different
    desmogleins, i.e., desmoglein-3 and desmoglein-1, respectively. The
    differential expression of these two desmogleins in the upper and
    lower epidermis could explain the different levels of acantholysis
    seen in the two pemphigus variants (Shimizu et al., 1995).

         Drugs may play a precipitating role in pemphigus.
    Penicillamine-D, thiopronine, ampicillin, rifampicin, phenylbutazone,
    captopril, pyrazolon, enalapril and piroxicam can all induce
    pemphigus. It would appear that the presence of certain chemically
    reactive groups in the drugs, in addition to the pemphigus
    susceptibility genes in the patient (HLA-DRB1*0402 or DQB1*0503;
    Matzner et al., 1995; Wucherpfennig et al., 1995), predispose for
    drug-induced pemphigus. Sulfhydryl groups (-SH), present in
    D-penicillamine, and active amide groups (-CO-N-), typically present
    in enalapril, are held responsible for the acantholytic effects in
    human skin cultures. In the group of penicillin and cephalosporins,
    this active amide group is probably more important for the induction
    of pemphigus than the sulfhydryl group (Wolf & Brenner, 1994).

         The mechanism by which the drugs induce pemphigus is still not
    completely understood. It is clear, however, that in addition to the
    direct acantholytic effects, which can be observed in human skin
    cultures  in vitro, an autoimmune reaction is being induced. The
    resulting autoantibodies appear to have the same antigenic
    specificity, i.e., to desmoglein-1 and desmoglein-3, as in idiopathic
    pemphigus patients (Korman et al., 1991). Together these findings
    would be in line with involvement of the same mechanism of
    drug-induced autoimmunity as described for autoimmune haemolytic
    anaemia.

         Antibodies towards antigens present in the lamina lucida of the
    basement membrane cause a less severe type of blistering disease,
    called bullous pemphigoid. Direct immunofluorescence of perilesional
    skin reveals the presence of autoantibodies along the dermo-epidermal
    junction. Concordantly, sub-epidermal blisters are being formed.

         The antigens in bullous pemphigoid have been identified as
    transmembrane proteins of 180 000 and 230 000 relative molecular mass,
    present in the hemidesmosomes of the basal keratinocytes (Korman,
    1995). These hemidesmosomes are believed to play a role in the
    epidermal-dermal adhesion. Also in bullous pemphigoid autoantibodies
    with the same specificity can be detected in the circulation, but
    their titres do not correlate with disease activity.

         The precipitating role of drugs for bullous pemphigoid is not
    well established, although the disease may occasionally follow drug
    ingestion (e.g., after furosemide).

    2.6.6  Myasthenia gravis

         Myasthenia gravis is an autoimmune disease that is mediated by
    IgG antibodies directed to the acetylcholine receptors in the
    postsynaptic membrane of the muscle (Vincent et al., 1995). The number
    of receptors can be considerably reduced by complement-mediated lysis
    and accelerated internalization. Additionally, the residual receptors
    may be blocked by autoantibodies directed to the acetylcholine binding
    site, thus leading to further impairment of the transmission from
    nerve to muscle. As a consequence, the disease is characterized by
    weakness and fatigue of the striated muscles. In some patients only
    few muscles are affected; a well-known localized form of the disease
    is ocular myasthenia (Weinberg et al., 1994).

         In young patients with myasthenia gravis (40-50% of patients,
    usually female) the thymus is an important site of autoantibody
    production and T-cell activation. Within the hyperplastic thymus,
    formation of lymphoid follicles can be observed, with germinal centres
    surrounded by T-cells. The acetylcholine receptor antigens are here
    presented to the immune system by muscle-like myoid cells, which bear
    MHC-class II molecules. As therapeutic treatment, in addition to
    immunosuppression, thymectomy is beneficial in these patients, since a
    substantial source of both antigen and antibody-producing plasma cells
    is thus removed. On the other hand, in late onset (usually male)
    patients (15-20%), the thymus is rather atrophic and autoantibody
    production by thymic cells is relatively low. In another minority of
    patients (15-20%) thymoma may develop. In this last group of patients,
    autoantibodies to striated muscles are typically found in addition to
    the acetylcholine receptor autoantibodies.

         Like pemphigus, myasthenia gravis can be induced by a number of
    drugs. D-penicillamine, used for treatment of rheumatoid arthritis,
    has been most frequently reported as a trigger for myasthenia gravis.
    A few other drugs are suspected of inducing myasthenia gravis; among
    them are thiopronin and chloroquin.

         Drug-induced myasthenia is characterized by frequent involvement
    of facial and oropharyngeal muscles (Bonnet et al., 1995). The disease
    seldom generalizes or results in thymoma. Autoantibodies to

    acetylcholine receptors are measurable in the circulation in the
    majority of patients (approximately 80%), whereas almost half of them
    have blocking antibodies. Similar frequencies of these antibodies are
    found in idiopathic myasthenia gravis (Morel et al., 1991). The
    autoantibodies disappear upon discontinuation of the drug, and full
    recovery may be obtained within a few months.

    2.7  Type III hypersensitivity diseases

    2.7.1  Immune complex disease

         Immune complexes are formed every time antibody meets antigen.
    Generally they are removed effectively by the reticuloendothelial
    system but occasionally their formation can lead to a hypersensitivity
    reaction. Diseases resulting from immune-complex formation can be
    placed broadly into three groups.

    a)   The combined effects of a low-grade persistent infection
         (such as occurs with alpha-haemolytic  Streptococcus viridans or
         staphylococcal infective endocarditis, or with a parasite such as
          Plasmodium vivax, or in viral hepatitis), together with a weak
         antibody response, leads to chronic immune-complex formation with
         the eventual deposition of complexes in the tissues.

    b)   Immune complex disease is a frequent complication of
         auto-immune disease where the continued production of
         autoantibody to a self-antigen leads to prolonged immune-complex
         formation. The mononuclear phagocyte, erythrocyte and complement
         systems (which are responsible for the removal of complexes)
         become overloaded and the complexes are deposited in the tissues,
         such as occurs in systemic lupus erythematosus (SLE).

    c)   Immune complexes may be formed at multiple sites, such as in
         the lungs following repeated inhalation of antigenic materials
         from moulds, plants or animals. This is exemplified in Farmer's
         lung and Pigeon fancier's lung, where there are circulating
         antibodies to the actinomycete fungi found in mouldy hay or to
         pigeon antigens. Both diseases are forms of extrinsic allergic
         alveolitis, and they only occur after repeated exposure to the
         antigen. The antibodies induced by these antigens are primarily
         IgG, rather than IgE, as in immediate (Type I) hypersensitivity
         reactions. When antigen again enters the body by inhalation,
         local immune complexes are formed in the alveoli leading to
         inflammation. Precipitating antibodies to the inhaled
         actinomycete antigens are found in the sera of 90% of patients
         with Farmer's lung, but since they are also found in some people
         with no disease, and are absent from some sufferers, it seems
         that other factors are also involved, including Type IV
         hypersensitivity reactions.

         The sites of immune-complex deposition are partly determined by
    the localization of the antigen in the tissues and partly by how
    circulating complexes become deposited.

         Immune complexes trigger a variety of inflammatory processes.
    They can interact with the complement system leading to the generation
    of C3a and C5a (anaphylatoxins), which cause the release of vasoactive
    amines from mast cells and basophils, thus increasing vascular
    permeability. These anaphylatoxins are also chemotactic for
    polymorphs. Cytokines released from macrophages, particularly
    TNF-alpha and IL-1, are also important in localized immune-complex
    diseases, such as alveolitis, through a mechanism involving neutrophil
    recruitment. Platelets can also interact with immune complexes,
    through their Fc receptors, leading to aggregation and microthrombus
    formation and hence a further increase in vascular permeability due to
    the release of vasoactive amines. Platelets are a rich source of
    growth factors, and release of these may contribute to the cellular
    proliferation found in immune-complex diseases such as
    glomerulonephritis and rheumatoid arthritis.

         The attracted polymorphs attempt to ingest the complexes, but in
    the case of tissue-trapped complexes this is difficult and the
    phagocytes are therefore likely to release their lysosomal enzymes to
    the exterior, causing tissue damage. If simply released into the blood
    or tissue fluids, these lysosomal enzymes are unlikely to cause much
    inflammation, because they are rapidly neutralized by serum enzyme
    inhibitors. But if the phagocyte applies itself closely to the
    tissue-trapped complexes through Fc binding, then serum inhibitors are
    excluded and the enzymes may damage the underlying tissue. A classic
    example of this type of inflammatory response is the Arthus reaction
    (see section 2.1.3.1).

    2.7.2  Serum sickness

         Serum sickness is a Type III reaction that is seen in humans,
    although not as frequently as it used to be. Serum sickness results
    from passive immunization with animal anti-serum used to treat such
    infections as tetanus and gangrene, usually horse or bovine
    anti-serum. Approximately 50% of the individuals who receive a single
    injection develop the disease (Barrett, 1988). Generalized symptoms
    appear about 1 to 2 weeks after injection of the animal serum and
    include headache, nausea, vomiting, joint pain and lymphadenopathy.
    Recovery takes between 7 and 30 days (Terr, 1994b).

         In this disease, the sensitizing and the shock-producing dose of
    antigen are one and the same, as antibodies develop while antigen
    still present. High levels of antibody form immune complexes that
    deposit in the tissues. Usually this is a benign and self-limiting
    disease, but previous exposure to animal serum can cause
    cardiovascular collapse upon re-exposure (Terr, 1994b). Antibiotic use
    has diminished the need for this type of therapy.

    2.7.3  Allergic bronchopulmonary aspergillosis

         Allergic bronchopulmonary aspergillosis (ABPA) is a syndrome
    characterized by respiratory and constitutional symptoms caused by
    hypersensitivity reactions to fungal antigens of  Aspergillus 
     fumigatus. Allergic bronchopulmonary aspergillosis is characterized
    by episodic wheezing, pulmonary infiltrates, eosinophilia in sputum
    and blood, markedly elevated serum IgE levels, positive immediate and
    late skin tests to  A. fumigatus, serum precipitating antibody to
     Aspergillus, and sputum containing brown plugs or flakes. Not all of
    these changes may be present during active disease and a diagnosis of
    allergic bronchopulmonary aspergillosis is usually considered when
    asthma is complicated by radiographic or clinical evidence of
    recurrent pneumonic infiltrates, bronchiectasis or pulmonary fibrosis.

         A variety of disease-related immunological alterations have been
    reported in allergic bronchopulmonary aspergillosis. Antigen extracts
    of  A. fumigatus, A. niger and  A. clavatus have been shown to
    activate the alternative complement pathway in fresh human serum from
    healthy humans. Total serum IgE is elevated in most, but not all,
    instances of allergic bronchopulmonary aspergillosis and
    Aspergillus-specific IgE is substantially increased as measured by
    radioimmunoassay.

         The immune pathogenesis of allergic bronchopulmonary
    aspergillosis is thought to involve direct activation of complement by
    Aspergillus antigen, IgE-antibody production with subsequent release
    of vasoactive amines from mast cells, as well as IgG-antibody
    production and deposition of antigen-antibody complexes in the
    broncho-alveolar tree. Local deposition of immune complexes may
    activate the complement pathway and generate chemotactic factors for
    polymorphonuclear leucocytes in peripheral blood and produce a
    resultant immune complex-initiated Arthus-type reaction in lung
    tissues. Also "late phase" eosinophil-mediated IgE-dependent reactions
    have been suggested to be involved in the pathogenesis of the disease.

    2.7.4  Extrinsic allergic alveolitis

         Extrinsic allergic alveolitis (EAA) is usually defined in
    pathological terms as a granulomatous inflammatory reaction which
    predominantly involves the gas-exchanging parts of the lung and which
    is the outcome of a specific immunological response to an inhaled
    substance. The vast majority of reported cases have been caused by
    inhaled organic dusts, but a few cases have been attributed to inhaled
    isocyanates, particularly diphenyl methane diisocyanate (MDI) but also
    hexamethylene diisocyanate (HDI) and toluene diisocyanate (TDI). No
    reported case has been validated by biopsy evidence of the
    characteristic pathological appearances; cases have been identified on
    the basis of:

    a)        Characteristic clinical history;

    b)        Changes on chest radiograph;

    c)        Pattern of functional change following controlled isocyanate
              inhalation;

    d)        Proportions of cells received at bronchoalveolar lavage.

         Typically, patients present with a history of recurrent episodes
    of breathlessness associated with systemic symptoms of fever, malaise
    and chills. A few (but only a minority of reported cases) have had
    abnormal chest radiographs.

         In the majority of cases the diagnosis has been made by the
    response to inhalation testing or the pattern of cells recovered at
    bronchoalveolar lavage. Inhalation testing provoked the changes of an
    "alveolar reaction" with proportionate reduction in forced expiratory
    volume in 1 second (FEV1), forced vital capacity (FVC) and in
    transfer factor (TLCO) accompanied by a neutrophil leucocytosis and
    fever. The cells recovered at bronchoalveolar lavage have, as is
    characteristic of EAA, shown an increase in the proportion of
    lymphocytes, on occasion by more than 50%.

         In some cases IgG antibody to a human serum albumin conjugate of
    the relevant isocyanate - MDI-HSA, TDI-HSA and HDI-HSA - has been
    identified in serum. The outcome of EAA caused by isocyanates has been
    little reported, but most cases, even if showing significant
    functional impairment at the time of diagnosis, would seem to have no
    permanent residual disability after avoidance of isocyanate exposure.

    2.7.4.1  Farmer's lung

         Farmer's lung affects workers who handle mouldy hay or grain. It
    originates from poor conditions of storage, involving high dust levels
    and humidity. Microorganisms responsible for Farmer's lung are moulds,
    above all  Micropolyspora faeni, and  Thermoactinomyces vulgaris.
    Specific precipitins are found in the blood, especially antibodies of
    the IgG class. This disease is classified as a Type III
    hypersensitivity. Better storage and work practices reduce the
    incidence.

         Sensitization takes some time to occur. Clinically, patients
    suffer respiratory distress accompanied by fever appearing 8-10 h
    after handling mouldy hay, straw or grain and presenting with fever,
    shivering, chest pains, lassitude, sweating, headaches and coughing,
    sometimes accompanied by haemoptysis. Fine auscultatory chest
    crepitations may be present. In typical forms, the chest X-ray shows
    miliary infiltrates and micronodules. Later, pulmonary fibrosis
    appears progressively when the disease reaches a chronic stage. There

    is also impairment in alveolar gas diffusion (so called restrictive
    syndrome) and, in the most advanced cases, an alveolar-capillary
    block, which leads to chronic pulmonary heart failure.

    2.7.4.2  Bird-fancier's lung

         Bird fancier's lung is another disease of the same type, found
    especially among pigeon breeders, but also in those handling other
    birds. The disease is due to the development of precipitating
    antibodies against serum proteins of relevant avian species, e.g.,
    pigeons, parrots, chickens, pheasants and turkeys.

    2.8  Type IV hypersensitivity diseases

         Although cell-mediated immunity has fully developed in
    vertebrates for their benefit by facilitating effective eradication of
    microorganisms and abnormal cells, T-cell mediated reactions can,
    under certain conditions, also cause disease (Table 15). Although
    allergic contact dermatitis probably represents the most common T-cell
    mediated disease, a few other pathological conditions are briefly
    reviewed here.

        Table 15. Pathology caused by Type IV hypersensitivity
                                                                                         

    Type IV induced disease                 Antigens, chemicals
                                                                                         

    Allergic contact dermatitis             low relative molecular mass chemicals, drugs

    Protein contact dermatitis              proteins

    Granulomatous disease                   mycobacterial antigens, beryllium

    Autoimmune disease, e.g.,               autoantigens, e.g., pancreatic islet antigens
    diabetes mellitus Type I

    Hypersensitivity pneumonitis            toluene diisocyanate, beryllium, heavy metals

                                                                                         
    
         Protein contact dermatitis is another example of Type IV
    hypersensitivity.

         One of the more serious complications of Type IV hypersensitivity
    is the formation of granulomata. In general, T-cell immunity to
    infectious agents confers a long-lasting state of protective immunity.
    Macrophages, activated by the T-cell cytokines, can attack the
    pathogen and should be considered as important effector cells here. If
    the microorganisms are not readily killed and degraded, however,
    macrophages may become "frustrated". They fuse to form multi-nucleated

    giant cells or develop into large macrophages ("epitheloid" cells).
    Together these cells can form new structures, so-called granulomata,
    in which the macrophages with the foreign material are being isolated
    from the environment by a layer of surrounding T-cells producing
    cytokines and fibroblasts. The expansion and outgrowth of new
    granulomata, especially at vulnerable sites, may cause considerable
    tissue damage and loss of function.

         In general, granuloma formation occurs when Type IV reactivity is
    directed towards persistent indigestible antigens. A number of
    organisms can induce granulomatous disease:  Mycobacterium 
     tuberculosis and  M. leprae,  Treponema pallidum,  Schistosoma,
    and  Yersinia enterocolitica. Importantly, a number of exogenous,
    non-infectious agents can also evoke granulomatous reactions.

         T-cell-mediated immune reactions may also cause disease when the
    T-cell response is directed to autologous tissue. The crucial role of
    T-cells, for instance, in the breakdown of the insulin-producing
    beta-cells of the pancreatic Islets, leading to insulin-dependent
    diabetes mellitus, is well established.

         Other autoimmune diseases can sometimes be precipitated by Type
    IV reactions evoked by completely unrelated antigens. Psoriasis may be
    an example of an autoimmune disease that could be triggered by contact
    allergens. How exactly these chemicals trigger the disease is not
    completely clear. It is, however, most likely that any damage to the
    skin, either toxic, physical or immunological, that recruits
    sufficient lymphocytes from the circulation to include some
    auto-reactive, keratin-specific T-cells will trigger a local response
    in susceptible patients, resulting in a psoriatic lesion.

         T-cell-mediated immunity plays a crucial role in the pathogenesis
    of some lung diseases. Environmental organic chemicals like toluene
    diisocyanate and trimellitic anhydride, but also inorganic compounds
    as chromium and nickel are known sometimes to cause pulmonary disease.
    The extent to which Type IV-mediated immune responses are involved in
    these disorders is discussed in section 2.1.4.2.

         Allergic contact dermatitis is considered to be the most frequent
    pathological manifestation of Type IV reactivity. In allergic contact
    dermatitis, T-cells are sensitized to proteins, environmental agents
    and chemicals, entering the body via the skin. Repeated exposure to
    such chemicals results in persistent eczematous inflammatory reactions
    at the site of allergen contact. Although allergic contact dermatitis
    can be regarded as a prototype of delayed-type hypersensitivity, the
    sensitization process for chemical contact allergens, which already
    starts in the most superficial layers of the skin, is very special.
    The mechanism by which chemicals induce and elicit hypersensitivity
    reactions in the skin will, therefore, be described in more detail.

    2.8.1  Chronic beryllium disease

         Chronic beryllium disease is a systemic disorder with primary
    manifestations in the lungs. The pathogenic beryllium compounds
    include metallic beryllium, beryllium alloys and beryllium oxide fume
    (IARC, 1993). Inhalation of low levels of beryllium dusts or salts
    over months to years is associated with a chronic interstitial
    pulmonary granulomatous disorder clinically similar to sarcoidosis
    (Freiman & Hardy, 1970; Jones Williams, 1988; Williams, 1989). The
    skin manifestations of beryllium disease consist of contact dermatitis
    and subcutaneous granuloma formation with occasional ulceration.

         The concept that the granulomas of chronic beryllium disease are
    T-cell-mediated immune granulomas is supported by the observations
    that:

    a)   beryllium (i.e., the antigen) persists in the lung for long
         periods (Jones Williams & Wallach, 1989);

    b)   large numbers of T-cells and non-caseating granulomas are
         present in the lung (Williams, 1989);

    c)   in response to beryllium salts, lung and blood T-cells
         proliferate and release lymphokines  in vitro, a parameter also
         used diagnostically to distinguish beryllium disease from
         sarcoidosis (Williams & Williams, 1982; Rossman et al., 1988;
         Newman & Kreiss, 1992; Newman et al., 1994);

    d)   intradermal administration of beryllium salts induces a
         local granulomatous response in these individuals.

         In chronic beryllium disease, the lung T-cell population is
    predominantly of the CD4+ phenotype (Rossman et al., 1988; Saltini et
    al., 1989). These CD4+ T-cells, compared to blood T-cells from the
    same individual or compared to T-cells from normal individuals,
    exhibit increased proliferation in response to beryllium (Rossman et
    al., 1988; Saltini et al., 1989). The T-cells are activated,
    expressing HLA class II molecules and IL-2R and releasing IL-2
    (Pinkston et al., 1984; Saltini et al., 1989). Furthermore, the
    beryllium-induced lung T-cell proliferation is Class II-restricted.
    Chronic beryllium disease is strongly associated with HLA-DPB1 *0201,
    and all beryllium-specific (BeSO2) T-cell clones have been shown to
    be restricted by this allele.

         Analysis of T-cell lines and T-cell clones of individuals with
    this disease has confirmed that the beryllium-induced response is
    antigen- specific and that all the responder cells are CD4+ T-cells
    (Saltini et al., 1989).

         Thus, from the information available, it appears that chronic
    beryllium disease is a classic example of an immune granuloma host
    response. Why an element like beryllium should do this is not clear,

    but two not mutually exclusive hypotheses could explain it. Firstly,
    it is likely that most disease is caused by dusts of beryllium metal
    or salts, so that the particulate forms a nidus around which
    macrophages ingest, allowing the beryllium to be slowly released.
    Secondly, soluble beryllium salts interact with proteins, such that
    the beryllium becomes an immunogenic hapten in the context of the
    protein.

         In an epidemiological study of groups exposed to the combustion
    products of coal containing a high concentration of beryllium, Bencko
    et al. (1980) found elevated levels of IgG and IgA and increased
    concentrations of autoantibodies (anti-nuclear and anti-mitochondrial
    antibodies).

    2.8.2  Systemic autoimmune diseases

         Several organ-specific autoimmune diseases such as pemphigus and
    pemphigoid (section 2.6.5) and myasthenia gravis (section 2.6.6) have
    been discussed above. Many of the major rheumatological disorders are
    autoimmune in nature. Although systemic lupus erythematosus (SLE) can
    be ranked under Type III immune complex disorders, for other
    autoimmune diseases this categorization is less clear-cut.

    2.8.2.1  Systemic lupus erythematosus

         Systemic lupus erythematosus (SLE) is a chronic systemic
    inflammatory disease that follows a course of alternating
    exacerbations and remissions. Multiple organ system involvement
    characteristically occurs during periods of disease activity (Fye &
    Sack, 1991) (see also section 4.6.2). The disease predominantly
    affects women (female to male ratio of 9:1) of childbearing age;
    however, the age at onset ranges from 2 to 90 years. It is more
    prevalent among non-whites than Caucasians. Family studies have
    demonstrated a genetic susceptibility to the development of SLE.
    Autoantibody formation in SLE is partially genetically determined:
    patients with HLA-DR2 are more likely to produce anti-dsDNA
    antibodies, those with HLA-DR3 produce anti-SS-A and anti-SS-B
    antibodies, and those with HLA-DR4 and HLA-DR5 produce anti-Sm and
    anti-RNP antibodies. Reduced serum complement and the presence of
    autoantibodies to double-stranded (ds) DNA are hallmarks of active
    SLE, distinguishing this entity from other lupus variants. Antibodies
    to single-stranded DNA and particularly against histone proteins are
    characteristic of some drug-induced forms of SLE, such as
    procainamide-induced lupus (Rubin, 1989; Rubin et al., 1995).

         Although in most cases the etiology of SLE is unknown, a wide
    variety of medicinal and environmental agents have been associated
    with the elicitation of SLE at low incidence in susceptible
    individuals (Kammüller et al., 1989a; Adams & Hess, 1991; Uetrecht,
    1992).

    2.8.2.2  Rheumatoid arthritis

         Rheumatoid arthritis is a chronic, recurrent, systemic
    inflammatory disease primarily involving the joints (Fye & Sack,
    1991). It affects 1-3% of people in the USA, with a female to male
    ratio of 3:1. Constitutional symptoms include malaise, fever and
    weight loss. The disease characteristically begins in the small joints
    of the hands and feet and progresses in a centripetal and symmetrical
    fashion. Elderly patients may present with more proximal large-joint
    involvement and deformities are common. Extra-articular manifestations
    such as vasculitis, atrophy of skin and muscle, lymphadenopathy,
    splenomegaly and leucopenia, are characteristic of rheumatoid
    arthritis and often cause significant morbidity.

         The cause of the unusual immune responses and subsequent
    inflammation in rheumatoid arthritis is unknown. HLA-D4 and HLA-DR4
    occur in approximately 70% of patients with rheumatoid arthritis. Some
    patients who are negative for HLA-D4 and HLA-DR4 carry the HLA-DR1
    gene. It is possible that these and perhaps other genetic determinants
    impart susceptibility to an unidentified environmental factor, such as
    a virus, that initiates the disease process. The most important
    serological finding is the elevated rheumatoid factor titre, present
    in over 75% of patients (Fye & Sack, 1991).

    2.8.2.3  Scleroderma

         Scleroderma or progressive systemic sclerosis is a disease of
    unknown cause characterized by abnormally increased collagen
    deposition in the skin (Fye & Sack, 1991) (see also section 4.6.3).
    The course is usually slowly progressive and chronically disabling,
    but it can be rapidly progressive and fatal because of involvement of
    internal organs. It usually begins in the third or fourth decade of
    life. Children are occasionally affected. The prevalence of the
    disease is 4-12.5 cases per million population. Women are affected
    twice as often as men, and there is no racial predisposition.

         Scleroderma is a manifestation of various diseases, many of them
    autoimmune (Alarcon-Segovia, 1985). Two primary forms of scleroderma
    exist: localized or systemic. The systemic form, progressive systemic
    sclerosis (PSS), has in turn two variants: the diffuse and the CREST
    syndrome (acronym for Calcinosis, Raynaud's phenomenon, Esophageal
    involvement, Sclerodactyly and Telangectasia). Autoantibodies to DNA
    topoisomerase I (scl 70) and centromere may be useful serological
    markers for these respective diseases. The occurrence of progressive
    systemic sclerosis with features previously considered characteristic
    of SLE, rheumatoid arthritis, dermatomyositis and Sjögren's syndrome
    and associated with high titres of antibodies to nuclear
    ribonucleoprotein has been termed mixed connective tissue disease
    (MCTD) (Alarcon-Segovia, 1985). In contrast to other autoimmune
    diseases, cellular infiltration in scleroderma is minimal or absent in

    all organs except the synovium, where impressive collections of
    lymphocytes and plasma cells can be seen. Unfortunately, research on
    the pathogenesis of scleroderma is severely hampered by the absence of
    an animal model.

         It has been suggested that certain chemicals may be associated
    with some forms of scleroderma, e.g., tri- and perchloroethylene
    (Sparrow, 1977; Saihan et al., 1978; Flindt-Hansen & Isager, 1987),
    vinyl chloride (Lange et al., 1974; Ward et al., 1976; Black et al.,
    1983), silicone (Rose & Potter, 1995) and epoxy resins (Yamakage et
    al., 1980).

    2.8.2.4  Sjögren's syndrome

         Sjögren's syndrome is a chronic inflammatory disease of unknown
    cause characterized by diminished lacrimal and salivary gland
    secretion resulting in keratoconjunctivitis sicca and xerostomia (Fye
    & Sack, 1991). There is a dryness of the eyes, mouth, nose, trachea,
    bronchi, vagina and skin. In one-third of the patients, the disease
    occurs as a primary pathological entity (primary Sjögren's syndrome).
    In the remaining patients, it occurs in association with rheumatoid
    arthritis or other connective tissue disorders such as SLE. Ninety
    percent of patients with Sjögren's syndrome are female. Although the
    mean age at onset is 50 years, the disease also occurs in children.

         Patients with Sjögren's syndrome have an abnormal immunological
    response to one or more unidentified antigens characterized by
    excessive B-cell and plasma cell activity, manifested by polyclonal
    hypergammaglobulinaemia and the production of rheumatoid factor,
    antinuclear factors, including antibodies to SS(A) and SS(B),
    cryoglobulins, and anti-salivary duct antibodies. Both B- and
    Th-lymphocytes and plasma cells infiltrate involved tissues. No single
    immunological test is diagnostic for Sjögren's syndrome, although a
    spectrum of nonspecific immunological abnormalities occurs in these
    patients. Histological demonstration of lymphocytic infiltration in a
    biopsy specimen taken from the minor labial salivary gland is the most
    specific and sensitive diagnostic test for Sjögren's syndrome (Fye &
    Sack, 1991).

    2.8.2.5  Hashimoto's disease

         Hashimoto's disease, autoimmune thyroiditis, is the classical
    example from which much of the knowledge of autoimmune disorders has
    come (Gell et al., 1975; Roitt et al., 1998).

         Antibodies are formed to several antigens in follicular cells of
    the thyroid, including specific domains of thyroglobulin, thyroid
    peroxidase and certain surface receptors. Delayed-type cellular
    hypersensitization also occurs. The consequence is often initial
    stimulation of the thyroid, followed after a variable period by
    progressive destruction of the follicular cells, infiltration by

    lymphocytes and plasma cells, often containing germinal centres, and
    eventual fibrosis. The clinical disease, which is much more common in
    women than in men, may be marked by initial thyrotoxicosis, which is
    invariably followed by progressive hypothyroidism and myxoedema.

         Thyroid autoantibodies and variable lymphocytic infiltration are
    common in many other autoimmune diseases, so other tissues and organs
    may also be affected and antibodies against these are frequently
    found.

         The cause of Hashimoto's disease is rarely known but it may
    sometimes follow an overt viral infection of the thyroid and it has
    been associated with high exposure to iodine.

         Thyroid autoantibodies of several types are found in many
    apparently healthy individuals and are common in patients suffering
    from other autoimmune diseases.
    

    3.  FACTORS INFLUENCING ALLERGENICITY

    3.1  Introduction

         Allergens can be defined as antigens that give rise to allergy
    (Sherrill et al., 1994). The molecular properties that distinguish an
    allergen from an antigen are not known, but certain features appear to
    be associated with allergens. Induction of allergic responses is
    highly dependent upon a number of exogenous, as well as endogenous,
    factors.

    3.2  Inherent allergenicity

         Most allergens are proteins. The structurally known allergens
    from pollen, mammals, insects and foods are all proteins (or
    glycoproteins) with a relative molecular mass of 10 000-40 000 (King
    et al., 1995). Regarding IgE-mediated allergy, it is known that the
    IgE antibodies are not formed to an entire allergen, but rather to
    certain epitopes on the molecule. IgE binding sites are referred to as
    B-cell epitopes. For a protein to be allergenic, it must be
    multivalent, expressing more than one B-cell epitope. This allows
    antigen to bind to more than one IgE molecule on the surface of a mast
    cell or basophil, and induce these cells to generate and release
    mediators that initiate the allergic reaction.

         B-cell epitopes usually involve 12-15 linear amino acids,
    although these epitopes may be non-contiguous. In the latter
    situations, tertiary folding of the molecule provides the epitopes
    (i.e., conformational epitopes). Allergens must also exhibit T-cell
    epitopes, the 6-8 amino acid fragments presented to T-cells by
    antigen-presenting cells such as macrophages. This interaction is
    necessary to initiate the process of antigen-specific IgE synthesis.

         Factors such as "foreignness", size and charge influence
    allergenicity and sensitization. Allergenic proteins do not possess
    physicochemical properties that distinguish them from non-allergenic
    proteins. Foreignness refers to the concept of "non-self". In general,
    the more foreign the substance, the greater is its immunogenicity. The
    relationship of foreignness to allergenicity is not known. The larger
    the antigen, the more likely it is to contain epitopes.

         Compounds with a relative molecular mass smaller than 1000
    typically are not immunogenic; those with relative molecular mass
    between 1000 and 6000 may or may not be immunogenic, whereas those
    with a relative molecular mass greater than 6000 are generally
    immunogenic (Benjamini & Leskowitz, 1991). Allergens with small
    relative molecular mass are termed "haptens". Such chemicals are
    believed to couple to macromolecules to become immunogenic. In
    general, the nature or identity of macromolecular "carriers" is not
    known.

         Certain inorganic chemicals are particularly potent sensitizers
    on exposure of the skin, e.g., nickel- and platinum-containing
    compounds, and, in some instances, of the respiratory tract (Rycroft
    et al., 1995; Vos et al., 1996). Cross-reactivity has been observed
    between allergic sensitization to nickel and chromium salts, and
    between platinum, palladium and related elements.

         Physicochemical complexity of a compound also favours
    immunogenicity, whereas homopolymers of amino acids, such as
    polylysine, are usually poor immunogens. When the complexity is
    increased, i.e., by attachment of moieties that of themselves are not
    immunogenic, the entire molecule becomes immunogenic (Landsteiner &
    Rostenberg, 1939). For example, attachment of dinitrophenol to
    polylysine renders the structure immunogenic, (Benjamini & Leskowitz,
    1991).

         Certain physical and chemical characteristics appear to be
    associated with allergens. Protein allergens tend to possess
    biological activity. Haptens tend to have chemical reactivity (or are
    metabolized into reactive compounds); contact allergens are often
    lipophilic. Such factors might have functional importance by
    facilitating access of the allergen to the immune system, and by
    interfering with regulatory mechanisms of the immune response. For
    example, many protein allergens have been shown to possess enzymatic
    activity (Stewart, 1994). The house dust mite allergen  Der p I is a
    serine protease (Chua et al., 1988). There is evidence that the
    proteolytic activity enhances penetration of the allergen through the
    mucosa (Herbert et al., 1995) and stimulates the synthesis and release
    of the Th2-associated allergy-promoting cytokine IL-4 from mast cells
    and basophils (Machado et al., 1996). Furthermore, it has been shown
    that  Der p I selectively cleaves the lymphocyte surface membrane
    molecules CD23 (Hewitt et al., 1995; Schulz et al., 1997) and
    CD25-alpha subunit (Schulz et al., 1998) and releases them into the
    fluids surrounding the cells. Whereas the low-affinity IgE receptor
    CD23 on the cell surface mediates negative feedback on IgE synthesis,
    released soluble CD23 promotes IgE synthesis. Thus, the enzymatic
    cleavage of CD23 by  Der p I will enhance the synthesis of IgE, a key
    mediator molecule in allergy. Furthermore, cleavage of the IL-2
    receptor CD25-alpha subunit will strongly inhibit the proliferative
    response and production of IFN-gamma in Th1-cells. Consequently, the
    immune response to  Der p I, and possibly other protein antigens
    simultaneously presented to the immune system, will be biased towards
    Th2-cells and an allergic response.

         Many of the respiratory chemical allergens possess distinctive
    functionalities that are thought to endow the chemical with
    allergenicity. Studies have been undertaken of structural features and
    physicochemical properties associated with respiratory allergens, and
    structure-activity relationship (SAR) models have been developed
    (Graham et al., 1997). Such factors include transport parameters,
    electron density and chemical reactivities. These models, as well as
    SAR models of allergic contact dermatitis, are discussed in chapter 6.

         The ability of the immune system to recognize and distinguish
    specific spatial regions (epitopes) on molecules has resulted in the
    development of reagents and methodology to map these epitopes on
    molecules such as drugs, proteins and microorganisms (Saint-Remy,
    1997). The immune system can distinguish between structures that are
    almost identical, i.e., that differ from one another by a single amino
    acid substitution, or by a conformational change. Epitope mapping is
    performed by generating panels of antibodies of known specificity.
    Examples of the use of such antibodies are: a) in physiology to
    identify structures that allow molecules to interact with their
    receptor, b) in pathology to identify particular T- or B-cell epitopes
    on antigens, c) in design of vaccines to either increase efficacy or
    stimulate certain types of responses, such as T-cell responses, d) in
    microbiology to aid in typing microorganisms.

    3.2.1  Inherent properties of chemicals inducing autoimmunity

         A variety of medicinal drugs with a relative molecular mass of
    less than 1000 can elicit systemic hypersensitivity reactions and
    autoimmune disorders in susceptible individuals at low incidence
    (Adams & Hess, 1991). Chemical agents, drugs in particular, with a
    documented potential to induce autoimmune disorders such as SLE,
    belong to different chemical classes. These include, among others,
    derivatives of aromatic amines, hydrazines, hydantoins, thioureylenes,
    oxazolidinediones, succinimides, dibenzazepines, phenothiaines,
    sulfoamides, pyrazolines, amino acids (Kammüller et al., 1989a; Adams
    & Hess 1991; Uetrecht, 1992), amines (Nilsson & Kristofferson, 1989),
    halothane (Gut et al., 1995), mercuric chloride (Pelletier et al.,
    1994), gold preparations (Sinigaglia, 1994), occupational or
    environmental chemicals such as tri- and perchloroethylene (Sparrow,
    1977; Saihan et al., 1978) and vinyl chloride (Ward et al., 1976;
    Black et al., 1983) (see also section 4.4). Environmental nitrophenols
    have been suggested to be able to elicit or perpetuate human
    autoimmune disorders (Lauer, 1990). Many of these compounds are
    heterocyclic and contain at least one aromatic group, suggesting that
    particular chemical entities may favour induction of immune
    dysregulation.

         From a pharmacological point of view, the majority of autoimmune
    disease-inducing drugs are beta-adrenergic-receptor-blocking 
    compounds, drugs acting on the central nervous system (CNS), 
    anti-thyroid agents and anti-infective agents. In view of the tight 
    functional connectivity between immune, nervous and endocrine systems, 
    which is at least partially effected by shared receptors and mediators 
    among the systems, it is possible that CNS drugs modulate immune 
    responses by acting at these receptors or inducing common mediators.

         Lupus-inducing compounds have the capacity to be oxidized by the
    extracellular myeloperoxidase-H2O2 system of activated neutrophils,
    despite their chemical and pharmacological heterogeneity (Uetrecht,
    1992; Jiang et al., 1994). Despite this substrate promiscuity of
    myeloperoxidase, analogues of lupus-inducing drugs with blocked or
    missing functional groups such as -NH2, -NHNH2-, -SH, -Cl or OHC3
    are not metabolized by myeloperoxidase (Jiang et al., 1994).

         In order to become antigenic to T-cells, haptens must bind
    carrier proteins, and whether or not T-cells may require covalent
    modification of MHC molecules for hapten recognition is a matter of
    debate. Investigation of mechanisms of allergic and autoimmune
    reactions has pointed to a major role of trinitrophenol- and
    gold-hapten-modified MHC-associated peptides as T-cell-antigenic
    structures (Martin & Weltzien, 1994; Sinigaglia, 1994; Weltzien et
    al., 1996).

    3.3  Exogenous factors affecting sensitization

    3.3.1  Exposure

    3.3.1.1  Magnitude of exposure

         The development of sensitization and the responses in individuals
    depend upon the frequency and intensity of acute symptomatic episodes
    (Friedmann et al., 1983; Ollier & Davies 1994). Clinical and
    experimental evidence indicates that exposure concentration is of
    critical importance for the development and exacerbation of allergy.
    For dermal and respiratory sensitization, in animal and human studies,
    the dose-response concept has been shown to operate at both the
    induction and elicitation phases of sensitivity.

         The role of dose in induction of contact sensitization has been
    demonstrated in animal models, including guinea-pigs and mice (Chan et
    al., 1983; Stadler & Karol, 1985). Data revealed a relationship
    between the amount of chemical applied epicutaneously to the animals
    and both the severity of the ensuing reaction and the percentage of
    animals responding. In both species, and with all chemicals tested, a
    no-effect concentration was also observed.

         Both the induction and elicitation phases of respiratory
    sensitization have been shown to be under the influence of the dose
    (concentration) of allergen. With protein allergens, sensitization to
    detergent enzymes was found to diminish as the workplace atmospheric
    levels of the enzyme dust were reduced (Juniper et al., 1977). With
    chemical allergens, clinical studies have indicated an association of
    episodic high (accidental) exposure with development of sensitization
    (Brooks, 1982). In a study of isocyanate workers, a relationship was
    found between the number of spills and the percentage of workers
    displaying symptoms of allergic disease (asthma, bronchitis and
    decreased pulmonary function). With Western red cedar, an association
    was also noted between workplace exposure and either the incidence of
    pulmonary sensitization to the wood dust or the prevalence of
    occupational asthma (Brooks, 1982). A further indication of the
    importance of exposure concentration on sensitization is the reported
    decrease in the number of cases of toluene diisocyanate (TDI)
    sensitization coincident with the lowering of the permissible
    occupational exposure levels (Karol, 1992).

         Animal studies have established more precisely the relationship
    between the exposure concentration, the elicitation concentration, and
    development of respiratory sensitivity (Karol, 1994 a,b). Once again,
    the concentration of inhaled allergen was shown to be a prime factor
    controlling the development of sensitivity (Karol, 1983). Exposure of
    guinea-pigs to monitored concentrations of TDI vapour resulted in
    development of pulmonary sensitization only when the exposure
    concentration was > 0.25 ppm (> 1.8 mg/m3) (Karol, 1983).
    Exposure to lesser concentrations, even for extended periods of time,
    did not result in sensitization. Both a threshold concentration and a
    no-effect concentration were observed, suggesting the existence of a
    safe level of exposure for this potent allergenic chemical (Karol,
    1986).

         A threshold concentration for sensitization to the allergenic
    proteolytic enzyme, subtilisin, was also noted in animal studies
    (Thorne et al., 1986). Groups of guinea-pigs were exposed to
    atmospheres containing increased concentrations of the enzyme for
    15 min per day on each of 5 consecutive days. Sensitivity developed in
    animals exposed to the high concentrations but not in those exposed to
    the lesser ones. Even long-term exposure of animals to the lower
    concentrations failed to produce sensitization, although the animals
    had received a cumulative exposure comparable to that which regularly
    induced sensitivity when given over 5 days. This enzyme is believed to
    be a particularly potent allergen and has a threshold limit value of
    0.06 mg/m3. Clinically, workplace sensitization to the enzyme has
    been dramatically reduced by lowering workplace exposures, and by
    changing the formulation of the allergen to make it less readily
    airborne (Juniper et al., 1977; Thorne et al., 1986; Sarlo & Karol,
    1994).

    3.3.1.2  Frequency of exposure

         Increased frequency of inhalation exposure to allergen increased
    the sensitization rate (Karol, 1986). However, studies clearly
    demonstrated the importance of the exposure concentration exceeding a
    threshold level for the chemicals. Repeated inhalation exposure of
    guinea-pigs to sub-threshold concentrations of subtilisin (Thorne et
    al., 1986) or TDI (Karol, 1983) failed to sensitize the animals,
    whereas the same total exposure given over a shorter time span
    consistently resulted in sensitization. Long-term sub-threshold
    exposure to TDI resulted in neither respiratory sensitization nor
    production of specific antibodies (Karol, 1983).

         Clinically, chronic low-level exposure has been implicated in the
    development of respiratory allergy to some airborne chemicals, notably
    TDI (Karol, 1986). However, at that time the ability to measure low
    concentrations of TDI was limited. Long sampling periods were often
    required which eliminated the possibility of detecting sporadic high
    TDI concentrations (Karol, 1986). As a result, in such studies no
    conclusion can be drawn regarding the development of sensitization as
    a result of repeated low-level exposure.

         The influence of chronic low-level exposure to detergent enzymes
    on the development of occupational sensitization to these enzymes has
    been studied (Juniper et al., 1977). Using skin prick tests as an
    indication of sensitization, conversion to skin test positivity was
    observed following 20 months of employment for both high- and
    low-exposure groups. A reduction in the dust levels in the workplace
    was coincident with a decreased conversion rate (Juniper et al.,
    1977).

         In the platinum industry, respiratory sensitization to soluble
    platinum salts has occurred under conditions where exposure is below
    the official workplace limit. Maynard et al. (1997) examined the
    possibility that high short-term exposures might be responsible but
    found there was no evidence for this. In a cross-sectional study of
    respiratory and dermal sensitization to platinum salts in a population
    of precious metals refinery workers, skin reactivity was found in
    workers exposed to permissible levels of platinum salts and was
    associated with respiratory and dermal sensitization, but not with
    atopic status (Baker et al., 1990). Merget et al. (1994), in a study
    of platinum refinery workers, found that in workers who developed
    immediate-type asthma caused by platinum salts both nonspecific and
    specific bronchial responsiveness did not decrease after removal from
    exposure.

         Repeated exposure of guinea-pigs to contact allergens resulted in
    reduced local reactions (Boerrigter et al., 1987) with eventual
    diminution such that the skin reactions were almost non-existent.
    However, the state of unresponsiveness disappeared upon
    discontinuation of the repeated allergen exposures.

         In humans, repeated exposure may also down-regulate the local
    inflammatory response in the skin. This phenomenon is termed
    "hardening". However, the individual remains sensitized. By contrast,
    repeated systemic exposure could also "desensitize". This effect is
    thought to be due to the high total dose administered.

    3.3.1.3  Route of exposure

         The route of exposure has an influence on the outcome of exposure
    to an allergen. In general, exposure by the inhalation or dermal route
    favours sensitization, whereas exposure by the oral route favours
    tolerance (unresponsiveness ). Immunological unresponsiveness can be
    induced in animals by non-cutaneous exposure. Induction of "tolerance"
    in humans to nickel as a result of exposure to nickel-releasing
    orthodontic braces during early age has been suggested (Van
    Hoogstraten et al., 1991).

         Systemic unresponsiveness after ingestion of antigen has now been
    described for a large variety of T-cell-dependent antigens (Mowat,
    1987). Proteins such as ovalbumin and bovine serum albumin (Silverman
    et al., 1982; Domen et al., 1987), particulate (erythrocyte-bound)
    antigens (Kagnoff, 1982; MacDonald, 1983; Mattingly, 1984),

    inactivated viruses and bacteria (Stokes et al., 1979; Rubin et al.,
    1981), autoimmune-related antigens (Thompson & Staines, 1990), as well
    as contact allergens, have been reported to induce oral tolerance
    (Asherson et al., 1977; Newby et al., 1980; Gautam et al., 1985).
    Generally, T-cell-mediated delayed-type hypersensitivity responses and
    IgE production are the types of immune responses most readily
    tolerized. Persistent tolerance can be induced with relatively low
    antigen doses of proteins (Heppel & Kilshaw, 1982; Jarrett, 1984;
    Jarrett & Hall, 1984) and contact allergens (Asherson et al., 1977;
    Polak 1980; van Hoogstraten et al., 1992; Hariya et al., 1994). The
    apparent ability of the intestinal immune system to prevent allergic
    hypersensitivity to soluble, non-replicating antigens seems an
    important pathway to prevent enteropathies (Challacombe & Tomasi,
    1987; Mowat, 1984, 1987). Abrogation of oral tolerance to, for
    instance, ovalbumin was found to lead to hypersensitivity responses in
    the intestinal mucosa and gut-associated lymphoid tissues, resembling
    those observed in food-sensitive enteropathies, e.g., coeliac disease
    (see section 1.5.1.3).

         If mucosal cells in the respiratory tract are the site of initial
    exposure, the result is frequently production of IgA and IgE
    antibodies and predisposition to Type I allergic reactions. Initial
    exposure of mucosal cells in the gastrointestinal tract may have the
    same effect but often produces tolerance. By contrast, skin exposure
    favours Type IV sensitization. It appears that the route of first
    encounter with the chemical allergen determines whether the outcome is
    sensitization or unresponsiveness.

         Once an individual is sensitized via the skin, subsequent oral
    exposure does not tolerize, but might contribute to further
    sensitization by boosting the ongoing immune response. It is even
    possible to induce systemic allergic reaction via the oral route in
    skin-sensitized individuals. Overall, all of these factors are
    dependent upon the nature of the allergen.

    3.3.2  Atmospheric pollution

         The effect of indoor and outdoor air pollution on allergic
    disease has received considerable attention. Environmental pollutants
    have been reported to contribute to the prevalence of allergic
    disease, the precipitation of allergic symptoms, and their intensity
    (Ollier & Davies, 1994). Both epidemiological and experimental studies
    have demonstrated that a variety of atmospheric substances (including
    sulfur dioxide (SO)2, nitrogen dioxide (NO2), ozone (O3) and
    particles) influence the induction and elicitation phases of the
    allergic response. Effects have included adjuvant activity for
    allergen-specific IgE production, modulation of mediator release from
    inflammatory cells, and irritant effects on effector organs of the
    allergic response (Behrendt et al., 1995) (see sections 5.13 and
    5.14).

         The question of whether environmental factors may be involved in
    the observed increase in the prevalence of allergy is a matter of
    controversy (Ring et al., 1995b; Behrendt et al., 1995; Vos et al.,
    1996). There is no doubt that pollutants such as suspended particles,
    automobile exhaust, ozone, sulfur dioxide and nitric oxides can be
    measured in rather high concentrations in the air of many countries
    that show an increasing prevalence of atopic diseases. However, some
    of these pollutants, like sulfur dioxide, have shown a decrease in air
    concentrations during the last decades. In a controlled prospective
    trial comparing different living areas with various degrees of air
    pollution in western and eastern Germany, striking differences were
    shown with regard to the prevalence of respiratory atopic diseases,
    with higher values for western compared to eastern Germany (von
    Mutius, 1992; Schlipköter et al., 1992; Behrendt et al., 1993, 1996;
    Ring et al., 1995). In contrast to atopic respiratory diseases, there
    was a trend to higher prevalence rates of atopic eczema in eastern
    Germany. In the same study there was evidence of an increased risk of
    developing atopic eczema after exposure to natural allergens as well
    as air pollutants from outdoor and indoor sources (Ring et al., 1995;
    Krämer et al., 1996; Schäfer et al., 1996).

         The mechanisms by which air pollutants influence allergic
    reactions are not clear. Some pollutants may have a direct toxic
    effect on the respiratory epithelium leading to inflammation, airway
    hyperreactivity and the appearance of asthma-like symptoms in
    previously non-asthmatic individuals. In cell systems, certain
    pollutants have been shown to modulate degranulation and histamine
    release from basophils (Ring et al., 1995). Polychlorinated biphenyls
    enhance eicosanoid production by granulocytes and platelets (Raulf &
    Konig, 1991). Certain pollutants may have the ability to augment or
    modify immune responses to inhaled antigens or to enhance the severity
    of reactions elicited in the respiratory tract following inhalation
    exposure of the sensitized individual to the inducing allergen.

         High concentrations of air pollutants can have irritant effects
    and aggravate the symptoms of allergic respiratory and skin diseases
    (Ring et al., 1995; Behrendt et al., 1996). Laboratory studies suggest
    that certain air pollutants have the potential to stimulate
    broncho-constriction and airway inflammation. Exposure to SO2 is
    associated with chest tightness and bronchoconstriction, the
    concentration required to induce a response being dependent upon the
    degree of hyperresponsiveness of the individual. The effects of SO2
    may be augmented in the presence of other pollutants. It has been
    reported, for instance, that the sensitivity of mild asthmatics to
    SO2 is increased by prior exposure to O3. Ozone has been
    investigated extensively and has been found to cause bronchial
    hyperresponsiveness. In controlled clinical exposure studies,
    researchers have demonstrated that asthmatics are more responsive to
    O3 than normal people (Ball et al., 1993; WHO, in press). Exposure of
    asthmatics to O3 for 1 h caused an increase in airway responsiveness
    to inhaled allergen. The proportion of cynomologous monkeys that

    developed asthma and a positive skin test after inhalation of complex
    platinum salts was increased in those animals that inhaled O3
    concurrently (Biagini et al., 1986). The health relevance of oxides of
    nitrogen, and in particular NO2, has attracted some interest since
    the gas is present both outdoors and indoors. Some studies have
    suggested mild effects of NO2 in asthmatics at concentrations of less
    than 1 ppm (< 1.88 mg/m3); others have not found responses at levels
    up to 4 ppm (7.52 mg/m3). Particulate air pollutants, especially fine
    particles derived from combustion sources, are also of interest
    although there have been few controlled exposure studies apart from
    those involving acid aerosols.

         Bioaerosols to which an asthmatic is sensitized are well known to
    exacerbate asthma. Epidemiological studies describing the increase in
    mortality associated with inhaled particulate matter (PM-10) provide
    provocative evidence for adverse pulmonary health effects associated
    with particulate pollution. The association between particulate matter
    and acute mortality and morbidity has been demonstrated most strongly
    with elderly people who have chronic cardiopulmonary disease
    (Thurston, 1996).

         Studies have demonstrated an effect on allergic disease from
    substances adsorbed to airborne particles. Such substances were found
    to release histamine from human basophils and had a priming effect on
    anti-IgE-induced release of histamine and LTC4 (Behrendt et al.,
    1995). These  in vitro studies indicated that particle-adherent
    substances interfere with cells involved in inflammatory processes.

         There is evidence of an interaction between pollen and air
    pollutants. Pollen grains in polluted areas have been shown to be
    loaded with particles including heavy metals, such as lead, cadmium
    and mercury.  In vitro, these pollen grains were found to have
    altered surface features and increased ability to release cytosolic
    allergenic proteins (Behrendt et al., 1991).

    3.3.2.1  Tobacco smoke

         Passive exposure to tobacco smoke is a risk factor for childhood
    asthma (Seaton et al., 1994; Becher et al., 1996). Studies to detect a
    possible association between passive smoke and allergic disease in
    adults are much more difficult to design. Asthmatic patients
    frequently report exposure to passive smoke. In children, there is
    evidence that tobacco smoke increases the risk for development of
    wheezy bronchitis and asthma.

         Tobacco smoking is associated with an increased risk of
    developing IgE antibodies and asthma. The mechanism of this effect of
    tobacco smoke is unknown, but may be a result of injury to the
    respiratory mucosa. Several studies have indicated that subjects who
    smoke cigarettes have higher IgE levels (Zummo & Karol, 1996).
    Specific IgE antibody or an immediate skin test response was found to
    be 4-5 times more frequent in smokers exposed to tetrachlorophthalic

    acid (TCPA) and ammonium hexachloroplatinate. Initially smokers had
    IgE levels similar to those of controls, but, with age, IgE levels in
    smokers did not decline at the same rate as they did in the
    non-smokers (Sherrill et al., 1994). This may provide an explanation
    for the difference in IgE values observed in adult smokers. Moreover,
    a relationship was noted between the number of cigarettes smoked and
    the IgE level, suggesting causality. In female smokers, there was a
    trend toward increased IgE at older ages (i.e., > 50 years).

         Passive smoking has been found to be a risk factor for
    development of sensitization in children (Halken et al., 1995). The
    association does not necessarily imply an allergic mechanism, rather
    the association can be a result of direct irritation and inflammation
    of the respiratory tract. In children with atopic predisposition, a
    significant correlation was found between exposure to tobacco smoke
    and wheezing/persistent wheezy bronchitis. A prospective study of 94
    asthmatic children found significantly more asthma symptoms in those
    exposed to maternal tobacco smoke. A retrospective study with 199
    children with asthma found acute exacerbations of asthma increased
    with exposure to tobacco smoke. In children with past or present
    atopic dermatitis, asthma was found more frequently in cases where the
    mother smoked cigarettes (Halken et al., 1995).

    3.3.2.2  Geographical factors

         Exposure to airborne allergens, notably pollens, depends on
    location, climate and time of year (Emberlin, 1994). Certain types of
    air pollution reduce the amount of pollen produced, but they can also
    render the proteins on pollen more allergenic (Ruffin et al., 1986).

    3.3.3  Metals

         Nickel is a frequent cause of contact sensitization, having a
    sensitization rate of 15-50% in experimental studies. Most cases of
    nickel allergy can be attributed to exposure to nickel alloys in close
    skin contact, which release high concentrations of nickel when exposed
    to sweat. Similarly, chromate dermatitis often relates to exposure to
    hexavalent chromate in wet cement (Andersen et al., 1995).
    Investigations of monozygotic female twins, where one or both were
    nickel sensitive, have shown that only the twin with a history of
    contact dermatitis by nickel alloy exposures gives a positive
    diagnostic patch test to nickel (Menné & Holm, 1983).  In vitro 
    diagnostic testing failed to demonstrate subclinical nickel
    sensitization in family members of nickel-sensitive individuals
    (Silvennoinen-Kassinen, 1981).

    3.3.4  Detergents

         Reports of respiratory allergic reactions in workers involved in
    large-scale production of enzyme-containing detergents suggest that
    the detergent component may contribute to the sensitization to the
    enzyme component. The symptoms of rhinitis and/or asthma suggested a
    Type I sensitization. Experimental studies in guinea-pigs, using

    either inhalation or intratracheal dosing, indicated that detergents
    and proteolytic enzymes enhance sensitization to allergenic proteins
    (Ritz et al., 1993; Sarlo et al., 1997) when sensitization was
    assessed by production of allergic antibody and respiratory responses
    to allergen challenge.

    3.4  Endogenous factors affecting sensitization

    3.4.1  Genetic influence

    3.4.1.1  Contact sensitization

         Although significant genetic influences on contact sensitization
    have been reported, lack of reproducibility and smallness of these
    effects suggest their minor importance, as compared to exposure, in
    clinical contact sensitization. A few studies utilizing different
    inbred mice and guinea-pig strains noted differences in sensitization
    rates for some contact allergens (Parker et al., 1975; Andersen &
    Maibach, 1985). In humans, a well-controlled family study indicated
    that experimental contact sensitization in children was greater when
    both parents could be sensitized by the same substance compared to
    children where only one parent could be sensitized (Walker et al.,
    1967). A population-based twin study focusing on nickel allergy found
    a significant genetic effect for the risk of developing this contact
    sensitivity (Menné & Holm, 1983). However, twin studies, using other
    designs, have failed to show such an association. Also, studies on
    frequencies of HLA genes in contact hypersensitive individuals have
    not revealed consistent patterns (Menné & Holm, 1986). Comparisons
    between frequencies of sensitization in different ethnic populations,
    e.g., for nickel in black and Caucasian groups, revealed either
    similar or different rates, depending on the study designs (Menné &
    Wilkinson, 1995).

         Histamine releasibility from mast cells and basophils is a
    critical event in many allergic disorders. In twin studies, this event
    (which is related to the quantity of IgE present on the cells) was
    shown to be under genetic control (Bonini et al., 1994).

         Products of HLA class II genes are involved in allergen
    presentation by antigen-presenting cells. Since these genes are highly
    polymorphic, different HLA genes represent risk factors for
    development of allergic asthma. Increased responsiveness to the
    ragweed allergen Ra 5 was found to be associated with the HLA gene DR
    2/DW 2.

         There is evidence for a genetic contribution to sensitization to
    some allergens of low relative molecular mass.

    3.4.1.2  IgE-related allergy

         One of the characteristic features of atopy is the production of
    IgE in an exuberant and prolonged fashion to common largely innocuous
    environmental allergens, such as house dust mites and pollen. Most

    atopics are allergic to more than one common environmental allergen
    and this introduces the concept that the causation of atopy occurs at
    a variety of levels: generalized hyper-IgE responsiveness; IgE
    response to specific allergens or epitopes; clinical disease
    expression (Hopkins, 1997).

         The genetics of production of total serum IgE have been studied.
    In such studies consideration has to be given to the following
    factors, since each has been shown to affect IgE levels: allergic
    exposure, parasitic infection, age, sex and smoking. A correlation was
    found between the total serum IgE of parents and children, suggesting
    the involvement of one or more genes (Sherrill et al., 1994). However,
    agreement on the model of inheritance is lacking. Linkage of loci for
    total serum IgE and BHR to chromosome 5q has been reported (Sherrill
    et a1., 1994). Mapping of this area of the chromosome will be
    important for further progress. Total serum IgE appears to be under
    strong genetic control (Bonini et al., l994), even in the presence of
    environmental factors such as smoking. A gene for IgE response with
    maternal inheritance was identified at chromosome 11q (Cookson et al.,
    1989). High levels of IgE in cord blood appear to be a strong
    indicator of subsequent development of atopic disease.

         The genetic factors that determine the specificity of the
    IgE-mediated response are thought to be independent of those governing
    total serum IgE and may be linked to the human leucocyte antigen (HLA)
    complex (Sibbald, 1997). Products of HLA Class II genes are involved
    in allergen presentation by antigen-presenting cells. HLA Class II
    genes are highly polymorphic. Different HLA genes represent risk
    factors for the development of asthma associated with sensitization to
    allergens. Increased responsiveness to ragweed antigen (Ra5) was found
    to be associated with HLADR2/DW2, and response to ryegrass (Lol pI and
    Lol pII) with HLADR23 and DR5 (Marsh, 1990). Environmental factors,
    such as the quality, intensity, route and duration of allergen
    exposure appear to be more relevant than genetic factors in causing
    allergic reaction to specific allergens (Bonini et al., 1994).

         Twin studies have suggested polyfactorial control of allergy
    variables such as serum levels of total IgE and IgG4, mediator release
    from inflammatory cells, and target organ response. Clinical data from
    32 monozygotic and 71 dizygotic twin pairs yielded a concordance for
    allergic disease of 50.0% of monozygotic pairs (16/32) and 35.2% of
    dizygotic pairs (25/71). The difference was not statistically
    significant (Cockcroft, 1988). Histamine releasibility from mast cells
    and basophils is a crucial event in allergic disorders. In twin
    studies, this event (which is related to the quantity of IgE present
    on the cells) was shown to be under genetic control (Bonini et al.,
    l994).

         Development of respiratory allergies to small relative molecular
    mass chemicals, i.e., relative molecular mass less than 5000, such as
    isocyanates and acid anhydrides has not been found to be associated
    with atopy (Chan-Yeung, 1995), although atopy has been shown to be a

    risk factor for development of respiratory symptoms to some chemical
    allergens, such as hexachloroplatinate (Dally et al., 1980).

         Regarding low molecular mass, or chemical allergens, an
    association between sensitization to acid anhydrides and HLA-DR3
    haplotype has been reported (Young et al., 1993). An association of
    HLA class II alleles and isocyanate asthma was detected (Bignon et
    al., 1994). Twenty-eight patients with isocyanate-induced asthma (as
    documented by positive inhalation challenge) were compared with 16
    exposed individuals with no history of the disease. HLA DQB1*0503 and
    allelic combination DQB1*0201/0301 were associated with susceptibility
    to asthma. Conversely, allele DQB1*0501 and the
    DQA1*0101-DQB1*0501-DR1 haplotype conferred protection in exposed
    healthy subjects. No significant difference was detected in the
    distribution of HLA Class II alleles and/or haplotypes among the
    immediate, late or dual responders to TDI. These results are
    consistent with the hypothesis that immune mechanisms are involved in
    isocyanate asthma and that specific genetic factors may increase or
    decrease the risk of development of isocyanate asthma in exposed
    individuals.

    3.4.1.3  Other genetic factors

         Another factor that may contribute to susceptibility, or
    resistance, to sensitization relates to genes that control production
    of IL-4, a pleotropic cytokine that influences the development of both
    Th- and B-lymphocytes, the induction of Class II MHC antigens and
    immunoglobulin class switching from IgM to IgE. Genes for IL-3, IL-4,
    IL-5 and GM-CSF have been identified on chromosome 5 (Van Lee Uwen et
    al., l989). The IL-4 gene, as well as genes that regulate its
    expression, appear to be prime candidates for predisposition to atopy
    since there are reports that cells isolated from atopic individuals
    have the ability to overexpress the IL-4 gene relative to those from
    non-atopic individuals. In addition, the human IL-4 proximal promoter
    exists in multiple allelic forms, with one of the alleles having a
    markedly enhanced promoter activity (Song et al., l996b). This finding
    suggests a gene target to screen for genetic predisposition for atopy.

    3.4.2  Tolerance

         Allergenicity of a given compound may be strongly reduced in
    individuals who previously developed immunological tolerance. This has
    been frequently seen when the primary contacts with the allergen were
    at mucosal surfaces, e.g., by its presence in food. Principles and
    mechanisms of immunological hyporesponsiveness and tolerance have been
    dealt with in detail above (see section 1.5).

    3.4.2.1  Orally induced flare-up reactions and desensitization

         Strong and long-lasting oral tolerance can only be achieved in
    naive individuals, i.e., those who have not been previously exposed to
    the antigen via the skin. In mice, a single feed of ovalbumin was
    reported to fully suppress subsequent systemic immune responses, with

    this state of tolerance persisting for up to 2 years. In contrast,
    tolerance is hard to induce in primed animals but partial and
    transient unresponsiveness ("desensitization") may develop after
    prolonged feeding of the antigen. Similar results have been obtained
    in guinea-pigs with various chemical allergens, including
    dinitrochlorobenzene (DNCB) (Polak, 1980), nickel (van Hoogstraten,
    1994) and amlexanol (Hariya et al., 1994). Unfortunately, essentially
    similar results have been obtained in clinical trials aiming at the
    treatment of autoimmune diseases, e.g., rheumatoid arthritis and
    multiple sclerosis, by oral administration of putative autoantigens
    (Weiner et al., 1994). Another problem with oral tolerance induction
    in previously sensitized individuals arises from the tendency of
    former inflammatory sites to re-inflame ("flare-up reactions"). These
    reactions are likely to be due to allergen-specific effector T-cells,
    which can persist for periods of several months at former inflammatory
    sites (Scheper et al., 1983).

         The differences between immunological responses in naive and
    primed individuals may reflect changes in expression of cellular
    adhesion/homing molecules and lymphocyte maturation. A qualitative
    distinction exists between (difficult to stimulate/afferently acting)
    naive and (easy to stimulate/efferently acting) effector/memory cells.
    In contrast to naive lymphocytes, which only are activated by allergen
    (modified self constituents) if presented by professional dendritic,
    e.g., Langerhans cells, their progeny, known as effector/memory
    lymphocytes, can also be stimulated by other cell types presenting
    allergen-modified MHC class II-molecules, e.g., monocytes, endothelial
    cells and B-cells. Clearly, effector/memory cells display increased
    numbers of cellular adhesion molecules (CAMs), allowing for more
    promiscuous cellular interactions. Amongst these, the most prominent
    CAMs are the CD28 and LFA-1 molecules, with B7.1 and B7.2 and ICAM-1
    as their respective ligands on APC. In addition, priming of T-cells
    leads to the loss of homing receptors, such as L-selectin, which
    facilitate interactions with high endothelial venules in peripheral
    lymph nodes. Apparently, after sensitization, T-cells are less capable
    of recirculating through the lymphoid organs, but gain ability to
    migrate into the peripheral tissues. Interactions with endothelia
    within inflamed skin are facilitated by the enhanced expression of
    CAMs, such as the cutaneous lymphocyte-associated antigen CLA, and
    effector/memory T-cells largely distribute over the peripheral
    tissues, where conditions may be insufficient to convey effective
    tolerogenic signals.

    3.4.2.2  Non-specific and specific mechanisms of unresponsiveness

         A preliminary factor contributing to non-responsiveness and/or
    lack of hypersensitivity reactions at mucosal surfaces is the
    epithelial barrier function, preventing entry of potentially harmful
    allergens. Obviously, from an immunological point of view, this is a
    "null-event", and does not have implications to subsequent encounters
    with the same allergen. TGF-beta, a cytokine locally produced by
    epithelial cells and immunocytes, plays a pivotal role in maintaining
    epithelial barrier integrity. Importantly, the same cytokine also has

    broad nonspecific immunosuppressive functions, e.g., by antagonizing
    phagocytic effector cell functions of pulmonary alveolar macrophages.
    Similarly, other immunosuppressive cytokines may be locally released
    from epithelial cells and may act in concert with TGF-beta to
    down-regulate immune effector functions, such as epithelial
    cell-derived P15E-related factors which show sequence homology with
    retroviral envelope proteins (Oostendorp et al., 1993).

         In contrast, specific immunological tolerance depends on
    decreased responsiveness of specific B- or T- cells, or release of
    immunosuppressive mediators from these cells after specific challenge.
    So far, no methods of permanent desensitization have been devised.
    Nevertheless, how T-cells specifically recognize distinct allergens,
    and how these and other inflammatory cells interact to generate
    inflammation, is beginning to be understood. Exposure to high doses of
    antigens may induce clonal deletion or anergy of specific B- or
    T-cells by induction of apoptosis or antigen-receptor down-regulation
    (Jones et al., 1990; Schönrich et al., 1991; Ohashi et al., 1991;
    Melamed & Friedman, 1993).

         As IL-4 and IL-13 direct IgE isotype switching, one way to
    intervene in allergen-specific IgE synthesis and to inhibit or prevent
    IgE-mediated allergic disease is to inhibit IL-4 and IL-13 production
    by allergen-specific Th2-cells. In addition to TCR engagement by
    peptide MHC complexes, optimal T-cell activation and proliferation
    generally requires co-stimulatory signals provided by interaction
    between CD28 or CTLA-4 on T-cells and their ligands CD80 or CD86 on
    professional APC. Ligation of the TCR in the absence of these
    co-stimulatory signals can result in T-cell non-responsiveness. Human
    CD4+ Th2 clones specific for the house dust mite allergen  Der p I
    can be rendered non-responsive to subsequent  Der p I challenges by
    incubating them with  Der p I-derived peptides, representing the
    relevant minimal T-cell activation inducing epitopes, in the absence
    of professional APC (Yssel et al., 1994). The mechanisms underlying
    this T-cell unresponsiveness have not yet been determined. Although
    these cells cannot be activated through their TCR, they proliferate
    well in response to IL-2 or following activation by Ca++ ionophore
    and TPA, suggesting that TCR activation or signalling pathways
    immediately downstream of the TCR are disturbed.

         This type of tolerance is generally short-lasting, since
    (functionally) deleted lymphocytes are gradually replenished by newly
    arising clones in the bone marrow and thymus and, in experimental
    animal models, cannot be transferred to naive recipients, since these
    still contain a fully functional repertoire, compensating for any
    missing clones. On the other hand, mucosal contacts of naive
    individuals with relatively low amounts of antigens, such as can be
    the case with environmental or occupational exposure to chemical
    sensitizers, frequently induce a long-lasting state of specific
    tolerance. Transfer of lymphoid cells, in particular T-cells, from
    orally tolerized animals to syngeneic naive recipients prevents their

    capacity to subsequently mount immune responses to the same allergen,
    revealing the existence of so-called T-regulator or suppressor cells
    (Polak et al., 1980; van Hoogstraten et al., 1992, 1994; Weiner et
    al., 1994). 

         Although "professional" suppressor T-cells may not exist (Bloom
    et al., 1992; Arnon & Teitelbaum, 1993) available data support the
    development of specific "regulatory" T-cells that suppress distinct
    immune functions. Depending on the experimental models, such
    regulatory T-cells can belong to either or both the CD4+ or CD8+
    subsets (Bloom et al., 1992). Evidence is accumulating that regulatory
    T-cells most often exert their role, after antigen-specific
    activation, by releasing distinct cytokines antagonizing distinct
    effector T-cell functions.

    3.4.3  Underlying disease

         There is ample evidence that underlying diseases are able to
    influence the susceptibility of individuals to develop allergy. Both
    the induction and the manifestation of allergy may be affected.

         Conditions that promote sensitization include ongoing
    inflammatory reactions at the site of allergen contact. It has, for
    instance, been described that late-phase reactions of the respiratory
    tract and the associated state of hyperresponsiveness, may facilitate
    sensitization (priming) to other allergens (Connell, 1969). At skin
    sites, a pre-existing eczema provides a risk factor for acquiring
    contact sensitization. The most important factor here is probably the
    local disturbance of the skin barrier, allowing for an increased
    penetration of allergen. The fact that all components for an immune
    response (cytokines, T-cells) have already been attracted to the site
    of allergen contact may, however, additionally contribute to this
    increased risk for new sensitization.

         The most important diseases affecting the hosts' immune
    responsiveness, and thus allergic responsiveness, include infectious
    disease, neoplastic disease and immune deficiencies. The relation
    between infection and the development of allergic disease is quite
    complex. On one hand, respiratory viral infections are believed to
    contribute to the exacerbation of asthmatic disease (Busse, 1990).
    However, from clinical and epidemiological studies it would appear
    that under certain conditions viral infections can also protect
    against asthma. These studies include the observation of incidental
    spontaneous remission of asthma during hepatitis, fever or measles, as
    well as the finding of a general inverse relationship between
    infections and asthma or atopy (Matricardi, 1997; Serafini, 1997). In
    line with such a "protective" role it is believed that natural
    infections during early childhood would prevent the development of
    atopic disease later on, presumably by activation of the Th1
    lymphocytes through IFN-gamma (Serafini, 1997). Reduction in family
    size and increased hygiene could thus contribute to the increased
    frequency of atopic disease in developed countries. Interestingly,
    infectious diseases, which are known to be associated with a

    predominant Th2 immune responsivenesses, like parasite infections, do
    not seem to favour the development of atopic disease (Bell, 1996). In
    contrast, people suffering from severe parasite infection may have
    less severe reactions to other allergens, due to competition of IgE at
    the Fc epsilon receptor level on mast cells. Also in HIV-positive
    patients, where Th2 responses may become dominant, no clear evidence
    has been obtained for enhanced atopic sensitization, although allergic
    manifestations are frequently observed in these patients.

         Conditions that suppress allergic reactions have been extensively
    described, since contact sensitization has been applied as a method
    for immune status determination in different patient groups. It is a
    well-known fact that in clinical conditions associated with general
    immune suppression and anergy, such as malnutrition, immunosuppressive
    treatment, malignancies and severe physical trauma, Type IV reactivity
    to recall antigens as well as primary sensitization to contact
    allergens like dinitrochlorobenzene can be dramatically impaired.

         Finally, it should be noted that certain immunological
    conditions, such as those found in some immunodeficiency diseases,
    e.g., in the Wiskott-Aldrich syndrome, may predispose for the
    development of atopic eczema. Atopic disease is also commonly seen in
    IgA deficiency.

    3.4.4  Age

         Childhood asthma is becoming more common and doubled in the
    United Kingdom, New Zealand and Australia between 1970 and 1990.
    Because of their greater activity and their developing lungs, children
    may be more susceptible to sensitization as well as to adverse effects
    of irritants (Zummo & Karol, 1996).

         The ability to become sensitized to dinitrochlorobenzene has been
    shown to be largely unchanged with age. Patch testing with  Rhus 
    oleoresins in subjects with a history of poison ivy sensitization
    showed diminished responses in the elderly (Lejman et al., 1984).
    However, exposure differences as a function of age must always be
    considered (Menné & Wilkinson, 1995).

         IgE levels change with age. Peak levels occur in the first or
    second decades of life. A longitudinal study of more than 2000
    subjects conducted over a 20-year period found no gender difference in
    total IgE (Sherrill et al., 1994). Both sexes had their highest IgE
    levels as children. Levels fell gradually up to around age 40 and
    thereafter remained constant.

    3.4.5  Diet

         To explain the observed increase in incidence of allergy and
    asthma during the last two decades, it has been suggested that a
    change in host resistance to allergy may have occurred (Seaton et al.,
    1994). A change in the diet in several Western countries has been

    documented. Specifically, a 20-50% fall in consumption of fresh fruits
    and vegetables has been noted. Since these foods are sources of
    antioxidants such as vitamin C and beta-carotene, decreased consumption,
    together with that of red meat and fresh fish, would mean less
    ubiquinone and fewer cofactors (such as zinc and copper) for
    antioxidant defence (see section 5.10).

    3.4.6  Gender

         In general, women appear to have greater immune capability than
    men (Menné & Wilkinson, 1995). Animal and human studies have indicated
    a greater incidence of autoimmune disease in women compared with men,
    as well as higher IgG and IgM levels. Women have also been reported to
    produce greater cell-mediated immune responses.

         In a large, controlled study, men were found more susceptible to
    sensitization by dinitrochlorobenzene than women (Walker et al.,
    1967). However, women were more readily sensitized to
     p-aminodiphenyl aniline than were men (Walker et al., 1967). In
    these studies, the issue of previous exposure to the chemical, and
    therefore greater susceptibility, could not be dismissed. This factor
    may also explain greater female sensitization in clinical patch tests
    with nickel and cobalt. Male and female sensitization rates obtained
    by maximization testing were comparable (Leyden & Kligman, 1977).

         In a study of the influence of sex hormones on sensitization,
    response to dinitrochlorobenzene was enhanced in women receiving oral
    contraceptive hormones (Rea, 1979)
    


    4.  CLINICAL ASPECTS OF THE MOST IMPORTANT ALLERGIC DISEASES

         Allergic diseases give rise to symptoms in many different organ
    systems and involve many different medical disciplines. The most
    important allergic diseases comprise allergic contact dermatitis,
    atopic eczema, allergic rhinitis and conjunctivitis, asthma and food
    allergy, and autoimmune diseases associated with chemicals.

    4.1  Clinical aspects of allergic contact dermatitis

    4.1.1  Introduction

         Like the mucous membranes and the gut, the skin is an advanced
    part of the immune system. Together with the skin barrier, the immune
    system defends the body surface against microorganisms. Skin contact
    with small molecules (haptens) tends to induce cellular-mediated
    contact sensitization. The consequence of this contact sensitization
    is allergic contact dermatitis. If the same molecules are given orally
    before cutaneous contact, they may induce persistent immunological
    tolerance. Allergic contact dermatitis is a common disease and the
    prevalence at any given time varies between 2-4% (Fig. 9, 10, 11).
    Allergic contact dermatitis of the hands has particularly important
    implications for society as prolonged sick leave is common.

         Most contact allergens are small molecules with a relative
    molecular mass below 6000. Contact sensitization is not inborn but is
    always a consequence of earlier cutaneous contact. Contact
    sensitization is considered to be life-long, but might become weaker
    if exposure is avoided. Contact sensitized individuals are at risk of
    developing the skin disease allergic contact dermatitis if re-exposed
    to the specific chemical. The term dermatitis is used synonymously
    with eczema and describes either an acute skin disease with redness,
    oedema and vesicles (water blisters) or a more chronic type with
    hyperkeratosis, fissures and scaling. The most important differential
    diagnosis of contact dermatitis is psoriasis, dermatophytosis, and
    scabies. IgE-mediated immunological contact urticaria is covered
    briefly.

    4.1.2  Regional dermatitis

    4.1.2.1  Hand eczema

         Epidemiological studies including 20 000 individuals representing
    the general population showed a one-year prevalence of hand eczema of
    10% (Meding, 1990); 20% of cases were classified as caused by contact
    allergy. The average duration was 12.8 years and 22% had periods of
    sick leave. Allergic contact dermatitis on the hands is therefore both
    a common disease and costly for the society, and it can imply
    significant socioeconomic consequences for the individual.

    FIGURE 9


    FIGURE 10


    FIGURE 11


    In a survey of 564 cases of permanent disability caused by skin
    diseases, 222 of the 564 were caused by allergic contact dermatitis of
    the hands (Menné & Bachmann, 1979).

         Frequent causes of allergic hand eczema are nickel, chromate,
    rubber additives (Fig. 9) preservatives, and fragrances (Menné &
    Maibach, 1993). It can be acute or chronic, and it can be located on
    either the dorsal or volar surfaces, or only on the fingers. It can
    also present as a diffuse dermatitis. Spread to the face and forearms
    is common.

    4.1.2.2  Facial dermatitis

         The face is second to the hands in the frequency of allergic
    contact dermatitis. The exposure can be direct to airborne allergens
    or indirect by contact with allergens transferred from the hands to
    the face. Acute allergic contact dermatitis in the face is often
    dramatic with severe oedema particularly of the eyelid regions.
    Chronic cases frequently show patchy dermatitis even if the allergen
    is uniformly spread on the face. Cosmetics, particularly fragrances,
    are the most common causes of facial dermatitis. Allergic contact
    dermatitis from medicaments (e.g., eye drops) and airborne
    occupational dermatitis are seen. Severe oedema of the eyelids is a
    common pattern of plant dermatitis. Facial dermatitis causes distress
    to the individual because of pain, itching and disfiguration.

    4.1.2.3  Other types of dermatitis

         Stasis eczema and leg ulcers are a common disease among the
    elderly as complications of arterial and venous insufficiency and
    arteriosclerotic heart disease. Stasis eczema is a consequence of skin
    malnutrition and can be followed by chronic ulceration. Both entities
    are treated with topical medicaments such as emollients, steroids,
    antiseptics and antibiotics. These compounds generally do not have a
    high sensitizing capacity, but because they are used on damaged skin
    under occlusion for prolonged periods, contact sensitivity is not
    uncommon. Patch testing is routinely recommended in the work-up of leg
    ulcer and leg eczema patients. On average 50% of these patients have a
    positive patch test of actual or past relevance.

         Intertriginous areas such as the axillae, external ear and
    perianal area are also frequent sites of primary sensitization from
    topically used medicaments and fragrances because of the natural
    occlusion.

         Shoe dermatitis is located in the skin area in direct contact
    with the offending material, most frequently chromate-tanned leather,
    rubber and glues (Podmore, 1995).

         Allergic contact dermatitis from textiles gives a characteristic
    clinical pattern with dermatitis in areas where textiles are in close
    contact with the skin on the trunk and extremities. The offending
    sensitizers are textile dyes and formaldehyde-releasing textile resins
    (Fowler et al., 1992).

    4.1.3  Special types of allergic contact reactions

    4.1.3.1  Systemic contact dermatitis

         Systemic contact dermatitis can be seen in primary contact
    sensitized individuals when they are later exposed systemically to the
    chemical (or drug) either orally, intravenously, by inhalation or by
    transcutaneous absorption (Menné et al., 1994). The clinical symptoms
    can either be erythematous flare in areas with earlier contact
    dermatitis or a combination of symptoms including vesicular hand
    eczema and inflammatory skin reaction in the flexural and genital
    area. The explanation for the flare reaction is probably specific
    sensitized lymphocytes persisting at the site of earlier allergic
    contact dermatitis areas. The mechanism behind the other type of
    reactions is speculative. Histologically this widespread reaction does
    not have the picture of contact dermatitis but frequently presents the
    picture of a lymphocytic vasculitis. The pathogenesis may be
    circulating immune complexes or a general reaction to released
    cytokines.

         Systemic contact dermatitis is mostly seen in patients sensitized
    to topically used medicaments when they are systemically treated with
    the medicament or a cross-reacting medicament. Systemic contact
    dermatitis has been described for a large number of substances.

    4.1.3.2  Allergic photo-contact dermatitis

         Most substances that cause photo-contact allergy are halogenated
    aromatic hydrocarbons or sunscreen agents (White, 1995). The
    combination of light, predominantly ultraviolet (UV), and the specific
    chemical make the complete hapten. Clinical allergic photo-contact
    dermatitis will therefore present a dermatitis (often severe) in
    sun-exposed areas. This will typically be on the face, the forearms or
    the dorsal aspects of the hands. In cases where photo-contact allergy
    is suspected, patch testing is performed in duplicate and one site is
    exposed to UVA. If a positive patch test only appears on the
    UV-exposed site, photoallergy is likely.

    4.1.3.3  Non-eczematous reactions

         Allergic contact sensitivity in the skin can give rise to
    clinical reaction patterns other than dermatitis (Goh, 1995). These
    types of reactions are rare and to only a few chemicals. Even if these
    patients have a clinical reaction type other than dermatitis, they
    frequently have a positive patch test with the usual eczematous
    morphology. The most common types of non-eczematous contact reactions

    are erythema multiforme and lichen planus. Erythema multiforme-like
    reactions are caused by contact with plant allergens and the lichen
    planus type by contact with photographic chemicals.

    4.1.3.4  Allergic contact urticaria

         Contact urticaria is an immediate wheal reaction in the skin
    caused by vasodilatation, with subsequent oedema. Contact urticaria
    can either be allergic or non-allergic. In the non-allergic types
    chemical causes a degranulation of the mast cells without involvement
    of the immune system. The allergic types are mediated via IgE bound to
    specific receptors on the mast cells and basophil lymphocytes in the
    skin. The clinical types are similar with urticaria localized at the
    contact site. Generalized anaphylactic reactions are rare. Both
    organic and inorganic substances have now been described as causes of
    allergic contact urticaria (Amin et al., 1996).

         Contact urticaria is a frequent occupational disease among
    individuals handling animals and animal products. Allergic contact
    urticaria from proteins in rubber latex is a frequent and troublesome
    problem among workers, particularly health personnel, due to
    widespread use of rubber gloves (Taylor & Praditsuwan, 1996; NIOSH,
    1997). A sensitization frequency of 2.8 to 10.7% has been reported in
    health personnel (Turjanmaa, 1996). Individuals occupationally
    sensitized to rubber latex proteins can develop anaphylactic reactions
    if exposed to rubber gloves as patients.

    4.1.4  Allergic contact dermatitis as an occupational disease

         Occupational skin diseases are defined as skin diseases either
    wholly or partly caused by the patient's occupation (Rycroft, 1995).
    The epidemiology of occupational skin diseases, which mostly comprise
    contact dermatitis of the hands, is known from population and
    cross-sectional studies of specific occupational groups. Information
    from centralized notification systems exists in some countries, but
    the quality of data can be questioned. In particular, the problem of
    under-reporting is difficult to quantify.

         Skin diseases comprise between 20 and 40% of all occupational
    diseases, depending on geographical area. Approximately one-third is
    caused by allergic contact dermatitis and the rest mainly by irritant
    dermatitis. The principal occupational contact sensitizing chemicals
    are listed in Table 16. Not unexpectedly there is an overlap between
    exposure to chemicals in occupational and domestic environments (see
    section 4.1 and Table 19). The common high-risk occupations for
    allergic contact dermatitis, modified from Rycroft (1995), are given
    in Table 17 (Flyvholm et al., 1996). The prevalence of occupational
    contact dermatitis in these occupations varies from a few percent up
    to 15% (Rycroft, 1995).


        Table 16.  Main allergens related to occupational exposure
    (from Flyvholm et al., 1996)
                                                                                                                            

    Allergens                            Sources of exposure
                                                                                                                            

    Acrylates                            adhesives; bone cement; dental products; UV-curing lacquers, etc.
    Amines                               hardeners/curing agents for epoxy resin
    Chromate                             cement; leather; pigments
    Cobalt                               paints/lacquers
    Colophony                            adhesives; dental products; paper; tin solder, etc.
    Epoxy resin                          adhesives; paints; electric insulation
    Formaldehyde                         disinfectants; preservatives; laboratory chemicals; formaldehyde resins;
                                         funeral service
    Formaldehyde releasers               metal working fluids; paints; adhesives
    Formaldehyde resins                  adhesives; paints/lacquers; impregnated textiles and paper; inks
    Isocyanates                          adhesives; paints; fillings; polyurethane foams
    Medicaments                          human and animal health care workers
    Nickel                               coins; nickel plated objects; contaminated oils, etc.
    Paraphenylenediamine                 hair dyes; rubber additive
    Plastics/resins                      adhesives; paints; fillings, containers, etc.
    Preservatives                        water-based products: metal working fluids; paints; adhesives;
                                         cleaning agents; cosmetics; polishes; skin protection creams; process water, etc.
    Rubber additives                     rubber gloves; rubber tubing; washers, etc.
                                                                                                                            
    

        Table 17.  High-risk occupations for allergic contact dermatitis
                                                                     

    Adhesives/plastics workers           Horticulturalists
    Agriculturalists                     Leather tanners
    Cement casters                       Painters
    Construction workers                 Pharmaceutical/chemical workers
    Glass workers                        Rubber workers
    Graphic workers                      Textile workers
    Hairdressers                         Tilers
    Health care workers                  Wood workers
                                                                     
    
         It is difficult to give exact data concerning the costs of
    occupational allergic contact dermatitis, as the compensation
    regulation differs significantly from one country to another. However,
    in the United Kingdom in 1996 it was estimated that 84 000 people had
    occupational contact dermatitis, and 132 000 working days were lost
    with a cost to employers of 20 million pounds per year (HSE, 1996). 

    4.1.5  Diagnostic methods

    4.1.5.1  Patch testing

         The aim of patch testing is to diagnose contact sensitization to
    environmental chemicals. The patch test was introduced in 1896 by the
    Swiss dermatologist Jadahsson (Wahlberg, 1995). The technology is a
    biological test where contact allergy is proved by re-exposing the
    skin to the specific chemical under occlusion on a skin area of 0.5
    cm2 on the upper back for 2 days. A positive test is a reproduction
    of the clinical disease showing redness, infiltration and eventual
    vesicles. Standardization has taken place, particularly influenced by
    the Scandinavian and later the International Contact Dermatitis
    Research Group (ICDRG). The test should only be performed using
    standardized test materials. All patients are primarily tested with
    the Standard series including the most frequent sensitizing chemicals
    such as metals, preservatives, fragrances, rubber additives and
    topically used medicaments. Testing is frequently supplemented with
    substances present in the patient's private or occupational
    environments. Specially trained staff are necessary to obtain high
    quality outcome of the procedure.

         Sensitization can be quantified according to the degree of
    positive patch test reaction (+ to +++), patch test concentration
    threshold defined by dilution series, and finally by the "Use test".
    In the latter test the individual is exposed to the chemical
    simulating normal use.

         The outcome of patch testing defines whether contact allergy is
    present or not. Quantification of allergy combined with quantitative
    exposure data is the basis for individual and general risk assessment
    (Flyvholm et al., 1996).

         The frequency of positive patch test reactions in the general
    population (Nielsen & Menné, 1992) and in eczema patients tested at a
    dermatological clinic in the same area of greater Copenhagen, Denmark,
    is shown in Table 18. The allergens causing positive reactions most
    frequently in eczema patients were nickel, fragrance mix, cobalt
    chloride, colophony and balsam of Peru. For the general population,
    nickel and thiomersal were the most common causes of positive patch
    test reactions. Contact sensitization is generally more frequent among
    patients investigated at dermatological centres than it is in the
    general population.

    4.1.5.2  In vitro testing

         Several attempts have been made to develop  in vitro methods for
    testing contact sensitization (von Blomberg et al., 1990; McMillan &
    Burrows, 1995). Yet, logistical and technical complexities, including
    allergen toxicities, and the generally low frequencies of circulating
    allergen-specific T-effector-memory cells, mean that currently
    available methods are not appropriate for routine clinical use.
    Nevertheless,  in vitro tests, in particular the lymphocyte
    proliferation test (LPT), using patient-derived white blood cell
    samples, can be of considerable value in answering specific scientific
    questions, e.g., on the involvement of allergen-specific T-cells or on
    potential cross-reactivity patterns between allergens (Bruynzeel et
    al., 1985; Pistoor et al., 1995).

    4.1.6  Assessment of exposure

         To establish the diagnosis of allergic contact dermatitis, the
    outcome of patch testing needs to be combined with a detailed exposure
    history (Flyvholm et al., 1996). Both domestic and work-related
    exposures need to be elucidated. Factory visits are valuable but
    rarely done (Rycroft, 1995). The most common contact allergens are
    metals, preservatives, rubber additives, perfumes and medicaments. The
    main sources of exposure to contact allergens can be divided into
    groups of substances, products or use categories. Exposure to
    allergens occurs under many circumstances, such as occupational,
    domestic work, hobby and leisure time activities, topical medicaments,
    cosmetics, personal care products, clothing and shoes. Examples of
    such allergens are listed in Table 19 (Flyvholm et al., 1996). For
    examples of occupational exposure, see Table 16 (section 4.1.4).
    Exposure data can be obtained from databases, product labelling or
    chemical analysis, and by contact with manufacturers and suppliers.
    The prognosis for the individual patient depends upon the quality of
    diagnostic patch testing and the ability to prevent contact of the
    patient with the allergen.


        Table 18.  Comparison of frequencies of positive patch test reactions
    in the general population (Nielsen & Menné, 1992) and in eczema patients at a
    dermatological clinic in the same area of greater Copenhagen in 1990a
                                                                                                           

    Test substance              General populationb                        Dermatological clinicc
                                (% positive of tested)                     (% positive of tested)

                                Men        Women       Total               Men        Women         Total
                                (n=279)    (n=288)     (n=567)             (n=262)    (n=410)       (n=672)
                                                                                                           

    Potassium dichromate        0.7        0.3         0.5                 1.9        2.7           2.4
    Neomycin sulfate            0.0        0.0         0.0                 3.4        3.7           3.6
    Thiuram mixture             0.7        0.3         0.5                 4.6        2.7           3.4
    p-Phenylenediamine          0.0        0.0         0.0                 1.9        2.7           2.4
    Cobalt chloride             0.7        1.4         1.1                 2.3        2.7           2.5
    Benzocaine                  -          -           NT                  0.4        0.7           0.6
    Caine(R) (local             0.0        0.0         0.0                 -          -             NT
    anaesthetic) mix
    Formaldehyded               -          -           NT                  1.9        2.2           2.1
    Colophony                   0.4        1.0         0.7                 4.6        5.4           5.1
    Quinoline mix               0.4        0.3         0.4                 1.9        0.5           1.0
    Balsam of Peru              0.7        1.4         1.1                 3.4        5.4           4.6
    PPD black rubber mix        0.4        0.0         0.2                 1.2        0.0           0.5
    Wool alcohols               0.4        0.0         0.2                 1.2        1.7           1.5
    Mercapto mix                0.7        0.0         0.4                 1.2        0.2           0.6
    Epoxy resin                 0.4        0.7         0.5                 0.8        0.2           0.5
    Paraben mix                 0.4        0.3         0.4                 0.8        0.2           0.5
    p-tert-Butylphenol          1.1        1.0         1.1                 0.4        1.2           0.9
    formaldehyde resin
    Fragrance mix               1.1        1.0         1.1                 6.1        7.1           6.7
    Ethylenediamine             0.4        0.0         0.2                 0.8        0.7           0.7
    dihydrochloridee
    Quaternium 15               0.4        0.0         0.2                 0.0        0.0           0.0
    Nickel sulfate              2.2        11.1        6.7                 4.2        16.1          11.5
    MCI/MI                      0.4        1.0         0.7                 0.4        0.7           0.6
    (chloro-methyl- and
    methyl-isothiazolinone)

    Table 18.  (continued)
                                                                                                           

    Test substance              General populationb                        Dermatological clinicc
                                (% positive of tested)                     (% positive of tested)

                                Men        Women       Total               Men        Women         Total
                                (n=279)    (n=288)     (n=567)             (n=262)    (n=410)       (n=672)
                                                                                                           

    Mercaptobenzothiazole       0.4        0.0         0.2                 1.2        0.2           0.6
    Priminf                     -          -           NT                  0.4        1.5           1.0
    Thiomersalg                 3.6        3.1         3.4                 -          -             NT
    Carba mixh                  0.7        0.0         0.4                 -          -             NT
                                                                                                           

    a  Menné, unpublished (personal communication by T. Menné to the IPCS, 1997)
    b  Patch tested with the ready-to-apply TRUE test, Pharmacia (Sweden)
    c  Test substances from Hermal (Germany)
    d  Formaldehyde not included in TRUE test at the time of study
    e  Ethylenediamine dihydrochloride excluded from the European Standard series as of August 1992
    f  Primin not included in the TRUE test at the time of study
    g  Thiomersal not included in the European Standard series
    h  Carba mix excluded from the European Standard series as of January 1989

    Table 19.  Main allergens related to non-occupational exposure
                                                                                                      

    Allergens                                 Sources of exposure
                                                                                                      

    Domestic work

    Chromium                                  leather; footwear
    Colophony                                 shoe polish; crayons; plasticine; paper
    Flowers/plants                            gardening; house plants
    Nickel                                    nickel-plated objects
    Plastics/resins                           adhesives; paints; containers
    Preservatives                             cleaning agents; polishes; personal care products
    Rubber additives                          gloves; other rubber objects
    Wood                                      repairs; handicraft

    Hobbies and leisure time activities

    Chromium                                  leather; footwear
    Colophony                                 adhesive tapes; plasticine; paper; violin bow resin
                                              crayons; artists' paints; textiles
    Dyes/pigments                             gardening; house plants
    Flowers/plants                            textile resins; preservative in various products
    Formaldehyde                              nickel-plated objects
    Nickel                                    adhesives; paints; containers
    Plastics/resins                           paints; personal care products
    Preservatives                             gloves; sports equipment
    Rubber additives                          handicrafts
    Woods

    Cosmetics and personal care products

    Colophony                                 mascara.
    Dyes                                      hair dyes; miscellaneous cosmetics
    Fragrances
    Glyceryl thioglycolate                    permanent waving
    Lanolin

    Table 19 (cont'd)
                                                                                                      

    Allergens                                 Sources of exposure
                                                                                                      

    Paraphenylenediamine                      hair dyes; creams; lotions; shampoos;
                                              liquid soap, etc. (i.e., most cosmetic
                                              and personal care products)
    Preservatives, e.g.,
    formaldehyde releasers,
    isothiazolines parabens

    UV filters                                sunscreens

    Topical medicaments

    Antibiotics
    Antihistamines
    Antimicrobials
    Balsams
    Benzocaine
    Colophony
    Ethylenediamine
    Formaldehyde releasers
    Lanolin
    Parabens
    Preservatives
    Tars
                                                                                                      
    

    4.1.7  Treatment and prevention of allergic contact dermatitis

         The treatment of allergic contact dermatitis requires medical
    intervention. It usually involves the controlled use of emollients or
    corticosteroids as well as prevention of further exposure to the
    offending allergen (Wilkinson, 1995). A distinction is usually made
    between primary prevention, focusing on the induction of contact
    sensitization, and secondary prevention, focusing on the eliciting of
    contact sensitization. In many instances the preventive measures for
    the two different types overlap.

    4.1.7.1  Primary prevention

         In the 1960s an epidemic of contact dermatitis from dish-washing
    products occurred in Scandinavia. The epidemic was resolved by the
    concerted action of dermatologists and manufacturers. Extensive
    chemical analysis combined with animal predictive testing, identified
    highly sensitizing sultones to be present in some products (Magnusson
    & Gilje, 1973; Ritz et al., 1975). It was determined that these
    specific chemicals occurred as an impurity in the manufacturing
    process, when temperature control was not strictly maintained. The
    evaluation of the problem led to a solution, and there have been no
    recurrences.

         There are examples of exposure to hapten concentrations being
    legally regulated in an attempt to prevent contact sensitization
    (Hjorth & Menné, 1990). There is a complex European Union regulation
    on cosmetic products, forbidding certain substances and regulating
    others, i.e., preservatives, by a concentration limit (Council of the
    European Communities, 1976).

         Since the 1950s, chromate in cement has been know to be one of
    the main causes of allergic chromate dermatitis among construction
    workers. At the start of the 1980s the Scandinavian countries added
    ferrosulfate at a low concentration to cement to reduce the hexavalent
    chromate to trivalent chromate. The idea of this initiative was that
    the trivalent chromate is not absorbed, or only to a minor degree,
    through human skin, and therefore the risk of primary sensitization
    from this salt is significantly less than from hexavalent chromate.
    Epidemiological studies on construction sites performed at the
    beginning of the 1980s and at the end of the 1980s in Denmark,
    strongly suggest that this measure has been successful, as the
    frequency of allergic chromate dermatitis has been reduced in Denmark
    (Avnstorp, 1992).

         Nickel is a common contact allergen on a global scale. This
    allergy is caused by intimate skin contact with metal alloys,
    releasing nickel when exposed to human sweat. Under simulated use
    conditions, some alloys release high amounts and other alloys low
    amounts of nickel (Lidén et al., 1996). Based on such research, some
    Scandinavian countries have introduced regulations and quality
    criteria for nickel alloys intended to be in prolonged skin contact.

    It is believed that such measures might reduce significantly the
    frequency of nickel allergy in the population. Regulation of nickel
    exposure along similar lines has been adopted within the European
    Union (Council of the European Communities, 1994).

         In considering different glove materials to protect against skin
    irritation and mechanical skin damage, it should be noted that most
    small sensitizing chemicals rapidly penetrate most rubber and plastic
    gloves, and appropriate gloves should therefore be used (Estlander &
    Jolanki, 1988; Mellström et al., 1989; Roed-Petersen, 1989).

         There is no method of predicting an individual propensity to
    contact sensitization to a given chemical. When patch testing with
    strong sensitizing chemicals is performed, active sensitization from
    the test cannot completely be excluded. Pre-employment testing is
    therefore not a method of preventing contact sensitization.

    4.1.7.2  Secondary prevention

         The cornerstones of the secondary prevention of allergic contact
    dermatitis (elicitation of contact dermatitis) are based on sufficient
    diagnostic procedures and patient information systems. The
    availability of standardized patch test materials is essential.
    Furthermore, it is crucial that it is possible for the doctor to
    inform the patient where exposure to the specific allergen can be
    expected. Of course, it is even more crucial that the patient is able
    to understand and use the information over the following years to
    identify the allergen in the home and occupational environments. It
    seems obvious that this type of diagnostic follow-up will work, but it
    has only been evaluated in a limited number of studies. Edman (1988)
    found that the prognosis for patients sensitive to topical medicaments
    depended upon whether the patients were able to follow the doctor's
    advice on the occurrence of sensitizers in different products. Later
    studies have shown that patients with contact allergy to formaldehyde
    often continued to be exposed to formaldehyde (Cronin, 1991; Flyvholm
    & Menné, 1992). When a careful investigation was made, formaldehyde
    exposure could be demonstrated in nearly all the patients which seemed
    to be decisive for the prognosis of their hand eczema (Flyvholm,
    1997).

    4.1.7.3  Ways of preventing contact sensitization

         The following ways of preventing contact sensitization have been
    suggested.

    a)   replacement of certain chemicals or particular products;

    b)   regulation of exposure (concentration) to sensitizing
         chemicals, either general or in specific products, or during
         particular work processes;

    c)   optimal diagnostic and information systems; education of
         either groups or individuals;

    d)   individual oriented preventive methods; gloves, barrier
         creams, protective clothing.

         The problems of contact sensitization have been identified over
    many years, and different types of preventive measures have been
    tried. Some have been successful, but a number of chemicals still give
    problems to a significant number of people. Different strategies
    should be considered, whether it concerns common environmental
    chemicals or chemicals with rare specific exposures. Chemicals
    frequently used in both the domestic and occupational environment need
    to be regulated by society, either with suggestion of replacement or
    regulation of the exposure concentration. For rare chemicals it is
    often sufficient to focus on specific occupational processes and to
    educate the exposed individuals in no-touch techniques or introduce
    individually oriented preventive measures.

    4.1.8  Information needed for a preventative programme

         The prevention of allergic contact dermatitis should be based on
    preventing sensitization and, subsequently, on avoiding sufficient
    exposure to elicit a response in a person already sensitized. This
    requires information on the following aspects.

     a)  Occurrence of sensitizing substances

         Products used at work or domestically should be labelled to
    indicate the presence of substances capable of causing sensitization
    and their concentrations, so that the user may take appropriate
    precautions.

         If there are suitable alternatives there may be no need to use a
    sensitizing agent.

         At present, the potential of new substances to cause
    sensitization is determined from the results of tests on animals or
    sometimes on humans (Rycroft, 1995), after databases have been
    searched for relevant published information. Structure-activity
    relationships should be assessed and may give valuable indications of
    sensitizing potential for substances of a similar structure to known
    contact allergens.

         Comprehensive information about the composition of products and
    the allergenic activity of their ingredients should be collected in
    each country and be made available to health care professionals and
    users. This should include the results of surveys of standardized
    patch testing of humans so that trends in allergic sensitization can
    be followed.

     b)  Avoiding or minimizing exposure

         Induction of sensitization and eliciting an allergic disorder
    both follow dose-response relationships, albeit at very different
    concentrations.

         It is important to minimize initial exposure to sensitizing
    agents by restricting their availability or, if they cannot be
    avoided, by minimizing exposure. Exposure can be minimized by ensuring
    adequate ventilation and using personal protective equipment
    appropriate to the work situation or in the home, e.g., gloves, masks,
    etc. (see also chapter 7).

    4.2  Atopic eczema (atopic dermatitis)

    4.2.1  Definition

         Atopic eczema or atopic dermatitis is a chronic pruritic
    inflammatory skin disease characterized by a typical age-related
    distribution and skin morphology (Figs. 12, 13). The diagnosis of
    atopic eczema is based primarily on clinical grounds and the patient's
    history (Hanifin, 1983; Rajka, 1990). Onset at an early age, pruritus
    and excoriation, chronic or chronic relapsing course for more than
    6 weeks, age-related eczematous morphology and distribution, as well
    as a positive family history for atopic diseases (allergic bronchial
    asthma, allergic rhinitis and conjunctivitis or atopic eczema), form
    the most striking criteria. Together with allergic rhinitis and
    conjunctivitis and bronchial asthma, atopic eczema forms the classical
    triad of atopic diseases (Rajka, 1990; Ruzicka et al., 1991). Atopy
    can be defined as "familial hypersensitivity of skin and mucous
    membranes against environmental substances associated with increased
    IgE production and/or nonspecific reactivity" (Ring, 1991). This
    underlines two components held to be responsible for induction of this
    disease. Although it is genetically determined, environmental
    influences may play a role. During the last century many synonyms for
    atopic eczema/atopic dermatitis have been evolved, e.g.,
    neurodermatitis, prurigo Besnier, endogenous eczema and diffuse
    neurodermatitis (Ring, 1991). The diagnosis of this skin disease is
    based on clinical criteria, family history and/or demonstration of
    IgE-mediated sensitization.

    4.2.2  Epidemiology of atopic eczema

         Atopic eczema is a common disease among children and adults. In
    the 1950s the frequency of eczema was estimated to be between 1.1 and
    3.1% (Walker & Warin, 1956). In the 1980s and 1990s the frequency of
    atopic eczema was found to be up to 25% on the basis of questionnaires
    (Bakke et al., 1990) and up to 9.7% for dermatologically examined
    cohorts (Varjonen et al., 1992; Schäfer & Ring, 1997). Epidemiological
    studies on the prevalence of atopic eczema in Germany were conducted
    with questionnaire, physical and dermatological examination including
    allergy tests. In 1989-1991 8.3% of 988 Bavarian school children aged

    FIGURE 12


    FIGURE 13


    5 to 6 years suffered from atopic eczema (Schäfer et al., 1994), and
    in a study comparing eastern and western German areas in 1991 atopic
    eczema was diagnosed in 12.9% of 1086 pre-school children (Ring et
    al., 1995; Krämer et al., 1996; Schäfer & Ring, 1997). In studies in
    the United Kingdom, Denmark and Switzerland, the same methodological
    analyses were applied for a longer time interval to obtain figures on
    the changes in frequency of atopic eczema. These studies showed a
    dramatic increase in the prevalence of atopic eczema (Schäfer & Ring,
    1995). In the United Kingdom the figures for the prevalence of atopic
    eczema were 5.1% in 1946, 5.3% in 1964, 7.3% in 1958, and around 12%
    for 1970-1989. Similarly, in Denmark the prevalence in 1964-1969 was
    3.2% compared with 11.2% for 1970-1979. In Switzerland there was an
    increase from 2.2% in 1968 to 2.8% in 1981.

    4.2.3  Clinical manifestations and diagnostic criteria

    4.2.3.1  Age-dependent clinical manifestations

         In most patients with atopic eczema, the disease begins in
    infancy between 3 and 12 months of age (Hill & Sulzberger, 1935) as an
    erythematous, squamous or papulo-vesiculous inflammation, which may
    worsen to the point of exudation. It is often found on the face, the
    extremities (especially extensor aspects) and finally the trunk.
    Oozing and crusted lesions can often be found on the scalp (cradle
    cap). More and more, itching becomes an essential feature; the infant
    may be irritable, restless and tries to scratch the affected areas
    (after 3rd month of life). The course is chronically persistent or
    relapsing. Later, between 2 and 5 years, the appearance of the lesions
    changes. They become nummular and infiltrated. The localization
    changes and affects flexures of popliteal and antecubital fossae, the
    nape of the neck and the backs of the hands and feet. In severe cases
    there may be an involvement of the entire skin surface. Dry skin
    becomes another characteristic feature especially in the adult phase
    and creates itching followed by scratching. This may lead to severe
    excoriation with nodule formation and perpetuation of the inflammatory
    reaction ("Prurigo Besnier"). Chronic inflammation produces thickening
    (lichenification) of the skin, especially in flexural regions.

    4.2.3.2  Diagnosis of atopic eczema

         Many diagnostic systems have tried to collect reliable criteria
    for this disease. The features listed by Hanifin & Rajka (1980) are
    those referred to most often in the literature. A combination of a
    number of major and minor criteria allows the establishment of the
    diagnosis. A more simple selection of criteria for practical purposes
    has been proposed (Ring, 1991). Williams et al. (1994a) proposed a new
    arrangement of diagnostic criteria, primarily for epidemiological
    studies. However, it must be kept in mind that all these diagnostic
    systems have their drawbacks in this heterogenous disease. Clinical
    criteria, as well as the patient's history and presence of
    IgE-mediated sensitizations must be considered together and are the
    mainstay for establishing the diagnosis. However, minimal forms exist

    and sometimes do not meet the required criteria. Papular or nodular
    variants as well as localized forms (e.g., exfoliating cheilitis,
    infra-auricular rhagades, nipple eczema, finger pad or toe eczema)
    constitute minimal expressions of this disease (Wüthrich, 1991).
    Typical eczematous lesions may not only be triggered by IgE-mediated
    allergic reactions in patients with a positive family history of
    atopy, but can also be triggered by food additives. In most patients,
    establishing the diagnosis is not too difficult. In selected cases,
    clinical findings, history and IgE-mediated sensitization have to be
    regarded critically and all important differential diagnoses have to
    be ruled out thoroughly.

    4.2.3.3  Stigmata of the atopic constitution

         The diagnosis of atopic eczema often depends on further
    additional features. Stigmata of atopic constitution are prevalent in
    many patients with atopic eczema, although they are not specific for
    this disease. Dry skin, hyperlinearity of palms and soles,
    intraorbital fold, white dermographism, facial pallor, orbital
    darkening, low hairline and thinning of the lateral portions of the
    eyebrow are found more often in this group of patients (Przybilla,
    1991). They are typical constitutional markers, which may add another
    clue in establishing the diagnosis (Ring, 1988).

    4.2.3.4  Prognosis

         Variability and chronic relapses are characteristics of the
    course of atopic eczema. Atopic eczema most frequently begins during
    infancy (Hanifin, 1983; Rajka, 1990). In about two-thirds of infants
    with atopic eczema, the disease clears during childhood. In the
    remaining patients it persists into adult life. Minimal forms and
    stigmata of the disease often remain throughout life (Vickers, 1991).
    Sometimes atopic eczema starts only in adulthood. A definite prognosis
    about the course of an individual patient cannot be made; there is
    controversy about prognostic factors (Vickers, 1991).

    4.2.4  Etiology

         The manifestation of atopic eczema is subject to a multifactorial
    genetic predisposition as well as to environmental provocation
    factors.

    4.2.4.1  Genetic influence

         There is no doubt about the existence of a genetic component
    favouring the manifestation of atopic eczema (Schnyder, 1960; Küster
    et al., 1990). Twin studies show a concordance in homozygous twins of
    83 and 86%, compared to 28 and 21% in heterozygous twins (Niermann,
    1964; Schultz-Larsen, 1991). The chance of developing atopic eczema
    depends on the family history of atopy. Whereas about 10-15% of

    children without a family history of atopy develop atopic eczema, with
    a positive history of one parent the risk rises to 25-30% and, with a
    positive history of both parents, to 50-75% (Schultz-Larsen et al.,
    1986; Björksten & Kjellman, 1987).

    4.2.5  Environmental provocation factors

         The activity of atopic eczema can be influenced by a large number
    of environmental provocation factors (Table 20). These can either act
    specifically in the sense of individual hypersensitivity, primarily
    IgE-mediated allergy or, more often, as unspecific provocation factors
    irritating the skin or affecting emotional status. The question of the
    possible involvement of environmental atmospheric pollution in the
    increase in the prevalence of atopic eczema remains controversial (see
    section 3.3.2).

    4.2.6  Pathophysiology

         Although knowledge concerning components of the immune system and
    inflammatory responses in patients with atopic eczema has increased
    widely in recent decades, the pathophysiology of atopic eczema also
    remains controversial (Marchionini, 1960; Rajka, 1990, 1996).

    4.2.6.1  Dry skin

         Dry or rough skin is a major feature of skin alteration in
    patients with atopic eczema. Although a number of studies have
    investigated the pathophysiology of dry skin, there is no consensus
    (Melnik & Plewig, 1991; Lindskov & Holmer, 1992). An attractive
    hypothesis is that even clinically non-inflamed "dry skin" shows
    histologically a mild inflammatory infiltrate, and this is supported
    by skin biopsies in atopic patients (Uehara, 1991). There seems to be
    an intimate relation between dry skin, irritability and itch (Rajka,
    1990; Ruzicka et al., 1991).

    4.2.6.2  Autonomic dysregulation

         In addition to immunological abnormalities, signs of
    dysregulation of the autonomic nervous system have been described
    (Szentivanyi, 1968; Ring et al., 1988; Ring & Thomas, 1989; Hanifin,
    1993). Elevated phosphodiesterase activity in mononuclear leukocytes
    seems to correlate with increased IgE production and vasoactive
    mediator secretion (Butler et al., 1983; Cooper et al., 1985).

    4.2.6.3  Cellular immunodeficiency

         First described by Kaposi in 1895, patients with atopic eczema
    are more susceptible to infection with viruses (e.g.,  Herpes 
     simplex, Human papilloma) and bacteria (especially  Staphylococcus
     aureus) (Kaposi, 1895). Earlier reports about decreased frequencies
    of allergic contact sensitization in atopic eczema are contradicted by

    Table 20. Important environmental provocation factors in atopic eczema
    (adapted from Ring et al., 1996)
                                                                        

    Unspecific provocation factors:
                                                                        

         Irritants

         Microbial skin colonization or infection

            e.g.,  Staphylococcus aureus

                   Pityrosporum ovale

                   Herpes simplex (Eczema herpeticum)

         Psychological stress, emotional factors

    Specific provocation factors (individual hypersensitivity):

         IgE-mediated allergy

            e.g.,  Food

                   House dust mite

                   Animal dander

                   Pollen

                   Microbial colonisation?

         Contact allergy

         Pseudo-allergy (idiosyncrasy) and intolerance

            e.g.,  preservatives in foods

                   citrus fruits
                                                                        

    others who claim that the tendency to develop contact allergy is
    increased (Rajka, 1990). However, Enders et al. (1988) reported that
    the prevalence of positive patch test reactions for contact allergy in
    patients with atopic eczema was almost equal to that of patients with
    allergic contact dermatitis.

    4.2.6.4  Increased IgE production

         Serum IgE levels are elevated in the majority of patients with
    atopic eczema (Ogawa et al., 1971). They tend to correlate with the
    extent and severity of the disease (Johansson & Juhlin, 1970;
    Wüthrich, 1975). Specific antibodies can be measured against common
    environmental allergens (Rajka, 1990; Ruzicka et al., 1991). Although
    often the clinical significance of these antibodies is lacking, in
    some patients eczematous skin responses can be provoked by
    aeroallergens (grass pollen, house dust mite or animal dander), a
    procedure that has been called "atopy patch test" (Reitamo et al.,
    1986; Adinoff et al., 1988; Ramb-Lindhauer et al., 1990; Ring et al.,
    1991a,b; Platts-Mills et al. 1991; Vieluf et al., 1993). IgE
    antibodies to foods are frequently found in patients and may induce
    urticaria as well as eczematous reactions. Well-controlled clinical
    trials showed that in a high number of patients with atopic eczema,
    skin lesions were exacerbated after specific oral provocation with
    certain foods in double-blind studies (Sampson & Albergo, 1984). Apart
    from aeroallergens and foods, microbial allergens ( Staphylococcus 
     aureus, Pityrosporum ovale) might play a role. Chronic colonization
    of atopic skin could provide a continuing cause of allergen
    stimulation (Leyden et al., 1974; Ring et al., 1992, 1995; Neuber et
    al., 1995; Kröger et al., 1995). After allergen stimulation of
    IgE-bearing mast cells or basophils, the released vasoactive mediators
    (such as histamine, eicanosoids, etc.) might induce itching, and also
    eczema via a late-phase reaction (Dorsch & Ring, 1981). Langerhans
    cells in the epidermis express high affinity receptors for IgE as well
    as CD23 and IgE binding-protein (Bieber & Ring, 1992). Allergen
    contact might result in the generation of Th2-helper cells, a subset
    producing IL-4 and IL-5, thereby maintaining the allergic
    inflammation. Also other cell types might be involved in the
    inflammatory process; lymphocytes might act directly through
    cytokines, and eosinophils through release of pro-inflammatory
    mediators (Jakob et al., 1991; Kapp, 1995).

    4.2.6.5  Psychosomatic aspects

         It is well known from clinical experience that psychological and
    emotional factors can greatly influence the clinical course of this
    skin disease (Borelli & Schnyder, 1962; Jordan & Whitlock, 1972, 1974;
    Ring et al., 1986; Cotterill, 1991). There is no convincing evidence
    that psychological factors  per se are the primary cause for atopic
    eczema; however, it is clear that psychological factors may influence
    existing eczematous lesions or even trigger new exacerbations of
    eczema in many patients (Rajka, 1990; Ruzicka et al., 1991). For

    children, the family situation, e.g., the interaction between parents
    and the affected child, seems to be of particular importance (Ring et
    al., 1976; Niebel, 1995).

    4.2.7  Diagnostic approach

         In atopic eczema diagnosis not only comprises the identification
    of the disease but should also focus on individual provoking factors
    able to trigger disease activity (Ring et al., 1991a,b; Morren et al.,
    1994). Each patient may be susceptible to an individual set of
    provocation factors. Often, exacerbations can be prevented or the skin
    condition can be directly improved by avoidance of these factors (Ring
    et al., 1996). Diagnostic procedures used are intended to reveal
    provocation factors for the individual patient. Specific provocation
    of atopic eczema often is the result of an individual
    hypersensitivity. Although diagnostic tests normally differ from the
    natural exposure with allergens, they provide useful information in
    the hands of a trained allergist (Ring, 1988). Allergy diagnosis is
    based on the four foundations: the patient's history, skin tests,
     in vitro (laboratory) tests and provocation tests.

    4.2.7.1   Medical history

         The patient's history forms the backbone of allergy diagnosis.
    Often the patient notices associations between disease activity and
    specific conditions or actions (e.g., intake of foods, seasonal or
    daily variations, contact with animals, heavy pollen emission). These
    observations are very valuable in revealing individual provocation
    factors. On the other hand, positive allergy tests must always be
    verified for their clinical significance for the patient's disease by
    comparing them with the history.

    4.2.7.2  Skin tests

         Skin test methods are divided into percutaneous (skin-prick,
    intradermal) tests and epicutaneous (patch) tests (American Medical
    Association, 1987a). Percutaneous tests search for immediate-type
    IgE-mediated hypersensitivity and are especially indicated in atopic
    eczema. The skin-prick test (prick puncture test) has gained the
    widest acceptance because of its high convenience and safety (Dreborg,
    1989). A drop of the test extract is placed on the volar surface of
    the forearm and the solution is introduced into the epidermis with a
    disposable hypodermic needle. After 15 min the reactions are graded in
    relation to the erythema and wheal that are induced. In intradermal
    testing 0.02 to 0.05 ml of the test extract is injected intradermally
    with a syringe. Scratch tests (applying the extract to a superficial
    scratch) and rub tests (rubbing of the skin with native allergen) are
    other variants applied only for special indications. Because of the
    danger of producing anaphylactic reactions these tests should be

    performed only by trained allergists with experience in emergency
    treatment. In patients with atopic dermatitis, percutaneous tests are
    widely used for the detection of hypersensitivity against
    environmental aeroallergens and foods (Ring, 1988).

         Epicutaneous tests primarily focus on the detection of contact
    allergy by cell-mediated immunity. The extract is put in an aluminum
    chamber and fixed onto the skin of the patient for 48 h. The test
    reaction is graded after 48 and 72 h. An eczematous response is
    regarded as positive. Since it has been shown that in patients with
    atopic eczema, eczematous skin responses can be elicited by epidermal
    application with aeroallergens (especially the house dust mite),
    epicutaneous testing with the atopy-patch test is gaining wide
    acceptance (Adinoff et al., 1988; Vieluf et al., 1990; Darsow et al.,
    1995). Although the definite mechanism is still unknown, this test
    might fill the gap between IgE-mediated hypersensitivity and an
    eczematous response.

    4.2.7.3  Laboratory tests

         In the serum of patients with atopic eczema, hypersensitivity can
    be detected by laboratory methods. In atopic eczema the most important
    hypersensitivity reactions are thought to be IgE-mediated. IgE
    antibodies can be determined by binding to an allergen in a solid
    phase and radioactive, enzymatic or fluorometric labelling (Ring,
    1988). Specific antibodies against environmental allergens are
    detected by the RAST (Radio-Allergo-Sorbent Test) and expressed
    semi-quantitatively in different classes. Positive reactions must be
    interpreted with regard to their clinical relevance (Pastorello et
    al., 1989).

    4.2.7.4  Provocation tests

         Oral provocation tests and elimination diets are often necessary
    for the evaluation of the clinical relevance of a suspected food
    hypersensitivity (Przybilla & Ring, 1990). Also, allergy-like symptoms
    to food additives, medications, etc., may be produced by
    non-IgE-mediated mechanisms ("pseudo-allergy") (Vieluf et al., 1990).
    In these cases elimination diets and provocation tests are performed.
    Foods unlikely to produce adverse reactions can be screened by
    elimination diets or open challenges. Oral provocation by double-blind
    placebo-controlled food challenges is regarded as the "gold" standard
    for the diagnosis of food allergies (Sampson, 1983; Bruijnzeel-Koomen
    et al., 1995). However, there are pitfalls and problems with this
    procedure (Bindslev-Jensen, 1994a).

    4.2.8  Therapeutic considerations

         The disease can be effectively controlled by a combination of
    avoidance procedures, basic dermatological therapy and
    anti-inflammatory therapy for exacerbations (Przybilla et al., 1994;
    Ring et al., 1996). However, the patient has to accept that there is

    no simple therapy allowing permanent cure. The integration and active
    cooperation of the patient in the therapeutic concept ("patient
    management") is a prerequisite for an effective therapy. In atopic
    eczema, diagnostic and therapeutic approaches are intimately
    connected.

    4.2.8.1  Avoidance of provocation factors

         During the first year of life food allergies are frequent. Later,
    sensitization to aeroallergens becomes more important (Guillet &
    Guillet, 1992). Food allergies were found in 63% of children with
    extensive atopic eczema (Sampson, 1982).

         Eggs, cow's milk, wheat, seafood and nuts present the most
    important food allergens. Citrus fruits and preservatives in foods
    often affect patients via non-allergic mechanisms (Przybilla & Ring,
    1990). Individual provocation factors (hypersensitivity) have to be
    revealed by allergological diagnostic procedures. Therapy consists in
    the elimination of the relevant allergens from the diet. If extensive
    interventions are planned, the help of a dietitian is needed.
    Controversy exists about the value of prophylactic dietary
    manipulations. Exclusive breast feeding for six months, maternal
    avoidance of allergens during lactation, and delay of solid food
    feeding seem to have a protective influence in postponing or avoiding
    atopic eczema (Kajosaari & Saarinen, 1983; Arshad et al., 1992;
    Saarinen & Kajorsaari, 1995).

         Sensitization to aeroallergens is frequently found in older
    children and in adults. As shown by atopy patch tests, in some
    patients direct contact with house dust mite allergen, animal dander
    and pollen on intact skin results in eczematous skin lesions (Ring et
    al., 1991a,b; Darsow et al., 1995). In the case of a suspected allergy
    to house dust mites, reduction procedures should include encasing of
    bedding with impermeable synthetic material and removal of carpets and
    upholstered furniture (Platts-Mills & Chapman, 1987; Platts-Mills et
    al., 1991; Lau et al., 1995). When allergy to animal dander is shown,
    contact with the animal must be avoided. In case of exacerbation of
    atopic eczema due to aeroallergens, rehabilitation in
    aeroallergen-poor climates (sea level or high altitude mountains) has
    been recommended (Borelli, 1981). In patients with severe atopic
    eczema without adequate improvement of skin condition despite therapy,
    additional contact allergy should be suspected and excluded by
    epicutaneous (patch) testing.

         Furthermore, there are various nonspecific provocation factors
    influencing the disease activity in patients with atopic eczema. The
    skin of these patients is highly susceptible to irritants, such as
    wool, coarse fabrics, soap, detergents, frequent bathing,
    disinfectants, wet working conditions and others. Patients need to be
    educated about avoidance of these factors (Ring et al., 1996).

         Chronic microbial colonization of the skin (e.g.,
     Staphylococcus  aureus, Pityrosporum ovale) and superinfection are
    possible additional provocation factors and should be treated (Cooper,
    1994). Psychological factors such as stress are well-known triggering
    factors for a subgroup of patients. In these patients, psychosomatic
    intervention has been proven successful and psychosomatic approaches
    should be supported (Cotterill, 1991; Ehlers et al., 1995).

    4.2.8.2  Basic dermatological therapy

         In patients with atopic eczema there is a defective skin barrier
    against exogenous substances (Ruzicka et al., 1991; Schöpf et al.,
    1995). Regular basic therapy with emollients with or without addition
    of moisturizers and bath oils is needed for the treatment of the
    irritable dry skin to prevent the itch/scratch cycle.

    4.2.8.3  Anti-inflammatory therapy

         Recurrent relapses are a characteristic feature of atopic eczema.
    Anti-inflammatory therapy of exacerbations is aimed to control
    effectively disease activity and permit a return to basic
    dermatological therapy as soon as possible. Topical corticosteroids
    are the drugs of choice for acute exacerbations.

    4.2.9  Conclusion

         Atopic eczema is one of the most common skin diseases in many
    countries of the world with an increasing prevalence. Prevalence rates
    range between 10 and 20% of school children. Owing to the immense
    suffering caused by the skin disfigurement and the often unbearable
    itching, as well as the large number of people affected, it presents a
    major health problem. The role of allergy in this skin disease has
    been controversial but it has been shown that in the majority of
    patients, allergic reactions -- preferentially by IgE-mediated
    sensitization -- seem to play a clinically relevant role in eliciting
    and maintaining eczematous skin lesions.

    4.3  Allergic rhinitis and conjunctivitis

    4.3.1  Introduction

         Allergic reactions can occur in the respiratory tract and ocular
    conjunctiva. In the respiratory tract allergic reactions occur in:

    a)   the upper respiratory tract predominantly involving the 
         nose -  rhinitis;

    b)   bronchial airways -  asthma;

    c)   gas exchanging parts of the lung -  extrinsic allergic 
          alveolitis.

         Allergic reactions in the nose and airways are characterized by
    mucosal infiltration with eosinophils and T-lymphocytes, diseases now
    considered to be the manifestation of a local Th2-lymphocyte-dependant
    eosinophilic inflammation. In contrast, extrinsic alveolitis is
    characterized by granulomata and mononuclear cell inflammation within
    alveoli, centred upon bronchioles; the disease is considered to be the
    manifestation of a local Th1-dependant granulomatous inflammation.
    Both patterns of reaction are predominantly induced by agents
    suspended in the air, such as dust or fume particulates, aerosol
    droplets or vapour, inhaled into the respiratory tract. In general
    larger particles will be deposited and soluble chemicals dissolved in
    the upper respiratory tract; smaller particles (<5 µm aerodynamic
    diameter) and insoluble chemicals can penetrate into the gas
    exchanging parts of the lung.

    4.3.2  Definition

         Allergic rhinitis and conjunctivitis are common allergic
    inflammatory conditions induced by hypersensitivity to environmental
    allergens affecting the nasal (rhinitis) and/or conjunctival mucosa
    (conjunctivitis) (Mygind, 1986, 1989). Rhinitis, characterized by one
    or more of the symptoms of nasal congestion, rhinorrhea, sneezing and
    itching, is defined as the inflammation of the lining of the nose
    (International Rhinitis Management Working Group, 1994). The symptoms
    of allergic conjunctivitis consist of redness, lachrymation, itching
    and burning of the conjunctiva (Ring, 1991). There is an increased
    likelihood of the development of asthma in these patients.

    4.3.3  Clinical manifestations

    4.3.3.1  Seasonal allergic rhinitis and conjunctivitis (hay fever,
             pollinosis)

         Seasonal allergic rhinitis and conjunctivitis consists of
    paroxysms of sneezing, nasal itching, nasal congestion and rhinorrhea
    (Druce, 1993; Mygind, 1986). In severe cases the conjunctiva and
    mucous membranes of the Eustachian tube, middle ear and paranasal
    sinuses also may be involved. In these cases additional symptoms
    usually present with low-grade itching, lacrimation, burning,
    stinging, photophobia, redness and watery discharge, as well as ear
    fullness, ear popping and pressure over the cheeks and the forehead.
    This may be complicated by malaise, weakness and fatigue. Symptoms
    typically show a periodic distribution manifesting at individual time
    intervals during the pollen seasons of tree, grass and weed pollen
    between spring and autumn months. About 20% of patients have asthmatic
    symptoms as well (Smith, 1983). Food allergy, often manifesting as
    "oral allergy syndrome" due to cross-reacting allergens, may also be
    present (see section 4.5.2).

    4.3.3.2  Perennial allergic rhinitis and conjunctivitis

         In perennial allergic rhinitis and conjunctivitis, indoor
    allergens are the main cause of symptoms, which are similar to those
    of seasonal allergic rhinitis and conjunctivitis although nasal
    blockage is more pronounced and itching of the eyes is a common
    problem. Among the indoor allergens, house dust mites, cockroaches,
    animal dander and moulds are important. The chronic and persistent
    symptoms can present as a "permanent cold" and may be accompanied by
    secondary complaints, such as mouth breathing, snoring and sinusitis
    (Lucente, 1989). Occupational hypersensitivity to an airborne allergen
    at the workplace may lead to symptoms only during the week with a
    disease-free interval at weekends, for example in laboratory animal
    workers.

    4.3.3.3  Prognosis

         The peak prevalence of allergic rhinitis and conjunctivitis is in
    adolescents and young adults. The first manifestations of seasonal
    allergic rhinitis and conjunctivitis develop before 20 years of age in
    most patients. After 30 years of age, disease severity usually
    moderates and is only occasionally a problem in the elderly. Repeated
    exposure to allergens may cause nasal hyperreactivity also to other
    allergens, thus broadening the spectrum of hypersensitivity (Connell,
    1969). A proportion of patients will develop asthma in the course of
    their disease (Evans, 1993).

    4.3.4  Etiology

         Symptoms of allergic rhinitis and conjunctivitis are provoked by
    environmental aeroallergens. Typical seasonal allergens are tree
    pollen in the spring, grass pollen in the early and mid summer and
    weed pollen in the late summer. In temperate climates of the Northern
    hemisphere the most important tree pollens derive from birch, alder
    and hazel; among grass pollens timothy and ryegrass, and among weed
    pollens mugwort and ragweed are the most important. However, regional
    differences are also of importance, e.g., cedar pollen in Japan,
    parietaria pollen in the Mediterranean area and ragweed pollen in the
    USA being the most important allergens. Sometimes mould spores, e.g.,
     Cladosporium and  Alternaria, cause symptoms during summer and
    autumn months. In perennial allergic rhinitis and conjunctivitis
    mainly indoor allergens present in the environment throughout the year
    are relevant triggers. The house dust mites  Dermatophagoides 
     pteronyssinus  and  Dermatophagoides farinae, in Southern countries
     Bloomia tropicalis, animal dander from horses, cats, dogs and other
    pets, cockroaches, and moulds such as  Aspergillus species are the
    most important allergens.

         Epidemiological studies indicate a significant increase in the
    prevalence of allergic rhinitis and conjunctivitis. There is evidence
    that outdoor air pollution plays a role in the increasing morbidity

    from allergic rhinitis and conjunctivitis. The disease seems to be
    more common in urban than in rural areas (Broder et al., 1974a,b).
    There is evidence that air pollutants may interact directly with
    pollen with a possible impact on allergenicity (Behrendt et al.,
    1992).

         Beside exogenous factors, the association of allergic rhinitis
    and conjunctivitis with other atopic diseases, such as atopic eczema
    or asthma and a positive family history for atopy clearly demonstrates
    the genetically determined susceptibility (Coca & Cooke, 1923). 

    4.3.4.1  Allergic rhinitis and conjunctivitis caused by contact with
             chemicals

         Allergic rhinitis and conjunctivitis caused by contact with
    chemicals is less common than by contact with proteins. The prevalence
    is unknown. The scope of the problem is probably underestimated
    because of diagnostic failure (Mygind, 1986). The majority of cases
    reported in the literature are in association with occupational
    diseases. Upper respiratory tract hypersensitivity involving the nose
    often coexists with asthma, conjunctivitis, bronchitis, and
    occasionally with contact dermatitis, allergic alveolitis or fever.
    Occupational chemicals may be haptens, allergens, mediator- releasing
    or pharmacological agents and irritants. Eliciting agents that
    sometimes are shown to induce an immediate-type IgE-mediated
    hypersensitivity include anhydrides, metallic salts, dyes,
    diisocyanates and antibiotics. In many, but not all, workers with
    trimellitic acid-induced rhinitis and asthma, specific IgE antibodies
    and positive skin tests can be found, suggesting Type I and Type III
    allergic mechanisms (Bernstein et al., 1982a). In isocyanate workers
    with rhinitis, conjunctivitis, asthma, bronchitis, chronic obstructive
    lung disease, cutaneous reactions or fever, 26% had positive
    skin-prick tests and in 14% specific IgE antibodies could be detected
    after conjugation of isocyanates with serum albumin (Baur et al.,
    1984). In the majority of cases with occupational rhinitis,
    conjunctivitis and asthma caused by platinum salts, a Type I
    hypersensitivity was proved by skin tests,  in vitro histamine
    release and passive cutaneous anaphylaxis (Schultze-Werninghaus et
    al., 1978). In textile workers exposed to reactive dyes, who had
    respiratory complaints, skin-prick tests and patch tests were positive
    (Alanko et al., 1978; Estlander, 1988). It is thought that these small
    molecule chemicals are haptens that combine with proteins to form
    antigenic determinants.

         Symptoms caused by chemicals may also be due to a contact-allergy
    and delayed-type hypersensitivity. This applies more often for ocular
    allergy. Rubbing of the eyes after handling detergents or other
    chemicals may provoke a contact conjunctivitis. Positive patch tests
    are found to chemicals such as antibiotics, thiomersal, benzalkonium
    chloride, solutions for contact lenses, and metallic salts. In these
    cases a Type IV hypersensitivity seems to be the primary allergic
    mechanism.

         However, allergies have to be differentiated from toxic and
    irritative mechanisms. Strongly toxic chemicals may elicit symptoms by
    directly damaging the mucosa after single contact. Milder irritants,
    such as sulfur dioxide, urea formaldehyde, detergents, solvents or
    dusts may cause hyperreactivity after repeated (cumulative) contact.
    Exposure to cotton defoliants causes asthma, rhinitis and
    conjunctivitis, which is thought to be a result of direct histamine
    release. It is important to note that there is often an overlap
    between allergic and irritative processes. Hyperreactivity to
    irritants occurs predominantly after repeated contact in patients with
    pre-existing atopic diseases, with or without an allergic basis.
    Chemicals are often not only irritants but also allergens.

    4.3.5  Pathophysiology

         Allergens transported by the air come into contact with the
    mucosal surface. Contact with mast cells or basophils leads to
    IgE-dependent activation and degranulation of mast cells. Preformed
    mediators stored in the granules (e.g., histamine, tryptase) are
    released rapidly and elicit immediate symptoms. Other mediators are
    eluted slowly (e.g., heparin) or are synthesized  de novo (e.g.,
    prostaglandins, leukotrienes) (Bachert et al., 1995). Afferent nerve
    stimulation may provoke an axon reflex, and the release of
    neuropeptides (substance P, tachykinins) may amplify this reaction
    (Barnes et al., 1991). Mediators that are released slowly induce a
    late- phase reaction after 6 to 12 h, which results in local
    accumulation of inflammatory cells including CD4+ T-lymphocytes,
    eosinophils, basophils and neutrophils (Dvoracek et al., 1984). These
    cells and mast cells release cytokines and proteins (e.g., eosinophil
    basic proteins) that perpetuate the reaction (Bachert et al., 1995).
    Inflammatory cytokines (e.g., IL-4) may selectively recruit
    eosinophils by increasing the expression of adhesion molecules on the
    vascular endothelium (VCAM-1, ICAM-1).

         The late-phase reaction results in an increased
    hyper-responsiveness, which may be specific for an allergen
    ("priming") or nonspecific to a variety of irritant triggers (Connell,
    1969).

    4.3.6  Diagnostic techniques

         Diagnostic techniques are applied for differential diagnosis and
    verification of a definite diagnosis. The patient's history, physical
    examination with rhinoscopy and allergy testing represent the basic,
    readily accessible diagnostic techniques. Rhinomanometry with
    assessment of nasal resistance and nonspecific provocation tests
    demonstrating hyperreactivity of the nasal mucosa are also often used
    for evaluation of clinical relevance (International Rhinitis
    Management Working Group, 1994).

    4.3.6.1  Medical history

         A careful history of seasonal and/or perennial symptoms provoked
    by specific exogenous factors is most important for the diagnosis of
    allergic rhinitis and conjunctivitis. The conditions that precipitate
    or aggravate symptoms should be asked for in detail. In particular,
    the presence of allergens in the patient's environment and the
    possible causal relationship to the symptoms should be evaluated.
    Exposure factors, such as contact with air pollutants, automobile
    exhaust emissions or detergents, a history of atopic diseases and the
    family history provide further important information. The severity of
    the disease may be estimated by the frequency, distribution and
    severity of symptoms. Standardized questionnaires are useful in
    obtaining detailed information.

    4.3.6.2  Clinical examination

         Special devices are usually unnecessary for examination of the
    eyes, whereas rhinoscopy is obligatory for the examination of the
    nose. The use of indirect laryngoscopy and full endoscopic
    ear-nose-throat examination are not mandatory, but may be of value in
    special patients (International Rhinitis Management Working Group,
    1994). The nasal mucosa is usually reddened, oedematous and produces
    large quantities of a clear mucous discharge. The periorbital tissues
    may be oedematous. Cyanosis, conjunctival injection, increased
    lacrimation and mucous discharge of the eyes are further symptoms. The
    quality and quantity of the secretions should be noted.

    4.3.6.3  Allergy testing

         Immediate hypersensitivity skin tests (skin-prick test,
    intracutaneous test) are the primary diagnostic tool, skin-prick tests
    being the method of choice for the majority of cases (Dreborg, 1989;
    Ring, 1991). Skin testing with commercially available aeroallergens
    generally has a high reliability. The number of skin tests that should
    be performed is confined to a few common environmental allergens
    tested routinely but should be extended specifically if the individual
    patient's history indicates a role of other allergens.

         The determination of total serum IgE is of limited value for this
    disease but tests for specific IgE antibodies (e.g., RAST) are useful.
    Positive results of the skin-prick test and determination of specific
    IgE antibodies (sensitizations) should always be evaluated in
    combination with the patient's history. Nasal and conjunctival
    challenges with commercially available allergens should be used
    whenever the clinical relevance of a sensitization to an allergen
    cannot otherwise be estimated. However, there is no universally
    accepted standard for this technique. As all  in vivo tests are
    potentially dangerous, with the risk of anaphylaxis, tests should be
    carried out only by personnel trained in cardio-pulmonary
    resuscitation.

    4.3.7  Therapeutic considerations

         The therapeutic repertoire of antiallergic therapy includes
    environmental control to minimize exposure to the allergen responsible
    for provoking symptoms, symptomatic medications, and immunotherapy
    under strict medical supervision (Druce, 1993).

    4.4  Clinical aspects of allergic asthma caused by contact with
         chemicals

    4.4.1  Introduction

         Asthma is by far the most frequently reported outcome of
    an allergic respiratory reaction to inhaled chemicals, primarily
    occurring as the consequence of exposures experienced at work, i.e.,
    occupational asthma. Allergic rhinitis is generally caused by the same
    agents and may occur in isolation or in association with asthma.

    4.4.2  Importance of occupational asthma

         The contribution of occupational causes to the prevalence of
    asthma in the community is not generally known. Estimates in different
    countries have varied between 2% and 15% but their basis is not
    secure. In Spain, occupational causes accounted for between 1 in 15
    and 1 in 20 of cases of asthma in young Spanish adults aged between 20
    and 44 years. Information in the United Kingdom is limited to the
    numbers awarded compensation and the number of cases reported to
    voluntary surveillance schemes, both of which are likely to
    underestimate the true frequency of the disease.

         In the United Kingdom a surveillance scheme for work-related
    diseases (SWORD) with voluntary reporting of new cases of occupational
    lung disease by respiratory and occupational physicians reported 2101
    new cases in 1989 of which 554 (26%) were asthma. The agents most
    frequently reported to cause occupational asthma were isocyanates,
    which accounted for 22% of cases, and grain, wood dusts and laboratory
    animals, which together accounted for a further 17% of cases. The
    annual incidence rate for occupational asthma in the working
    population was estimated to be 22 per million. The highest rates in
    the occupational groups occurred primarily among those encountering
    chemicals at work, i.e., coach and spray painters, chemical
    processors, plastics making and processing, metal making and treating,
    and welders (Table 21).

         The incidence reported in this survey is lower than that reported
    in Finland by Meredith & Nordman (1996); Finland is one of the few
    countries where occupational lung diseases are registered. The
    incidence in 1981 of occupational asthma in Finland was estimated to
    be 71 per million (compared to the rate in the United Kingdom of 22
    per million). However, within the United Kingdom there was
    considerable regional variation in reported rates, and the area of
    highest incidence, West Midlands Metropolitan Area, had a rate of 63

    per million, similar to the reported incidence in Finland. Meredith &
    Nordman (1996) suggested that the differences in regional rates might
    at least in part be due to differences in ascertainment and reporting,
    and that the true incidence of occupational asthma in the United
    Kingdom was three or more times that reported.

    4.4.3  Chemical causes of occupational asthma

         Many different chemicals encountered at work can stimulate a
    hypersensitivity response and cause asthma. The more prevalent causes
    are shown in Table 22.

    4.4.3.1  Isocyanates

         Diisocyanates are bifunctional molecules used commercially to
    polymerize polyglycol and polyhydroxyl (polyols) compounds to form
    polyurethanes. Because each diisocyanate molecule has two reactive
    isocyanate (NCO) groups, they link adjacent polyols to form a
    three-dimensional lattice. Isocyanates also react with water to evolve
    carbon dioxide, a reaction exploited in the manufacture of flexible
    polyurethane foam. The urethane reaction is exothermic and the heat
    generated sufficient to evaporate diisocyanates with high vapour
    pressures, such as toluene diisocyanate (TDI) and hexamethylene
    diisocyanate (HDI). Diphenyl methane diisocyanate (MDI) and
    naphthalene diisocyanate (NDI), whose vapour pressures are lower,
    evaporate in significant amounts when heat is applied.

         It is estimated that approximately 5% of workers regularly
    exposed to TDI develop asthma, which may be manifested as immediate
    and/or late onset responses. TDI can act as a direct irritant, can
    stimulate nerve reflexes, and, in the minority of patients, elicit an
    IgE antibody response and occasionally an IgG response (Baur &
    Fruhmann 1981; Baur et al., 1994). In addition, persistent activation
    of T-cells and continuous expression of pro-inflammatory cytokines
    seems to maintain a state of chronic inflammation (Maestrelli et al.,
    1995).

         Polyurethanes have widespread applications, and exposure to
    isocyanates occurs in many different occupations. These include the
    manufacture of flexible and rigid polyurethane foam, the application
    of two part polyurethane paints by brush and by spray painting, and in
    flexible packaging production where isocyanates are used in inks and
    as laminating adhesives.

        Table 21. Incidence of occupational asthma in high-risk occupational
    groups reported to the United Kingdom Surveillance of Work-Related
    and Occupational Respiratory Disease Project (SWORD) in 1989
    (Meredith, 1993)
                                                                                  

    Occupational Group                 Cases      Population    Incidence/106/year
                                                                                  

    Coach and spray painters           35         54 737        639
    Chemical processors                31         73 189        424
    Bakers                             29         70 839        409
    Plastics making and processing     27         66 005        409
    Metal making and treating          14         56 270        249
    Laboratory technician and          26         127 478       204
    assistant
    Welders/solderers electronic       35         220 068       159
    assemblers
    Working population                                          22
                                                                                  

    Table 22.  Examples of occupational chemical respiratory allergens associated
    with positive bronchial provocation challenges
    (adapted from Karol et al., 1996)
                                                                                    

    Isocyanates                               Dyes
                                                                                    

    Diphenylamine-4,4'-diisocyanate (MDI)     Brilliant orange GR
    Hexamethylene diisocyanate (HDI)          Carminic acid
    Isophorone diisocyanate (IPDI)            Reactive orange 3R
    Naphthalene-1,5-diisocyanate              Rifafix red BBN
    Toluene 2,4-diisocyanate (2,4 TDI)        Rifazol black GR
    Toluene 2,6-diisocyanate (2,6 TDI)

    Amines                                    Acid anhydrides

    Dimethyl ethanolamine                     Phthalic anhydride
    Ethanolamine                              Tetrachlorophthalic anhydride 
    Ethylenediamine                           Trimellitic anhydride
    Triethylenetetramine

    Others

    Abietic acid                              Glutaraldehyde
    6-Aminopenicillanic acid                  Iso-nonanoyl sulfonate oxybenzene
    7-Aminocephalosporanic acid               Methyl-2-cyanoacrylate
    Ampicillin                                alpha-Methyldopa
    Azocarbonamide                            Phenylglycine acid chloride

    Table 22 (continued)
                                                                                    

    Isocyanates                               Dyes
                                                                                    

    2-(n-Benzyl-N-tert-butylamino)4'-hydroxy  Piperacillin
    3'-hydroxymethylacetophenone diacetate    Piperazine
    Benzylpenicillin                          Plicatic acid
    Cephalexin                                Spiramycin
    Chlorhexidine                             Styrene
    Complex platinum salts
    Ethyl cyanoacrylate                       Tylosin
    Natural rubber latex
                                                                                    
    
         Inhaled isocyanates have been reported to cause four different
    respiratory reactions:

    a)   Toxic bronchitis and asthma caused by isocyanate inhalation
         at toxic concentrations. Exposure to TDI at an atmospheric
         concentration of 0.5 ppm (3.6 mg/m3) causes irritation of
         mucosal surfaces - eyes, nose and throat (Henschler et al.,
         1962). Persistent asthma and reactive airways dysfunction
         syndrome (RADS) has been reported following a single inhalation
         of TDI at toxic concentrations (Luo et al., 1990).

    b)   Bronchial asthma caused by sensitization to isocyanates.

    c)   Accelerated decline of forced expiratory volume in 1 second
         (FEV1). The rate of decline of FEV1 in an isocyanate
         manufacturing plant workforce was similar in non-smokers with
         high cumulative exposures to toluene diisocyanate (TDI) to the
         rate observed in smokers in both the high- and low-exposure
         groups. The rate in non-smokers with low cumulative exposure was
         not different from that expected for control non-smokers. No
         additive effect of TDI with smoking was observed (Diem et al.,
         1982).

    d)   Extrinsic allergic alveolitis, which has been reported
         particularly in workers exposed to MDI (Zeiss et al., 1980) and
         also to HDI (Malo et al., 1983).

         Of the four reactions, bronchial asthma caused by
    hypersensitivity to isocyanates has been the most frequently reported
    and is the most important both in terms of prevalence and morbidity.
    TDI and MDI have been the most widely used isocyanates and are the
    major causes of asthma, although, with its increasing use in spray
    paints, HDI is becoming a more prevalent cause. A study of workers
    employed at a new TDI manufacturing plant identified 12 workers (4% of
    the total workforce) who had developed asthma during a 5-year period,

    with 9 developing it in the first year of employment. The average
    exposure to TDI monitored by paper tapes was 0.002 ppm (14 µg/m3)
    (Weill et al., 1981). Half of the cases had been exposed to spills;
    six were maintenance workers, one was a laboratory worker and only
    five were process workers. A cross-sectional study of a steel coating
    plant, where TDI had been introduced into the process some years
    before, identified 21 cases of asthma out of a total of 221, which was
    probably an underestimate of the true number of cases (Venables et
    al., 1985a).

         Inhalation challenge tests with TDI have shown that asthmatic
    responses may be provoked in sensitized workers by very low
    atmospheric concentrations, as low as 0.001 ppm (7 µg/m3) (O'Brien et
    al., 1979). Late asthmatic responses provoked by isocyanates are
    associated with the development of an increase in nonspecific airway
    responsiveness (Durham et al., 1987), and cells recovered from
    bronchoalveolar lavage during a late asthmatic reaction provoked by
    TDI have an increased proportion of neutrophils, identifying an
    inflammatory response in the airways provoked by TDI (Fabbri et al.,
    1987).

    4.4.3.2  Acid anhydrides

         Acid anhydrides are low relative molecular mass chemicals used
    industrially as curing agents in the production of epoxy and alkyd
    resins and in the manufacture of the plasticizer dioctyl phthalate.
    Epoxy and alkyd resins have widespread applications as paints,
    plastics and adhesives. Six acid anhydrides, i.e., phthalic anhydride
    (PA) (Maccia et al., 1976), trimellitic anhydride (TMA) (Fawcett et
    al., 1977; Zeiss et al., 1977), tetrachlorophthalic anhydride (TCPA)
    (Howe et al., 1983), maleic anhydride (MA) (Durham et al., 1987;
    Topping et al., 1986), hexahydrophthalic anhydride (Moller et al.,
    1985) and himic anhydride (Bernstein et al., 1984), have been reported
    to cause occupational asthma. Inhalation tests with the causal acid
    anhydride provoked asthmatic responses, and specific IgE or IgG
    antibodies, or both, to the specific anhydride conjugated to human
    serum albumin were identified in the sera of the great majority of
    cases, although this was less frequent with maleic than with the other
    anhydrides. Zeiss et al. (1977) suggested that four separate clinical
    syndromes were caused by TMA, for which they proposed separate
    immunological mechanisms: i) toxic airway irritation; ii) immediate
    IgE-mediated rhinitis and asthma; iii) IgG-mediated late asthma with
    systemic symptoms ("TMA flu"); iv) pulmonary haemorrhage-haemolytic
    anaemia syndrome as the outcome of antibody binding to circulating red
    blood cells and to pulmonary vascular cells. The distinction between
    "immediate" and "late" asthmatic reactions with influenza-like
    symptoms and their different pattern of immunological response has not
    been consistently observed by other investigators. It seems more
    likely that asthma caused by acid anhydrides, including TMA, may be
    associated with specific IgE or IgG, or both, although specific IgE

    and IgG4 seem to be more associated with asthma and IgG with exposure
    (Forster et al., 1988). A relationship between HLA DR3 and the
    development of specific IgE to TMA and possibly tetrachlorophthalic
    anhydride (TCPA), but not phthalic anhydride (PA), has been reported
    (Young et al., 1995). The pulmonary haemorrhage haemolytic anaemia
    syndrome is real but rare. It has been reported in individuals exposed
    to hot trimellitic anhydride fume and may be the outcome of a toxic
    reaction to inhalation of trimellitic anhydride at very high
    concentration rather than a hypersensitivity reaction.

    4.4.3.3  Complex platinum salts

         The complex platinum salt ammonium hexachloroplatinate is an
    essential intermediate in the refining of platinum, a corrosion
    resistant metal used as a catalyst and in jewellery. Allergy to
    platinum salts in refinery workers was first reported in 1945 (Hunter
    et al., 1945). Subsequently, inhalation of ammonium
    hexachloroplatinate was shown to provoke asthmatic responses and to
    elicit immediate skin test responses in sensitized individuals (Pepys
    et al., 1972).

         The incidence of occupational allergy in the platinum refining
    industry was high in United Kingdom in the mid 1970s. In a cohort
    study of 91 workers who entered employment in a platinum refinery in
    the two years 1973 and 1974 (Venables et al., 1989), 22 developed
    respiratory symptoms and an immediate skin test response to ammonium
    hexachloroplatinate. The risk was greatest in the first year of
    employment and smoking was more important than atopy as a predictor of
    developing a positive skin test reaction.

    4.4.4  Diagnosis of occupational asthma

         Accurate and early diagnosis of cases of occupational asthma is
    important. Remission of respiratory symptoms and restoration of normal
    lung function, including nonspecific airway responsiveness, can follow
    avoidance of exposure to the specific initiating cause. Furthermore,
    chronic asthma is more likely to develop in those who remain exposed
    to the initiating cause after the onset of symptoms. However,
    avoidance of exposure frequently requires a change of employment.
    Accurate diagnosis is also essential if those whose asthma is not
    occupationally caused are to avoid being advised unnecessarily to
    change or leave their work.

         The diagnosis of occupational asthma requires:

    a)   differentiation of asthma from other causes of respiratory
         symptoms, in particular chronic airflow limitation and
         hyperventilation;

    b)   differentiation of occupational cause from non-occupational
         asthma;

    c)   differentiation of asthma initiated by an agent inhaled at
         work from pre-existing or incidental asthma exacerbated by
         nonspecific irritants, such as sulfur dioxide and cold air,
         inhaled at work.

         The diagnosis of occupational asthma is usually suggested by the
    history. It commonly occurs in an individual exposed at work to an
    agent recognised to cause occupational asthma and only develops after
    an initial symptom-free period when the patient has been exposed
    without symptoms to concentrations in air that now provoke asthma.
    Respiratory symptoms occur during the working week, may increase in
    severity as the week progresses, and improve during absences from
    work, at weekends or during holidays. The patient may also be aware of
    others who have developed similar respiratory symptoms at the place of
    work.

         Nonspecific stimuli provoke asthmatic reactions that usually
    occur within minutes of exposure to them and resolve within 1-2 h of
    avoidance of exposure. Where work-related respiratory symptoms are due
    to the provocation of asthma by a respiratory irritant encountered at
    work, the onset of asthma will often have preceded initial exposure to
    the irritant and the severity of asthma does not significantly improve
    when away from work. Nonspecific irritants such as organic solvents,
    which often have a characteristic and unpleasant smell, may also
    provoke a hyperventilation response, when difficulty with breathing is
    associated with symptoms that are consequences of a low pCO2, such as
    tingling of the fingers, headaches and dizziness.

    4.4.4.1  Investigation of causes of occupational asthma

         In the majority of cases a confident diagnosis of occupational
    asthma induced by an inhaled chemical can be made from knowledge of
    exposure at work to a recognised cause of occupational asthma and a
    characteristic history. Where possible, these should be supported by
    objective evidence from serial measurements of peak expiratory flow
    (PEF) or immunological tests or both. Inhalation testing is reserved
    for occasions when the results of these investigations do not provide
    an adequate basis for advice about future employment.

    4.4.4.2  Serial peak expiratory flow (PEF) rate measurements

         Asthma can be attributed with confidence to an agent inhaled at
    work where exposure to it in the work place reproducibly provokes
    airway narrowing. Repeated measurements of airway calibre, most
    conveniently made as PEF rates, need to be made during a period long
    enough to allow observation of the consistency of any changes and
    their relationship to periods at work. Measurements need to be made
    repeatedly during each day for a period of several weeks and the
    patient can take measurements and record the results. Self-recording
    of PEF measurements is now widely used. Patients are lent a peak flow

    meter and asked to record the best of 3 measurements of PEF made every
    2 h from waking to sleeping over a period of one month in the first
    instance. To allow sufficient time for lung function to recover from
    exposure to an agent inhaled at work, it is helpful if the month
    includes a period away from work which is longer than a weekend,
    ideally a one or two week holiday. Self-recording requires patient
    compliance and honesty. The measurements may be conveniently
    summarized to show the maximum, minimum and mean peak flow
    measurements for each day and differences between periods at and away
    from work observed. This method of patient investigation has proved,
    in the hands of those experienced in its use, to be reliable and a
    relatively sensitive and specific index of occupational asthma.

    4.4.4.3  Immunological investigations

         The demonstration of specific IgE antibody is a helpful, but not
    conclusive, criterion to establish the diagnosis of occupational
    asthma or rhinitis, because the presence of specific IgE antibody is
    not unique to clinically allergic individuals. Antigen-specific IgE is
    rare (5-15% of clinical cases). The presence of specific IgE antibody
    can be detected by  in vivo tests such as skin-prick test and  in
     vitro tests such as radioallargosorbent test (RAST) or enzyme-linked
    immunosorbent assay (ELISA). Other  in vitro tests, such as histamine
    release from basophils, are less standardised. When allergens are not
    available, histamine release from basophils can be an alternative to
    direct measurement of IgE.

         The application of immunological tests in the investigation of
    occupational asthma caused by inhaled chemicals has widened with the
    preparation of hapten protein conjugates suitable for immunological
    testing (e.g., acid anhydride and reactive dye-human serum albumin
    conjugates) and the development of reliable methods for identification
    of specific IgE antibody in serum. Complex platinum salts such as
    ammonium hexochloroplatinate can elicit immediate skin-prick test
    responses without the need for conjugation to human serum albumin.

         The value of such tests in the diagnosis of occupational asthma
    depends upon their sensitivity and specificity in populations exposed
    to the particular cause. An immediate skin-prick test response and
    specific IgE antibody identified by RAST to conjugates of the acid
    anhydride tetrachlorophthalic anhydride (TCPA) with human serum
    albumin (Howe et al., 1983) were found to be associated with cases of
    asthma in exposed populations and not simply a reflection of exposure.

    4.4.4.4  Inhalation challenge tests

         Inhalation tests are rarely undertaken because they are
    potentially hazardous. Inhalation tests with occupational agents
    should only be undertaken by those experienced in conducting them and
    who have adequate hospital facilities for continuous monitoring of
    patients for 24 h after each test.

         There are 4 major indications for inhalation challenge testing in
    the diagnosis of occupational asthma:

    a)   Where the agent thought to be responsible for causing asthma
         has not previously been reliably shown to do so.

    b)   Where an individual with occupational asthma is exposed at
         work to more than one potential cause, and his future employment
         depends on knowledge of which one is responsible.

    c)   Where asthma is of such severity that further uncontrolled
         exposure in the work environment is not justifiable.

    d)   Where the diagnosis of occupational asthma remains in doubt
         after other appropriate investigations, including serial peak
         expiratory flow rate (PEF) and immunological tests, have been
         completed.

         The aim in an occupational-type inhalation challenge test is to
    expose the individual under single blind conditions to the putative
    cause of the asthma in circumstances which resemble as closely as
    possible the conditions of the exposure at work. Wherever possible,
    atmospheric concentrations of the inhaled agent should be based on
    knowledge of the concentrations experienced at work, and the physical
    conditions of exposure, (e.g., size of dust particles, whether vapour
    or aerosol, and temperatures to which the materials are heated),
    should be similar to those encountered at work.

         Measurements of airway responses provoked by inhalation challenge
    tests should ideally include measurements of both changes in airway
    calibre and in nonspecific airway responsiveness. Changes in airway
    calibre are most conveniently measured by regular measurements of
    forced expiratory volume in 1 second (FEV1) and forced vital capacity
    (FVC) or PEF rate before and at regular intervals after the test, for
    at least 24 h. Changes in airway responsiveness can be made by
    estimating the concentration of inhaled histamine or metacholine that
    provokes a 20% fall in FEV1 (PC20) before the test and at 3 and 24 h
    after the test. The changes in airway calibre and nonspecific
    responsiveness observed are compared to those following a control
    challenge test, each test being made on a separate day.

         The patterns of change in airway calibre provoked by inhalation
    testing are distinguished by their time of onset and duration.
    Immediate responses occur within minutes and resolve spontaneously
    within 1-2 h. Such reactions can be provoked by both allergic (e.g.,
    grass pollen) and non-allergic (e.g., inhaled histamine or sulfur
    dioxide) stimuli. The response depends upon the concentration of the
    provoking agent and the degree of pre-existing nonspecific airway
    responsiveness. Lone immediate responses (i.e., an immediate response
    not followed by a late response) are not usually associated with an
    increase in nonspecific airway responsiveness. Late responses develop

    one or more hours after the inhalation test exposure, usually after
    some 3-4 h, and may persist for 24-36 h. Unlike the immediate
    response, late responses are often associated with an increase in
    nonspecific responsiveness which can be identified 3 h post test prior
    to the onset of the late asthmatic response and less reliably at 24 h
    after the test (Durham et al., 1987).

         A dual response is an immediate response followed by a late
    response. Recurrent nocturnal responses may be provoked by a single
    inhalation test exposure with asthmatic responses occurring during
    several successive nights with partial or complete remission during
    the intervening days. Such responses are almost certainly a
    manifestation of a provoked increase in nonspecific airway
    responsiveness. The question to be answered from the results of an
    inhalation test is whether or not in the individual case the
    particular agent inhaled at work has induced asthma. The most reliable
    means of answering this question is to determine whether or not
    inhalation of the specific agent at concentrations to which exposure
    occurs at work reproducibly provokes a non-immediate asthmatic
    response and increases nonspecific airway responsiveness. In such
    cases the specific agent can be considered to be the inducing cause in
    that particular individual. Nonspecific irritants may provoke
    immediate responses in individuals with hyper-responsive airways, but
    do not provoke either an increase in nonspecific airway responsiveness
    or a late asthmatic reaction. Agents that induce specific IgE
    antibody, however, may provoke lone asthmatic responses; in these
    cases inferences from the inhalation test result should take the
    immunological test into account.

    4.4.5  Outcome of occupational asthma

         Asthma induced by an agent inhaled at work may become chronic,
    persisting for several years, if not indefinitely, after avoidance of
    exposure to its initiating cause. This seems particularly, although
    not exclusively, to occur with asthma caused by low relative molecular
    mass chemicals. Asthma caused by isocyanates and acid anhydrides has
    been reported to have persisted in over half of cases. Six cases of
    asthma caused by the acid anhydride tetrachlorophthalic anhydride
    (TCPA) were followed up 4 years after avoidance of exposure: all had
    chronic respiratory symptoms consistent with persistent airway
    hyper-responsiveness and a measurable histamine PC20 was present in
    the five in whom it was assessed. The rate of decline of specific IgE
    to a TCPA-human serum albumin conjugate during the period of avoidance
    of exposure was parallel in all 6 subjects and exponential with a
    half-time of one year, making it very improbable their continuing
    asthma was caused by further, albeit inadvertent, exposure (Venables
    et al., 1987).

         Chronic asthma in these cases is likely to be a manifestation of
    persistent airway inflammation which, although initiated by the agent
    inhaled at work, persists in its absence. Ten patients with
    TDI-induced asthma, who had continuing respiratory symptoms and airway

    hyper-responsiveness, were investigated for 4-40 months from their
    last exposure. Bronchial biopsies obtained from eight patients showed
    basement membrane thickening with infiltration of the mucosa by
    eosinophils, lymphocytes and neutrophils. In four patients in whom
    airway responsiveness had not improved, the proportion of eosinophils
    in fluid recovered by BAL was increased, whereas this was the case in
    only one of five patients whose airway responsiveness had improved
    (Paggiaro et al., 1990).

    4.4.6  Management and prevention of occupational asthma

         Reduction in the incidence of occupational asthma will follow
    adequate control of exposure to its causes (Table 22). Substitution of
    a different paint for one containing TDI halted an epidemic of asthma
    in a steel coating plant (Venables et al., 1985a). Measures to secure
    control of exposure to the majority of causes of occupational asthma
    have, however, been impeded by lack of knowledge of the nature of
    exposure-response relationships for sensitizing chemicals.

         The development of control measures that will significantly
    reduce the incidence of occupational asthma requires examination of
    exposure-response relationships and the effects of interventions in
    longitudinal studies.

         Management advice for patients with occupational asthma has been
    greatly influenced by the results of studies of outcome of
    occupational asthma which have found evidence of continuing asthma and
    airway hyper-responsiveness despite many years avoidance of exposure
    to the initiating cause, and particularly studies, such as those of
    azodicarbonamide workers, that have identified a relationship between
    the duration of symptomatic exposure and the risk of chronic asthma.
    The importance of accurate identification of the specific cause cannot
    be over-emphasised. Avoidance of exposure often involves relocation or
    change of employment. Misdiagnosis of occupational asthma can be as
    hazardous for individual patients as missing the diagnosis because of
    the implications for employment.

         Rigorous prevention of exposure is a key to the prevention of
    occupational asthma. Patients who develop occupational asthma in whom
    a specific cause is identified should be advised to avoid further
    exposure to the cause of their asthma. This seems particularly
    important where low relative molecular mass chemicals, such as
    isocyanates, plicatic acid or acid anhydrides, are the cause as these
    seem particularly, although not exclusively, associated with the
    development of chronic asthma and airway hyper-responsiveness.

         However, environmental changes, unless they involve substitution,
    are often not able to reduce exposures sufficiently to prevent
    continuing airway responses in sensitized individuals. Venables &
    Newman Taylor (1990) examined the relationship between the
    concentration of TCPA in air and the provocation of asthmatic
    responses in inhalation challenge tests in 4 sensitized individuals.

    They observed a log-linear relationship between the magnitude of late
    asthmatic responses and TCPA concentration, which passed through the
    origin, suggesting no threshold.

         When an individual remains in employment exposed to the cause of
    the asthma, either directly or indirectly, the effectiveness of
    relocation or of respiratory protection needs to be monitored. This
    can be conveniently done by serial self recordings of peak flow to
    determine whether or not asthma persists and, if so, whether or not it
    is work related.

    4.5  Food allergy

    4.5.1  Definitions

          Food allergy is an adverse reaction to food occurring in
    susceptible individuals, which is mediated by a classical immune
    mechanism specific for the food in question. Therefore, the "true"
    allergic reaction to a food is caused by an over-sensitive reaction of
    the body's immune system (in essence, "immunity gone wrong").  Food 
     intolerances are all non-immune-mediated adverse reactions to food.
    The subgroups of food intolerance are  enzymatic (resulting from an
    enzymatic defect, e.g., lactase deficiency),  pharmacological 
    (depending on the direct effect of certain substances found in foods,
    e.g., caffeine) and  undefined food intolerance (Bruijnzeel-Koomen et
    al., 1995) (Fig. 14). Food intolerances will not be described further
    since, as far as is known, they do not involve immune mechanisms.

         The foods of animal origin that most commonly cause allergic
    reactions are milk, eggs, cod fish and shrimps. Those of plant origin
    that most often cause allergic reactions are peanuts, soybeans,
    celery, apple, hazelnuts and wheat.

         In food allergy the best established mechanism is the presence of
    IgE antibodies against the offending food (Type I allergy). Other
    immune mechanisms may be involved in other clinical patterns of food
    allergy. For example, Type III and Type IV mechanisms may play a role
    in the genesis of food protein-induced enterocolitis of newborns and
    infants.

         A subgroup of patients with contact allergy (T-cell mediated) to
    nickel or balsam of Peru can develop cutaneous symptoms (haematogenous
    contact eczema) to double-blind, placebo-controlled food challenge
    (DBPCFC) (Veien et al., 1987; Menné & Maibach, 1991). Foods with very
    high amounts of nickel or containing natural flavours, cross-reacting
    with balsam of Peru, may provoke skin symptoms.

         In the following text only adverse food reactions with a proven
    involvement of the immune system are covered.

    4.5.2  IgE-mediated food allergy

         Clinical manifestations of IgE-mediated food allergy can remain
    localized at the site of the primary direct contact, i.e., the mouth
    and throat  (oral allergy syndrome) or the gastrointestinal tract
     (isolated gastrointestinal food allergy). However, after ingestion
    of the specific food, food-allergic patients often exhibit symptoms in
    various organs: the skin, the respiratory tract, the gastrointestinal
    tract and the cardiovascular system  (systemic anaphylaxis) are the
    most frequently affected. Usually, the patients show involvement of
    two or more organ systems. Among 402 patients with a systemic
    IgE-mediated allergy to one or more specific foods -- the oral allergy
    syndrome was not included -- diagnosed over a 10-year period at the
    Allergy Unit in the Zurich University Hospital, the affected organ was
    most often the skin (46%), followed by the respiratory tract (25%),
    the gastrointestinal tract (20%) and the cardiovascular system
    (10%)(Wüthrich, 1993). Twenty percent of the food allergic patients
    had skin symptoms exclusively, 11% had isolated gastrointestinal
    manifestations and 8% isolated respiratory symptoms. In only 7% of all
    cases was food allergy responsible for a chronic condition such as
    chronic urticaria, perennial asthma or gastroenterocolitis.

    4.5.2.1  Oral allergy syndrome

         Mainly patients with pollinosis may describe, spontaneously or by
    exact questioning, itching of the lips, mouth, palate and throat
    immediately after intake of some fresh foods, namely fruits and
    vegetables. Hoarseness and/or swelling of the lips, tongue, uvula and
    larynx can occur infrequently. Oral allergy syndrome must be carefully
    differentiated from the beginning of a generalized anaphylactic
    reaction in which itching of the mouth and throat may be the first
    symptoms. These symptoms were described in the USA in
    ragweed-sensitive patients (6.2% in one series) after ingestion of
    melon and banana (Anderson et al., 1970; Ross et al., 1991).

         Cross-reactivity of pollen-sensitized subjects to various food
    allergens can occur (Fig. 15). Most patients allergic to birch pollen
    react to apples, hazelnuts and numerous other vegetables and fruits
    from the Rosaceae family such as cherries, peaches, pears and almonds
    (Eriksson et al., 1982; Dreborg & Foucard, 1983; Pauli et al., 1992) ;
    mugwort pollen sensitive patients may react to celery root (celeriac)
    and spices, the so called  mugwort-celery-spices-syndrome (Wüthrich &
    Hofer, 1984; Wüthrich et al., 1990). Grass pollen allergic patients
    may react to cereals or tomatoes (de Martino et al., 1988). It has
    been shown by prick, RAST studies and RAST inhibition experiments that
    a celery thermolabile allergen seems to be involved in
    celery-birch-pollen allergic patients whereas a thermostable allergen
    is involved in celery-mugwort-allergic patients (Wüthrich et al.,
    1990). Patients with chestnut and banana oral allergy syndrome may be
    sensitized to natural rubber latex (M'Raihi et al., 1991). Many
    patients are able to identify the offending fruit or vegetable.

    FIGURE 14

    Cooking often destroys the reactivity (Wüthrich et al., 1990). Shared
    epitopes and cross-reactive IgE antibodies are the most probable
    explanation for the observed clinical symptoms in cases of association
    between pollen allergy and food hypersensitivity (Calkhoven et al.,
    1987).

    4.5.2.2  Allergic reactions after ingestion of food

         The main symptoms of gastrointestinal food allergy are vomiting,
    nausea, diarrhoea and abdominal pains (colics or cramps) (Anderson,
    1981; Atkins et al., 1985a,b). Skin reactions include local or
    generalized pruritus, flush, urticaria, angioedema, morbilliform
    exanthema and flare-up of atopic dermatitis. The symptoms of the upper
    and the lower respiratory tract are rhinitis (sneezing, pruritus of
    the nose, nasal stuffiness, nasal obstruction), larynx oedema, cough,
    wheezing and bronchial asthma. Itching, redness and watering eyes
    (conjunctivitis) is often associated with the above symptoms, but can
    also appear as an isolated manifestation of food allergy.

         Systemic anaphylaxis (cardiovascular collapse) always involves
    other organ symptoms, e.g., of the gastrointestinal tract, the skin or
    the respiratory tract. A particular subtype is food-dependent
    exercise-induced anaphylaxis (Maulitz et al., 1979).

    4.5.2.3   Allergic reactions following skin contact with food

         Urticarial lesions can be provoked by contact with certain foods,
    such as fish, shrimps, flour and pork meat (Maibach, 1976). Chronic
    contact with food may induce protein contact dermatitis in food
    handlers (Hjorth & Roed-Petersen, 1976).

    4.5.3  Non-IgE-mediated immune reactions

         Immune complexes with IgG antibodies and milk antigen inducing
    complement-mediated damage have been suggested to induce Heiner's
    syndrome and haemorrhagic gastroenteritis in childhood (Heiner et al.,
    1962; Gryboski, 1967). Other non-IgE-mediated immune reactions to food
    include gluten-sensitive enteropathy (coeliac disease), food-induced
    colitis, and cutaneous allergic vasculitis (purpura).

    4.5.3.1  Gluten-sensitive enteropathy (coeliac disease)

         Gluten sensitive enteropathy or coeliac disease occurs in
    susceptible individuals. It is characterized by damage to the small
    intestinal mucosa with symptoms of malabsorption (Kagnoff, 1992). The
    prevalence of coeliac disease has been estimated to be 0.2-0.5% with
    considerable geographical variation (Logan, 1992).

         Affected individuals develop specific immunological reactions to
    gliadin, a protein that is a major alcohol-soluble fraction of gluten,
    present in wheat, rye, barley and oat. Humoral cell-mediated immunity
    and genetic factors seem to be involved in the pathogenesis.

         The peak incidence of symptoms is in infancy after the
    introduction of cereals, and a second peak occurs during the third
    decade. In children it is often a semi-acute disease. The symptoms of
    coeliac disease in children are recurrent abdominal pain, loose
    stools, anorexia, short stature and delayed puberty. Symptoms of
    malabsorption are unexplained nutritional deficiencies such as iron
    and folic acid deficiency, anaemia and rickets. Dental enamel
    hypoplasia and recurrent aphthae are also associated with coeliac
    disease.

         Patients with coeliac disease must avoid gliadins and related
    proteins for life. As little as 100 mg gliadin has been described as
    causing intestinal damage.

    4.5.4  Diagnosis of adverse food reactions

    4.5.4.1  Case history and elimination diet

         The diagnosis of food allergy or food intolerance is the result
    of a careful case history and clinical examination, and of several
     in vitro  and  in vivo diagnostic procedures. A record of food
    intake and its relation to clinical symptoms usually provides useful
    information in the case of acute reactions, occurring from a few
    minutes to a few hours after ingestion. History is, however, less
    reliable when symptoms are chronic and caused by food that generally
    is consumed daily, such as milk, egg, wheat and meat or by additives.
    An elimination diet based on rice, potatoes and mineral water over a
    week is useful in patients with chronic symptoms when a food allergy
    to other foods, hidden allergens, spices or additives are suspected.
    If there is no evident improvement of the symptoms after the diet
    period, the role of food is practically excluded. In case of suspicion
    of an enzymatic intolerance, appropriate laboratory tests must be
    performed, as well as careful gastrointestinal examinations including
    mucosal biopsies in the case of a chronic gastrointestinal disease.

    4.5.4.2  Skin tests

         Besides the case history, skin testing with various methods
    (skin-prick tests with commercial glycerol extracts, prick-prick or
    scratch tests with raw food or intracutaneous tests with aqueous
    extracts) with a panel of routine or selected food is the normal
    screening procedure for diagnosing food allergy (Metcalfe & Sampson,
    1990; Sampson & Metcalfe, 1992). However, the allergen source is of
    major importance and the standardization of commercial food extracts
    is not optimal. When reviewing the literature on sensitivity and
    specificity of skin-prick tests, as compared with the outcome of
    double-blind placebo-controlled food challenge (DBPCFC), widely
    variable results are found, depending on the allergen source
    (commercial extracts) and the different food items (Sampson & Albergo,
    1984; Atkins et al., 1985a,b; Pastorello et al., 1989). Using fresh or
    raw food, e.g., raw milk, apple, celery, carrot, increases the
    sensitivity of skin-prick tests (Ortolani et al., 1989). A typical

    FIGURE 15

    case history (e.g., oral allergy syndrome) or a severe,
    life-threatening immediate Type I reaction after ingestion of a
    defined food (e.g., apple or peanut), supported by a clear positive
    specific skin test with the suspected food, establishes the diagnosis
    (Metcalfe & Sampson, 1990). The demonstration of IgE antibodies
    towards the culprit food underlines the IgE-mediated pathogenesis
    (Johansson et al., 1984).

    4.5.4.3  Specific serum IgE

         The determination of food-specific serum IgE antibodies with
    different techniques (e.g., RAST = Radio-Allergo-Sorbent-Test, ELISA =
    Enzyme-Linked Immuno-Sorbent-Assay, FEIA = Fluoro-Enzyme Immuno-Assay)
    has become a routine diagnostic tool in many Allergy Centres and among
    practitioners (Johansson et al., 1984). However, a positive IgE
    determination as well as a positive skin test do not mean actual food
    allergy, but only a food sensitization. Cross-reactive IgE antibodies
    to inhaled allergens (birch or mugwort pollen, cat dander, cow
    epidermis) (Aalberse et al., 1981; Calkhoven et al., 1987) or to
    botanically related food (e.g., legumes and peanut, soy and chickpea)
    can be observed in negative challenge tests (Bernhisel-Broadbent &
    Sampson 1989; Bernhisel-Broadbent et al., 1989). On the other hand,
    some food allergic patients who show positive results in skin tests
    have negative results in RAST. Like skin testing, the diagnostic value
    of IgE determination is hampered by the lack of standardized food
    extracts. Also the cut-off limit of IgE determination, i.e., the
    minimal value in kU/litre or class, that should be considered
    clinically relevant is still a matter of debate.

    4.5.4.4  IgG determination

         Specific IgG antibodies against food (Wüthrich, 1990) can be
    found in many different physiological and pathological conditions.
    Their determination does not prove the existence of a clinically
    relevant immune reaction (Dannaeus & Inganäs, 1981).

    4.5.4.5  Other in vitro tests

         The Histamine Release Test (HRT) (Clinton et al., 1986) and the
    Cellular Allergen Stimulation Test (CAST) (de Weck et al., 1993),
    which determine the histamine or the sulfidoleucotrienes release from
    basophil leucocytes in the peripheral blood, are of considerable
    scientific interest but are too complicated and time-consuming for
    daily routine use.

    4.5.4.6  Oral challenge tests

         At present, double-blind, placebo-controlled, food challenge
    (DBPCFC) is considered as the "gold standard" for diagnosis of adverse
    reactions to foods (Bernstein et al., 1982b; Bock et al., 1988).
    Although several procedures for performing DBPCFC have been developed,
    their application in normal clinical practice is hampered by their
    heavy demands on resources and time.

         The use of DBPCFC is necessary to assess objectively food
    allergy/food intolerance, because several studies have demonstrated a
    tremendous discrepancy between subjective perception of food
    allergy/food intolerance and the results of DBPCFC. However, there are
    difficulties in conducting studies of this nature in large population
    samples. For example, challenge tests are dangerous with the risk of
    severe anaphylactic reactions in the case of positive skin-prick tests
    and/or positive serum IgE (RAST/CAP) against the offending food.
    Moreover, there may be several potential hazards in the procedure and
    in the interpretation of food challenges (Bindslev-Jensen et al.,
    1994b), namely:

    a)   The nature of the food being tested (raw, freeze-dried,
         cooked, food allergen extract).

    b)   The amount of food necessary to provoke objective signs.

    c)   DBPCFC may bypass important sites, e.g., mouth when using
         capsules.

    d)   Additive or synergistic effect of multiple
         hypersensitivities (e.g., concomitant sensitization to spices or
         inhaled allergens such as pollen) or trigger factors (e.g.,
         drugs, aspirin, alcohol, exercise, stress).

    e)   If the patient only reports subjective symptoms, e.g., itch,
         headache, it may be necessary to increase the number of challenge
         tests (three active and three placebo provocations) to avoid
         false positive reactions and to obtain statistical significance
         (Young et al., 1994).

         In clinical practice it is often convenient to apply an open or a
    single-blind food challenge: a negative test is of high predictive
    value. A positive test, with the presence of objective signs (e.g.,
    urticaria, asthma) or laboratory markers, increase of serum histamine,
    tryptase or ECP (Eosinophil Cationic Protein) levels after challenge,
    is usually sufficient to verify the diagnosis.

    4.5.5  Therapeutic considerations

         The only proven therapy for food allergy is to avoid the allergen
    causing the disease. To be able to avoid, for instance, wheat flour,
    milk or egg in the daily diet, professional advice is necessary both
    to avoid hidden allergens in processed food and to ensure that the
    diet is nutritionally adequate.

    4.5.6  Prevalence

    4.5.6.1  Introduction

         It is possible to make more or less well-documented estimates of
    the prevalence of different adverse reactions to foods. Frequency and
    duration of breast feeding, eating habits, and flora (e.g., birch
    trees) are factors influencing the prevalence. It is not known whether
    the prevalence of food allergy is increasing. The prevalence of pollen
    rhinitis is increasing (Wüthrich et al., 1995), and it is possible
    that the prevalence of pollen-related food allergies may also be
    rising.

    4.5.6.2  Children

         In many countries cow's milk is the first food allergen newborns
    meet. The prevalence of cow's milk allergy is 2-5% in one-year-old
    babies (Jakobsson & Lindberg, 1979; Bock, 1987; Host et al., 1988;
    Hill & Hosking, 1997). This figure declines rapidly during the first
    three years of life (Host & Halken, 1990).

         The overall prevalence of food allergy is somewhat higher with a
    maximum around one year. Bock (1987) followed a cohort of North
    American children from birth to their third birthday; 7.7% had adverse
    reactions to food not including fruit and fruit juices and 85% of the
    adverse reactions were found during the first year of life. By the
    third birthday less than 1% had adverse food reactions. The cumulative
    prevalence of food allergy/intolerance diagnosed by elimination and
    open challenge in a group of unselected Danish infants 18 months of
    age was 6%. In the same study the prevalence in high-risk infants was
    17% (Halken et al., 1992).

         In a British study of a birth cohort of children age 4 years,
    0.7% had allergy to peanuts or tree nuts and 0.5% had allergy to
    peanuts alone. These figures were based on skin-prick tests and
    positive history (Tariq et al., 1996).

         The estimated prevalence of hypersensitivity reactions in
    Australia to common foods in infancy and childhood is: cows's milk 2%,
    egg 3.2%, and peanut 1.9%. Based on food challenge studies from
    Australia, Japan, Indonesia, Malaysia and the Philippines, milk and
    egg are the commonest food allergens. Soy, wheat and peanut
    hypersensitivity are next commonest, but rice allergy is rare (Hill,
    1997).

         In a study of allergic children, Crespo et al. (1995) found that
    allergy to cow's milk, egg and fish predominantly began before the
    second year of age, whereas allergy to fruit, legumes and vegetables
    predominantly began after the second year. This is in accordance with
    the findings in a Danish study (Saval et al., 1993) where in older
    children an increased prevalence of allergic pollen rhinitis was

    followed by an increase in oral itch, the symptom characteristic for
    allergic reactions to fruit and vegetables. In the 14-16 year old the
    prevalence of oral itch was 2-2.9%.

    4.5.6.3  Adults

     a)  IgE-mediated allergy

         Allergic reaction to foods that cross-react with pollen is
    probably the most common food allergic reaction in adults, at least in
    countries where tree pollen allergy is common. In a study of Swedish
    medical students, Foucard (1991) found that 78% of the students with
    allergic rhinitis and positive skin test to birch pollen reported
    clinical sensitivity to nuts and/or apples or other fruits. In another
    Swedish study 70% of the patients with birch pollen allergy and 19% of
    the patients with allergy to other pollen had food-related symptoms
    (Eriksson et al., 1982). These estimates were mainly based on
    self-reported symptoms.

         In Switzerland approximately 10% (9.1-11.2%) of the adult
    population currently suffers from hay fever. The prevalence of
    sensitization measured by skin-prick test, but not necessarily
    followed by clinical symptoms, is 12.7% for grass pollen and 7.9% for
    birch pollen (Wüthrich et al., 1995). Other reports of the prevalence
    of pollen rhinitis vary from 2-15% depending on, among other things,
    the age group studied (Sibbald, 1993).

         Applying Swedish figures for pollen-related reactions to the
    Swiss data, taking into account the prick test results, approximately
    50% of hay fever patients would have adverse reactions to
    pollen-related foods, suggesting a prevalence of 3-5% in the adult
    population. In a study by Kremser (1989) only about 10% of subjects
    allergic to birch pollen had more severe reactions than oral itch.
    Combining this with the Swiss figure gives a prevalence of
    pollen-related reactions other than oral itch of approximately 0.5%.

         In adults, the prevalence of allergy to non-pollen-related food
    allergens such as milk, egg, shrimp, meat is probably about 0.1%. In a
    study by Wüthrich (1993) of 402 patients with food allergy,
    approximately 75% of the positive reactions were caused by
    pollen-related food and 25% of the reactions were caused by
    non-pollen-related food. Patients where the only food-related symptom
    was itching of the mouth were not included in the study.

         In some patients IgE allergy to natural rubber latex causes
    allergic reactions to ingested banana, avocado, chestnut, etc. (Blanco
    et al., 1994). The prevalence of natural rubber latex allergy in
    European health-care workers screened with skin-prick tests has ranged
    from 2.8 to 10.7%. About half of patients allergic to natural rubber
    latex have experienced symptoms after eating banana (Turjanmaa et al.,
    1996). In a study by Beezhold et al. (1996) 36% of a group of patients
    allergic to natural rubber latex had clinical reactions to related
    foods. The majority of these patients had anaphylaxis.

         In the Netherlands an attempt has been made to estimate the total
    prevalence of adverse reactions to foods and additives in adults. The
    study used double-blind placebo-controlled food challenge (Niestijl
    Jansen et al., 1994) and 12.4% of the studied population reported food
    allergy or intolerance to specific food. Food groups mentioned most
    frequently were fruit, chocolate and vegetables. The prevalence of
    food allergy or intolerance in the adult Dutch population was
    estimated to be 2.4%. If the positive reactions to food additives,
    including menthol and glucose, is disregarded, the estimated
    prevalence of food allergy/intolerance is 1.2%.

         In a British study (Young et al., 1994) approximately 20% of a
    survey population reported adverse reactions to foods. Eight foods
    commonly perceived to cause sensitivity were canned or made into candy
    bars and used for double-blind placebo-controlled challenge. The foods
    were: cow's milk, hen's egg, wheat, soya, citrus fruits (orange),
    fish/shellfish (prawn), nuts (peanut, brazil nut, walnut and hazel
    nut) and chocolate. These eight foods accounted for 49.3% of reactions
    reported in the study questionnaire. Adding some of the persons with
    severe reactions and estimating the number of theoretically positive
    in the non-challenged groups, the estimated prevalence of food
    allergy/intolerance to the eight foods was 1.4-1.8%. As the eight
    foods used for challenge accounted for approximately 50% of reported
    reactions, the total prevalence estimate must be 3-4%.

     b)  Contact allergens

         In dermally sensitized subjects, ingestion of the contact
    allergen may cause skin flare reactions or other symptoms, for
    instance, from the gastrointestinal tract. It has been reported that
    10% of Danish women have contact allergy caused by nickel (Menné &
    Holm, 1983). Up to 10% of these may benefit from a nickel-restricted
    diet (Veien et al., 1993). The prevalence of systemic reactions via
    the food from other contact allergens is not known. The chemicals of
    fragrances and of food flavours, natural or synthetic, are often
    identical. Veien et al. (1987) found an effect of a diet low in
    flavours in patients having a flare of dermatitis after oral challenge
    with balsam of Peru.

    4.5.6.4  Conclusions

         Milk and egg seems to be the most common food allergens in
    infants and children worldwide. Peanuts are also reported to be common
    food allergens in the United Kingdom, USA, Australia and some Asian
    countries. The overall prevalence of food allergy/intolerance in young
    children may be around ó-8%. This figure declines before 3 years of
    age. In older children and young adults the prevalence of allergy to
    pollen-related fruits, nuts and vegetables increases.

         The results from the epidemiological studies, combined with the
    knowledge on pollen and latex cross-reactions, systemic reactions to
    contact allergens, and coeliac disease (prevalence estimate 0.2-0.5%),
    point to an estimated prevalence of food allergy in the adult
    population of 3-5%, giving that some of the reactions may be
    coincident. These figures are based on the existing prevalence data,
    which are mainly European. It is not yet known whether prevalence data
    in the rest of the world are comparable.

    4.6  Autoimmune diseases associated with drugs, chemicals and
         environmental factors

    4.6.1  Introduction

         The autoimmune connective tissue diseases include conditions such
    as systemic lupus erythematosus (SLE), systemic sclerosis, Sjögren's
    syndrome, rheumatoid arthritis and the systemic vasculitides such as
    Wegener's granulomatosis, Churg Strauss syndrome and polyarteritis
    nodosa. The etiology of these conditions remains largely unknown but
    there is a consensus that several factors may be important, including
    genetic, ethnic and hormonal; many of these conditions have a female
    preponderance (Table 23). Some of these conditions may also have an
    environmental component to their pathogenesis and this particularly
    applies to systemic sclerosis. Many drugs have been associated with
    the development of SLE and cutaneous vasculitis. There has been some
    interest in the possible development of connective tissue diseases
    associated with silicone breast implants.

    4.6.2  Systemic lupus erythematosus

         SLE is predominantly a disease of young women (9:1 female: male
    ratio) and is commoner amongst certain ethnic groups such as
    Afro-Caribbeans and Orientals (Beeson 1994). Clinically, the disease
    manifestations include arthritis, serositis, photosensitivity, oral
    ulceration, malar rashes, recurrent thromboses, glomerulonephritis and
    central nervous system involvement, e.g., epilepsy, psychoses.
    Serologically, SLE is characterized by autoantibody production to
    nuclear components such as anti-nuclear antibodies, antibodies to double
    stranded DNA and antibodies to extractable nuclear antigens.

        Table 23.  Age and sex associations of some autoimmune diseases
    (adapted from Beeson, 1994)
                                                                         

                                            Age                 % Females
                                                                         

    1.  Autoimmune diseases that may
        appear during childhood

    Systemic lupus erythematosus            2-12                70
    Dermatomyositis                         1-15                70

    Table 23.  (continued)

                                                                         

                                            Age                 % Females
                                                                         

    Sydenham's chorea                       5-15                65
    Rheumatoid arthritis                    1-10                65
    Rheumatic fever                         5-15                50
    Thrombocytopenic purpura                2-5                 50
    Polyglandular syndromes                 2-15                50
    Bullous pemphigoid                      1-15                50
    Diabetes mellitus                       2-15                45
    Henoch-Schonlein purpura                2-5                 40
    Post-streptococcal nephritis            5-15                35

    2.  Autoimmune diseases that usually
        appear during early adult life

    Systemic lupus erythematosus            15-40               90
    Erythema nodosum                        15-30               90
    Takayasu's arteritis                    10-30               85
    Myasthenia gravis                       20-30               75
    Thrombocytopenic purpura                15-45               75
    Addison's disease                       20-50               75
    Rheumatoid arthritis                    20-40               65
    Multiple sclerosis                      20-35               60
    Sarcoidosis                             20-40               55
    Ulcerative colitis                      15-40               50
    Erythema multiforme                     20-40               45
    IgA nephropathy                         10-30               35
    Polyarteritis nodosa                    20-50               30
    Ankylosing spondylitis                  20-30               25
    Goodpasture's syndrome                  15-35               15
    Ankylosing spondylitis                  20-30               25
    Goodpasture's syndrome                  15-35               15
    Thromboangiitis obliterans              20-40                5

    3.  Autoimmune diseases that usually appear during mature adult life

    Sjogren's syndrome                      40-60               95
    Primary biliary cirrhosis               30-75               90
    Hashimoto's thyroiditis                 30-50               85
    Thyrotoxicosis                          30-50               85
    Scleroderma                             30-50               80
    Chronic active hepatitis                30-50               65
    Polymyositis/dermatomyositis            40-60               55
    Polychondritis                          40-60               50
    Pemphigus vulgaris                      50-80               50
    Wegener's granulomatosis                10-80               50
    Henoch-Schonlein purpura                30-65               45

    Table 23.  (continued)

                                                                         

                                            Age                 % Females
                                                                         


    Membranous nephropathy                  30-60               35
    Amyotrophic lateral sclerosis           40-70               35
    Tabes dorsalis                          30-50               20

    4.  Autoimmune diseases that usually
        appear late in life

    Giant cell arteritis/polymyalgia        50-90               65
    Pernicious anemia                       40-80               60
    Bullous pemphigoid                      60-75               50
    Rapidly progressive glomerulonephritis  50-70               45
    Myasthenia gravis                       50-80               40
    Fibrosing alveolitis                    40-70               35
                                                                         
    
         The precise etiology of SLE remains obscure but some environmental
    factors are known to exacerbate the disease. One of the most important
    of these is ultraviolet light. In particular UV-B in the 295 to 305 nm
    range is known to be toxic to SLE patients (McGrath et al., 1994).
    Furthermore, patients with antibodies to Ro (SSA) are sensitive to UV
    light, which provokes a photosensitive skin rash and may be followed by
    a generalized disease flare (Gilliam & Sontheimer, 1982). Being in a
    lower socioeconomic group may also increase the risk for the development
    of glomerulonephritis in patients with SLE (McAlindon et al., 1993),
    although the reasons for this remain obscure.

         Many patients with SLE are allergic to a variety of substances
    including drugs and chemicals, and a case-control study has supported
    this clinical observation (Sequeira et al., 1993). Patients with lupus
    and particularly those with Sjögren's syndrome appear to be sensitive to
    cotrimoxazole and other sulfonamide-containing drugs. Oral contraceptive
    pills, especially those with high oestrogen doses, may provoke flares of
    SLE, and these agents should generally be avoided (Jungers et al.,
    1982). The progesterone-only pill is associated with fewer flares.

        Table 24.  Drugs associated with the development of
    systemic lupus erythematosus
                                                                          

         Drugs commonly inducing systemic lupus erythematosus
              Debrisoquine
              Hydralazine
              Quinidine
              Procainamide

         Drugs with good evidence for inducing systemic lupus erythematosus

              Carbamazepine
              Chlorpromazine
              Hydrazine
              Isoniazid
              Minocycline
              Methyldopa
              Penicillamine
              Phenytoin
                                                                          
    
         The majority of cases of SLE are idiopathic but certain drugs are
    known to cause SLE in genetically predisposed individuals. Of the 70 or
    so drugs reported to be associated with drug-induced lupus, some of
    which are shown in Table 24, only procainamide and hydralazine have been
    studied in detail. Clinically, drug-induced lupus is similar to
    idiopathic lupus, but there are one or two striking differences. For
    example, serositis and pleuro-pulmonary involvement is much commoner in
    drug-induced lupus, whereas renal disease and central nervous system
    disease is less common in comparison to idiopathic lupus (Yung et al.,
    1995). Serologically, anti-nuclear antibodies and antibodies to both
    single-stranded and double-stranded DNA are found in both drug-induced
    and idiopathic lupus, but it is uncommon to find very high levels of
    anti-DNA antibodies in drug-induced lupus. Antibodies directed against
    certain components of histone are thought to be characteristic of
    drug-induced lupus. Although anti-histone antibodies may commonly be
    found in idiopathic lupus, they react to the H1 and H2B subunits of
    histone. In drug-induced lupus, the specificity appears to be against
    the H2A-H2B dimer in procainamide-induced lupus, or against H3 and H4 in
    hydralazine-induced lupus (Burlingame & Rubin, 1991).

         Idiopathic lupus erythematosus is associated with HLA DR2 or DR3,
    at least in Caucasian patients. One of the best clues towards an
    explanation for a genetic predisposition to drug-induced lupus comes
    from the study of the acetylator status of these patients. "Slow
    acetylators" are homozygous for this gene and have low levels of
     N-acetyltransferase in the liver. Both procainamide and hydralazine
    are metabolized by this pathway and slow acetylators have a higher risk
    of developing drug-induced lupus following exposure to these drugs
    (Strandberg et al., 1976; Sonnhag et al., 1979).

         The exact mechanisms by which drugs can induce lupus remain unknown
    and a number of possibilities are being considered. It may be that the
    ability of both procainamide and hydralazine to bind polynucleotides
     in vitro may render DNA and/or histones antigenic (Dubroff & Reid,
    1980; Tomura & van Lancker, 1988). A more specific mechanism has been
    suggested whereby drugs interfere with the normal process of methylation
    of DNA. Following DNA replication, cytosine residues are methylated at
    the 5-position by the enzyme DNA methyltransferase. Failure of
    methylation of regulatory sequences is associated with gene expression,
    whereas methylation is associated with suppression of gene
    transcription. Thus, DNA methylation is a mechanism regulating gene
    expression (Yung et al., 1995). Studies have shown that
    procainamide- and hydralazine-treated human T-cells show evidence of
    hypomethylation (Scheinbart et al., 1991). In particular, procainamide
    is capable of reversibly inhibiting T-cell DNA methyltransferase in a
    dose-dependent manner (Scheinbart et al., 1991). Furthermore, these
    T-cells become autoreactive in response to procainamide and hydralazine
    (Yung et al., 1995). These autoreactive T-cells are capable of inducing
    B-cells to differentiate into IgG-secreting cells without the addition
    of antigen or mitogen, thus providing a mechanism for polyclonal B-cell
    activation that is seen in both idiopathic and procainamide-induced
    lupus (Richardson et al., 1990).

         Experiments have shown that UV light is also capable of inhibiting
    T-cell DNA methylation, increasing LFA-1 expression and inducing
    autoreactivity (Richardson et al., 1994). Thus, drugs such as
    procainamide and hydralazine and environmental factors such as UV light
    may induce T-cell autoreactivity by inhibiting T-cell DNA
    methyltransferase, thereby altering T-cell gene expression and making
    autoimmunity more likely.

    4.6.3  Scleroderma: environmental and drug exposure

         Scleroderma (progressive systemic sclerosis) is a multisystem
    connective tissue disease of unknown etiology. The prevalence ranges
    from 47.9 to 290 per million among females, with an estimated incidence
    of between 3.6 and 16 per million per year (Silman et al., 1996). It is
    commoner in women (the sex ratio ranges from 3:1 to 8:1) and some ethnic
    minorities such as Afro-Caribbeans and is characterized by widespread,
    diffuse sclerosis affecting the peripheral vasculature, skin,
    gastrointestinal tract, heart and muscle. Raynaud's phenomenon is a very
    common early feature, and pulmonary and renal involvement may be serious
    and life-threatening. Serologically, antibodies to topoisomerase I
    (anti-Scl 70) and RNA polymerase III may predict prognosis in terms of
    more extensive disease.

         Scleroderma-like conditions (pseudoscleroderma) and scleroderma
    have been associated with a variety of chemical and environmental agents
    (Silman & Hochberg, 1996) (Table 25).

    4.6.4  Silicone breast implants

         Two studies have reported possible interactions between silicone
    breast implants and the immune system. Tenenbaum et al. (1997) showed
    the existence of a relationship between the level of anti-polymer
    antibodies in the serum and the severity of clinical complications
    (related to connective tissue) in silicone breast implant recipients.
    Although these antibodies were not directed against silicone polymers
    this might be the first objective marker to be used as a diagnostic
    feature in silicone breast implant patients. Smalley et al. (1995, 1997)
    reported on a lymphocytic response to silica (silicon dioxide) similar
    to that for silicone (polysiloxane polymer), a silicon derivative, and
    present in silicone breast implant material, in silicone breast implant
    patients and their children.

    Table 25.  Some agents associated with scleroderma
                                                          

         Organic chemicals:

              Toluene

              Benzene

              Xylene

              Aromatic mixes e.g., white spirit

              Vinyl chloride

              Trichloroethylene

              Perchloroethylene

              Naphtha-n-hexane

              Epoxy resins

              Metaphenylenediamine

              Urea formaldehyde foam insulation

         Drugs:

              Bleomycin

              Carbidopa

              L-5-hydroxytryptophan

    Table 25.  (continued)
                                                          

         Drugs:

              Pentazocine

              Cocaine

              Diethyl proprion

              Fenfluramine HCl
                                                          

         However, several thorough reviews of the subject, including a
    case-control study (Sanchez-Guerrero et al., 1994, 1995; Gabriel et
    al. 1994; Hochberg et al., 1996), concluded that there was little or
    no association between silicone breast implants and either connective
    tissue diseases or a unique arthralgia/myalgia/fibromyalgia syndrome.
    They suggested that the connective tissue diseases that had been
    reported were most likely to have been idiopathic in origin.

         In a large retrospective cohort study, the Women's Health Study
    sent questionnaires to 395 543 women and found 10 830 women who
    reported breast implants (Hennekens et al., 1996). This study showed a
    small but statistically significant increased risk of the combined
    end-point of any connective tissue disease in patients with breast
    implants, with a relative risk of 1.24 (95% confidence intervals
    1.08-1.41,  P = 0.0015). Of the specific connective tissue diseases
    studied, scleroderma was associated with a relative risk of 1.89 (95%
    confidence intervals 0.98-3.45) but this was not significant
    ( P = 0.06). For patients with scleroderma with implants less than 4
    years, the relative risk was significant: 2.68 (95% confidence
    intervals 1.11-6.51,  P = 0.029) The precise nature of the implants
    was not ascertained so it is not clear how many of the implants were
    silicone gel filled. Another possibility for bias is the publicity
    surrounding breast implants, which may have led to a degree of
    over-reporting amongst the cases.

         Other studies have supported the epidemiological view that there
    is no association between connective tissue disease and silicone
    breast implants (Nyren et al., 1998; Edworthy et al., 1998). Both were
    very large cohort studies that compared patients who had either
    undergone breast reduction (Nyren et al., 1998) or non-implant
    cosmetic surgery (Edworthy et al., 1998). Neither found an excess of
    any connective tissue disease among patients with silicone implants. A
    United Kingdom review (DOH, 1998) came to the same conclusion.

         Overall, apart from one retrospective study, there is no clear
    evidence that patients with silicone breast implants have any risk of
    developing a connective tissue disease but the area remains
    controversial.

    4.6.5  Toxic oil syndrome

         This epidemic started in May 1981 when large numbers of patients
    in the Madrid industrial area suffered acute respiratory illnesses
    that did not respond to antibiotics. The etiological agent was
    identified as being rapeseed oil that had been denatured with 2%
    aniline for industrial use but sold illegally by itinerant vendors for
    domestic food use. Initially 20 688 cases were recorded with 835
    deaths -- a cumulative mortality of 4%, but over the first 2 years of
    the epidemic the cumulative mortality was 2.3%. Oleyl-anilide proved
    to be an excellent marker for case-related oil specimens although the
    precise nature of the etiological agent has never been described. It
    has proved impossible to consistently replicate the syndrome in animal
    studies.

         Various clinical features have been described (Kammüller et al.,
    1984; Philen & Posada, 1993) including severe myalgia, arthralgia,
    Raynaud's phenomenon, scleroderma-like skin changes, carpal tunnel
    syndrome, joint contracture, pulmonary hypertension and peripheral
    neuropathy. The most consistent laboratory features were eosinophilia
    and high IgE counts and low levels of antibodies such as antinuclear
    antibodies. HLA DR3-DR4 was associated with the chronic phase of the
    disease. Histologically, the most consistent finding was a vascular
    lesion characterized by intimal proliferation with fibrosis, vascular
    occlusion and thrombosis.

    4.6.6  Eosinophilia-myalgia syndrome

         This epidemic was first identified in October 1989 in New Mexico
    (Hertzman et al., 1990). The report was followed by a case-control
    study that firmly linked the consumption of possibly contaminated
    L-tryptophan from one source with the eosinophilia-myalgia syndrome
    (Eidson et al., 1990). L-tryptophan was widely used as a
    non-prescription food supplement by health conscious individuals for a
    wide variety of minor ailments. The source of the epidemic was traced
    to a single manufacturer (Philen & Posada, 1993).

         The vast majority of cases occurred in the USA. The majority of
    these patients were white middle-class middle-aged females, probably
    reflecting the pattern of use of health supplements rather than any
    innate risk factor. The mortality rate was reported as 2.7% (37 deaths
    among 1370 known cases) (Philen & Posada, 1993).

         The clinical features resembled those of toxic oil syndrome,
    although scleroderma-like changes and Raynaud's phenemenon were not
    reported (Kaufman et al., 1991; Philen & Posada, 1993). These authors
    also noted that HLA DR4 was associated with an increased risk of
    chronic disease. Laboratory investigations consistently showed an
    eosinophilia early in the disease course, although this diminished

    spontaneously even when the patients continued to be ill (Philen &
    Posada, 1993). The main factor in treatment was the avoidance of
    further ingestion of L-tryptophan. Glucocorticoids were used widely
    and helped the myalgias and reduced the eosinophil count, but there
    was often a recurrence of symptoms on stopping the steroid treatment,
    and progression to chronic disease was not altered.

    4.6.7  Vinyl chloride disease (occupational acro-osteolysis)

         Vinyl chloride (CH2=CHCl) is a combustible colourless gas at
    room temperature that is used in the manufacture of a variety of
    plastics. Several methods can be used to polymerize the gas to make
    polyvinyl chloride (PVC). In the mid-1960s a new syndrome affecting
    workers involved in polymerizing vinyl chloride was recognised (Wilson
    et al., 1967). These patients developed paraesthesia of the fingers,
    cold sensitivity, Raynaud's phenomenon, pseudoclubbing of the fingers,
    skin oedema and thickening of the fingers, hands and forearms, and
    chest X-ray changes (Veltman et al., 1975). The risk of development of
    symptoms was related to cumulative exposures over time and work
    practices but was not related to handling the finished PVC product,
    and in Wilson's (1967) study it occurred in less than 3% of exposed
    individuals.

         An increased prevalence of HLA DR3 and DR3/B8 haplotypes has been
    noted in patients with vinyl chloride disease (Black et al., 1983).
    Vinyl chloride is a cause of non-cirrhotic portal hypertension and
    angiosarcoma of the liver.

         The skin changes of vinyl chloride disease resemble morphoea
    clinically and histologically, and vascular changes were often present
    with luminal narrowing of the digital arteries and subtotal occlusion
    of these vessels (Veltman et al., 1975). The most dramatic
    radiological change is acro-osteolysis seen in the terminal phalanges
    of the fingers; a transverse lytic band is seen across the distal
    phalangeal shaft. Ward et al. (1976) reported immunological
    abnormalities in vinyl chloride disease including polyclonal increases
    of IgG, cryoglobulins, evidence of complement activation and low titre
    anti-nuclear antibodies. Vascular endothelial, medial and sub-intimal
    deposits of IgG, C3, C4 and fibrin/fibrinogen were seen on histology
    of small and medium-sized arterioles. Reduced T-cell and modestly
    increased B-cell numbers were also observed (Ward et al., 1976).

    4.6.8  Systemic vasculitis: environmental factors and drugs

         Various drugs are associated with hypersensitivity reactions and
    the most common mechanism is an immune-complex-mediated vasculitis
    (Dubost et al., 1991). Drugs account for approximately 10-20% of
    dermal vasculitis and this figure is on the increase. The cutaneous
    lesions most commonly seen include palpable purpura, although
    urticarial lesions may be seen in 10% (Dubost et al., 1991). They
    usually occur symmetrically on the lower limbs extending to the thighs

    and buttocks (Mullick et al., 1979). In Mullick's series of 30
    patients, 19 had disseminated vasculitis with other organ involvement,
    including renal disease, synovitis, pleuropulmonary and cardiovascular
    disease with coronary vessel vasculitis and cardiac failure. More than
    80% of patients had constitutional features such as fatigue, malaise
    and fever.

    4.6.9  Conclusion

         The vast majority of the autoimmune connective tissue diseases
    have no known etiological agents. Certain drugs, occupational
    exposures, and UV radiation have been shown either to exacerbate a
    known autoimmune disease or, occasionally, to trigger the onset of a
    syndrome that closely resembles one of the established diseases.
    

    5.  EPIDEMIOLOGY OF ASTHMA AND ALLERGIC DISEASE

    5.1  Introduction

         Asthma and allergies, such as atopic diseases (i.e., bronchial
    asthma, allergic rhinitis, atopic dermatitis) and allergic contact
    dermatitis, are common medical problems.

         Asthma, allergic rhinitis and atopic dermatitis are conditions
    that have a variety of clinical similarities and epidemiological
    connections (Montgomery-Smith, 1983). Asthma has frequently been
    classified as of allergic or non-allergic origin. Different atopic
    conditions often occur together in the same person. A family history
    of atopic disorders has consistently been found to be an important
    predisposing factor for the development of atopic diseases (Croner &
    Kjellman, 1990). However, epidemiological studies suggest that
    exogenous factors also play an important role in their etiology
    (Newman Taylor, 1995). There have been epidemiological studies on
    geographical variation and time trends in prevalence rates of these
    disorders, and knowledge about the effects of a variety of
    environmental factors, including outdoor exposures, indoor exposures,
    diet and occupational exposures, is increasing.

    5.2  Definition and measurement of allergic disease

    5.2.1  Asthma

    5.2.1.1  Definition

         Asthma is a respiratory disease that is not well defined. It is
    characterized by variable airflow limitation due to bronchial
    hyper-responsiveness and often by inflammatory changes in the airways
    (see section 2.5.1)

    5.2.1.2  Assessment

     a)  Questionnaires

         In epidemiological studies many different questionnaires have
    been used to investigate in populations the prevalence of asthma
    symptoms such as wheezing and shortness of breath, as well as
    diagnosed asthma. These include the MRC (United Kingdom Medical
    Research Council), the ECSC (European Coal and Steel Community), the
    ATS-DLD (American Thoracic Society and the Division of Lung Disease),
    and the IUATLD (International Union against Tuberculosis and Lung
    Disease) questionnaires (Toren et al., 1993).

         When questionnaires are used in epidemiological studies to
    compare prevalence rates between populations, findings may be
    influenced by differences in language, interpretations of the concept
    of wheeze in different communities, or the general awareness of the

    disease in the community (Strachan et al., 1990). Nevertheless, the
    IUATLD questionnaire has been shown to provide valid and comparable
    data even when translated (Burney et al., 1989a). A similar written
    questionnaire was developed for the International Study of Asthma and
    Allergies in Childhood (ISAAC) (Pearce et al., 1993; Asher et al.,
    1995; Shaw et al., 1995). In addition, a video questionnaire depicting
    (in five scenes) adolescents with different symptoms of asthma has
    been developed to overcome problems with the translation of
    questionnaires (Asher et al., 1995). The sensitivity and specificity
    for predicting bronchial hyperresponsiveness of the video
    questionnaire was similar to the respective questions in the IUATLD
    questionnaire (Shaw et al., 1992a,b; Shaw et al., 1995). By providing
    data relatively free from biases due to language, culture, literacy or
    interviewing techniques, the video questionnaire may prove
    particularly useful for comparisons of prevalence and severity of
    asthma in different populations (Asher et al., 1995).

     b)  Bronchial hyperresponsiveness

         In epidemiological settings, a major limitation of using
    bronchial hyperresponsiveness (BHR), measured by challenge, as a gold
    standard for the definition of asthma is that a considerable
    proportion of subjects with bronchial hyperresponsiveness report no
    respiratory symptoms (Sterk et al., 1993). Thus, bronchial
    hyperresponsiveness cannot be used synonymously as a diagnosis of
    asthma (Sterk et al., 1993). The thresholds used to define bronchial
    hyperresponsiveness -- usually a fall in the forced expiratory volume
    in one second (FEV1) of more than 15% or 20%, or a fall in peak
    expiratory flow rate (PEF) of more than 10% -- were chosen
    arbitrarily. Various methods including exercise tests and inhalation
    of metacholine, histamine or cold air have been described to measure
    bronchial hyperresponsiveness (Sterk et al., 1993). It is now well
    recognized that a change in osmolarity of the periciliary fluid is a
    potent stimulus to airway narrowing and may be a common cause for
    provoking an attack of asthma (Anderson & Smith, 1991). This has led
    to the use of hypertonic saline aerosols to document bronchial
    hyperresponsiveness in epidemiological studies (Riedler et al., 1994).
    However, a negative response to any of these provocation methods does
    not exclude asthma. In general, measuring bronchial
    hyperresponsiveness in epidemiological studies has a moderate
    specificity and a relatively low positive predictive value (Sterk et
    al., 1993).

    5.2.2  Rhinitis

         There are no clear-cut criteria for defining hay fever and
    perennial rhinitis. Rhinitis is frequently underdiagnosed and
    misdiagnosed (Sibbald & Rink, 1991a,b). Patients are generally
    classified according to the suspected etiology of their conditions.
    Rhinitis is labelled "allergic" when a causal allergen can be
    identified, otherwise it is labelled "non-allergic". Subjects with
    seasonal symptoms are twice as likely to be labelled as having

    allergic rhinitis by their doctors. The usual complaints of allergic
    rhinitis are "summer flu", series of sneezing and a stuffy nose. The
    classification is based primarily on nasal smears and skin-prick tests
    compared with the patient's history (Weeke, 1987). Perennial allergic
    rhinitis in patients who are allergic to pets is easy to diagnose if
    symptoms such as sneezing and itchy eyes occur immediately after
    contact with pets. If the symptoms are caused, for instance, by mites,
    it is more difficult to obtain a clear-cut medical history. Symptoms
    of patients with non-allergic rhinitis are often different from those
    with allergic causes. The main symptoms in non-allergic cases are
    stuffy nose and loss of sense of smell, while in the allergic type,
    sneezing and watery secretion are more prominent. A standardized
    questionnaire for assessing the prevalence of rhinitis in children in
    epidemiological studies has been developed (Asher et al., 1995).

    5.2.3  Atopic dermatitis

    5.2.3.1  Definition

         Atopic dermatitis is a disease that is difficult to define,
    because of the frequently subtle clinical manifestations, the lack of
    an identifying laboratory marker, and the lack of a distinguishing
    primary lesion. Generally, atopic dermatitis is a chronic cutaneous
    inflammatory disease with a strong tendency of patients to overproduce
    IgE (Hanifin, 1987).

    5.2.3.2  Assessment

         The absence of diagnostic criteria for atopic dermatitis prompted
    Hanifin & Lobitz, (1977) and Hanifin & Rajka (1980) to suggest major
    and minor diagnostic criteria for atopic dermatitis based on clinical
    features. Disadvantages of these criteria were that many of them had
    no precise definition, that some were very infrequent, and that others
    were nonspecific. A minimum set of diagnostic criteria for atopic
    dermatitis was derived by the United Kingdom working party composed of
    dermatologists, family practitioners, paediatricians and
    epidemiologists (Williams et al., 1994a). These criteria included a
    history of flexural involvement, a history of dry skin, the onset
    under the age of 2 years, a personal history of asthma, a history of
    pruritic skin condition, and visible flexural dermatitis. The criteria
    were used for assessing the prevalence of atopic dermatitis in
    children in an international multiple cross-sectional study (Asher et
    al., 1995). The European Task Force (ETFAD, 1993) on atopic dermatitis
    has developed a scoring index for the severity of atopic dermatitis,
    which is based on a combination of symptoms such as erythema,
    oedema/papulation, oozing/crusts, excoriation and lichenification.
    Nevertheless, more objective tests, such as raised IgE and/or 
    skin-prick test positivity, remain important tools for epidemiological
    research.

    5.2.4  Skin-prick test and serum IgE

         Skin-prick testing is commonly used to assess allergic
    sensitization in epidemiological studies. The test is easy to perform;
    serious local or systemic reactions occur only very rarely, and
    different test devices are available. However, results of different
    studies are often not comparable due to differences in allergen
    concentrations, type of needles used for pricking, and criteria for
    defining a positive reaction (Nelson et al., 1993). The most common
    criterion is that the diameter of the allergen wheal be >2-3 mm after
    subtraction of the reaction to the negative control (Pepys, 1994).
    Even well-standardized skin-prick tests may be subject to measurement
    error arising from different field workers and variations in the
    degree of skin reactivity in different racial groups or under
    different environmental conditions. Measurements of total and specific
    serum IgE may therefore provide valuable additional information on the
    atopic susceptibility and of the atopic status of an individual. Total
    serum IgE, however, may also be raised in association with other
    conditions, such as parasitic infections.

    5.2.5  Allergic contact dermatitis

         Allergic contact dermatitis is the condition in which contact
    with haptens induces cell-mediated contact sensitization.

         Patch testing is used to diagnose allergic contact dermatitis.
    The test should be performed by a skilled operator using standardized
    test materials and also with substances present in the patient's
    domestic or occupational environment which are considered to be
    possible sensitizers.

    5.3  Asthma and atopy: prevalence rates and time trends in prevalence
         rates

         Asthma is the most common single chronic disease in childhood.
    Most of the prevalence studies on asthma have therefore been conducted
    in children and adolescents. Many of these studies have also
    determined the prevalence of rhinitis and atopic dermatitis. In the
    last 20 years prevalence estimates have been reported from many
    different geographical regions in all five continents. Several studies
    were repeated after a number of years applying the same methods at
    different points in time, thereby providing information on time trends
    in prevalence. Estimates on asthma prevalence were mostly derived by
    questionnaires, often in combination with a lung function test and
    determination of bronchial hyperresponsiveness. In addition to
    questionnaire data, prevalence rates of atopic sensitization were
    assessed by skin-prick tests and, to a lesser extent, by determination
    of total or specific IgE. The overview given below is not intended to
    be comprehensive, but will give some insight into the geographical
    variation and recent time trends. The comparability of prevalence
    estimates between study centres, however, is limited as most studies
    applied different methods.

    5.3.1  Europe

    5.3.1.1  Prevalences

         Between 1989 and 1992 a parental questionnaire, a skin-prick test
    and a cold air challenge test were administered to 9- to 11-year-old
    children in western and eastern Germany. Atopic sensitization was more
    frequent in western German children living in Munich (5.9%), compared
    to children living in Leipzig and Halle in eastern Germany (3.9%).
    Bronchial hyperresponsiveness was also more prevalent in western
    Germany (8.3%) than in eastern Germany (5.5%) (von Mutius et al.,
    1994b). Hay fever and rhinitis were reported less often in Leipzig
    than in Munich (2.4% and 16.6% compared to 8.6% and 19.7%), whereas
    bronchitis was more prevalent in Leipzig. In contrast to atopic
    respiratory disease, the prevalences for atopic eczema were similar in
    the two study areas (von Mutius et al., 1992).

         The occurrence of allergic diseases was studied during 1979 and
    1980 on the basis of a questionnaire sent to the parents of 20 000
    children 7, 10 and 14 years of age in three parts of Sweden with
    different climatic conditions (Aberg et al., 1989). The prevalence of
    asthma was significantly higher in the northern part of the country,
    and this higher prevalence could not be explained by other factors
    than by the cold and dry climate. In Viborg, Denmark, the frequency of
    rhinitis was 10.5%, of atopic eczema 7%, of urticaria 3.2%, and of
    asthma 4.5% among 5- to 16-year-old school children studied in 1990
    (Saval et al., 1993). Asthma and rhinitis were more frequent among
    boys, while atopic eczema was more frequent among girls. For both
    sexes, the frequency of rhinitis increased during their years at
    school, while the frequency of skin symptoms decreased.

         In a general practice in London, symptoms, atopic state and
    medical history were compared among 16- to 65-year-old patients with
    seasonal and perennial rhinitis (Sibbald & Rink, 1991b). The
    prevalence of rhinitis was 24%, 3% had seasonal symptoms only and 13%
    had perennial symptoms only. Distinguishing between atopic and
    non-atopic subjects by skin-prick testing with five common allergens
    revealed that subjects with seasonal rhinitis were more likely to be
    atopic. Moreover, subjects with seasonal rhinitis were also more
    likely to have eczema and to have a family history of hay fever.

         Rates of reported eczema during early childhood have been studied
    by health visitor interview in three national cohorts of children born
    in England, Scotland and Wales in 1946 (at age 6 years), 1958 (at age
    7 years), 1970 (at age 5 years) (Taylor et al., 1984). The reported
    rates in the three birth cohorts were 5.1%, 7.3% and 12.2%.

    5.3.1.2  Time trends

         From 1926 to 1961 the prevalence of asthma in Finnish men
    registered through the defence forces statistics ranged between 0.02
    and 0.08%, whereas from 1961 to 1989 the prevalence rose from 0.29 to
    1.79% (Haahtela et al., 1990).

         In the United Kingdom several studies were conducted in different
    epidemiological settings to estimate the prevalence and time trends in
    the prevalence of allergic disorders. The different settings comprised
    patients of general practices (Fleming & Crombie, 1987; Sibbald &
    Rink, 1991a,b), national birth cohorts (Taylor et al., 1984),
    representative samples of school children in England (Burney et al.,
    1990), in South Wales (Burr et al., 1989), in the London borough of
    Croydon (Anderson et al., 1983; Anderson et al., 1994), in Aberdeen
    (Ninan & Russell, 1992), in the South Thames region (Barbee et al.,
    1987), and hospital admission statistics (Strachan & Anderson, 1992;
    Anderson, 1989). The latter showed between 1978 and 1985 an increase
    in the number of hospital admissions because of asthma in infants up
    to 4 years old (186%) and in 5- to 16-year-olds (56%) in the South
    West Thames region (Anderson, 1989). Changes in mode of referral,
    severity on admission and readmission ratio were also explored, but
    little evidence was found for a reduction in severity or change in
    readmission rate since 1978. These findings contrast with findings of
    two identical surveys that were conducted in 1978 and 1991 to explore
    prevalence changes and use of medical services (Strachan & Anderson,
    1992). These data showed a substantial increase in self-referral
    together with an increase in readmission. A comparison of two surveys
    of morbidity carried out in 1970-1971 and 1981-1982 in general
    practices in England and Wales showed an increase in prevalence rates
    of asthma in men from 8.8 to 15.9% (Fleming & Crombie, 1987).

         Between 1973 and 1989 the lifetime prevalence of wheezing among
    12-year-old children in South Wales increased from 17 to 22%, a
    history of asthma rose from 6 to 12%, and current asthma from 4 to 9%
    (Burr et al., 1989). Increases occurred also in the frequency of
    history of eczema (5 to 16%) and of hay fever (9 to 15%), while
    wheezing not attributable to asthma remained constant. Increasing
    prevalences of asthma were also observed between 1973 and 1986 in
    English school children (relative increase in boys of 6.9% and in
    girls of 12.8%), which could not simply be explained by changes in
    diagnostic fashion (Burney et al., 1990).

         In Aberdeen, Scotland, the prevalence of wheeze among 8- to
    13-year-old school children rose from 10.4% in 1964 to 19.8% in 1989,
    and shortness of breath rose from 5.4 to 10.0%. Wheeze and shortness
    of breath were more prevalent in boys than in girls. Asthma rose from
    4.1 to 10.2%, hay fever from 3.2 to 11.9% and eczema from 5.3 to 12%
    (Ninan & Russell, 1992). Between 1978 and 1991 significant relative
    increases in the 12-month prevalence rates of attacks of wheezing and
    of asthma were found in the London borough of Croydon (Anderson et
    al., 1994). Results from a repeated cross-sectional study performed in
    1991-1992 and 1995-1996 in 10-year-old children in Leipzig, Germany,
    showed a rise in hay fever and atopic sensitization. However, no
    increase in the prevalence of asthma or bronchial hyperresponsiveness
    was observed over the 4-year period (von Mutius et al., 1998).

    5.3.2  Oceania

    5.3.2.1  Prevalences

         Several cross-sectional surveys were performed in Australian and
    New Zealand school children to assess the prevalence of respiratory
    symptoms and bronchial hyperresponsiveness. A comparison of 769
    children living in Wagga Wagga (inland Australia), and 718 children
    living in Belmont (coastal Australia) carried out in 1982, showed that
    respiratory symptoms, asthma, bronchial hyperresponsiveness, hay fever
    and atopy were all more common in the dry inland area than in the
    humid coastal area. In both areas 38% of the children were atopic
    (Britton et al., 1986).

         The prevalence of wheeze in Melbourne school children was 23.1%
    for 7-year-olds, 21.7% for 12-year-olds, and 18.6% for 15-year-olds.
    History of wheeze was more common for boys than for girls at age 7
    years, but not at age 15 years. A history of asthma among 7-year-olds
    was reported for 46% of the children in 1990 compared to 19.1% in 1964
    (Robertson et al., 1991).

         When comparing school children living in New Zealand with school
    children living in South Wales in 1990, the prevalence of a history of
    asthma at any time was higher in New Zealand (17%) than in South Wales
    (12%) (Barry et al., 1991). Wheeze ever and wheeze brought on by
    running were also higher in New Zealand than in South Wales. The sex
    ratio of asthmatic and wheezy children was similar. In a comparison of
    12- to 15-year-old school children living in five regions in four
    countries, the prevalence of wheezing during the last 12 months, as
    assessed by both a written and a video questionnaire, was similar in
    West Sussex, England (29% and 30%), Wellington, New Zealand (28% and
    36%), Adelaide, Australia (29% and 37%), and Sydney, Australia (30%
    and 40%), and was lower in Bochum, Germany (20% and 27%) (Pearce et
    al., 1993). One year prevalence of severe wheezing limiting speech was
    greater in Wellington (11%), Adelaide (10%) and Sydney (13%) than in
    West Sussex (7%) and Bochum (6%). In addition, the one year prevalence
    of frequent attacks of wheezing, frequent nocturnal wheezing, and
    doctor diagnosed asthma were higher in Australia and New Zealand than
    in the European centres.

    5.3.2.2  Time trends

         Between 1982 and 1984 the prevalence of bronchial
    hyper-responsiveness assessed by histamine challenge test among 2363
    Australian children was 17.9% (Salome et al., 1987). The prevalence of
    respiratory symptoms, bronchial hyperresponsiveness, severity of
    bronchial hyperresponsiveness and bronchial hyperresponsiveness
    combined with symptoms was compared between children living in
    Auckland, New Zealand and two locations in Australia: Wagga Wagga in
    the inland and Belmont on the coast (Asher et al., 1988). The
    prevalences were similar in Auckland and Wagga Wagga, but lower in

    Belmont. When the same study was repeated 10 years later with
    Australian 8-10 year olds, the prevalence of wheeze had increased from
    10.4% in 1982 to 27.6% in 1992 in Belmont and from 15.5 to 23.1% in
    Wagga Wagga. Bronchial hyperresponsiveness increased twofold up to
    19.8% in Belmont and 1.4 fold up to 18.1% in Wagga Wagga (Peat et al.,
    1994). The prevalence of atopy remained unchanged. The reported asthma
    or wheeze in a Maori population rose from 26.2% in 1975 to 34% in 1989
    (Shaw et al., 1990).

    5.3.3  Eastern Mediterranean

         The prevalence of asthma was studied in Israeli adolescents by
    means of computerized medical draft records of conscripts aged 17-18
    years who were born over a 9-year period and examined up to the end of
    1989 (Laor et al., 1993). Asthma was more prevalent in males than in
    females and the prevalence of asthma increased over time. Risk of
    asthma was higher for subjects of Western origin and lowest for those
    of African origin. By area of residence the risk for asthma was
    highest in coastal and lowest in inland regions.

    5.3.4  Africa

         In Africa, 694 children from a Cape Town township and 671 from a
    rural area in Transkei performed an exercise tolerance test. Of the
    children living in the urban area 3.2% had asthma compared to 0.14%
    living in a rural district (Van Niekerk et al., 1979). The prevalence
    of reversible airway obstruction after running for 6 min was studied
    in 7- to 9-year-old school children living in three areas in Zimbabwe.
    Prevalence of airway obstruction was 5.8% in northern Harare, an urban
    area with more people of higher socioeconomic status, 3.1% in southern
    Harare and 0.1% in Wedza, both more rural areas. Prevalence in white
    children living in northern Harare was similar to the prevalence in
    black children in that region, (5.3% and 5.9%, respectively) (Keeley
    et al., 1991).

    5.3.5  Asia

    5.3.5.1  Prevalences

         In 1992, the prevalences of hay fever, eczema and wheeze were
    estimated by parental questionnaire in secondary school students, ages
    12 to 19 years, in the three south-east Asian cities Hong Kong, Kota
    Kinabalu and San Bu (Leung & Ho, 1994). The prevalences for hay fever
    were 15.7%, 11.2% and 2.1%, for eczema 20.1%, 7.6% and 7.2%, and for
    wheeze 11.6%, 8.2% and 1.9%, in the respective cities. Skin test
    reactivity to one of five common allergens was common and present in
    49.0-63.9% of subjects. In Singapore, allergic rhinitis was studied in
    a cross-sectional population-based study of 2868 adults, aged 20-74
    years (Ng & Tan, 1994b). Allergic rhinitis was reported by 4.5% of
    subjects.

    5.3.5.2  Time trends

         Two surveys in 7- to 15-year-old school children in Taipei,
    Taiwan, were conducted in 1974 and 1985 (Hsieh & Shen, 1988). The
    prevalence of childhood asthma increased from 1.3% in 1974 to 5.1% in
    1985, with boys showing higher rates in both studies.

    5.3.6  North America

    5.3.6.1  Prevalences

         Significant variation in asthma mortality between different
    geographical regions was reported. Regions with elevated rates
    included the central plain states and three large urban metropolitan
    areas: Chicago IL, New York NY and Phoenix AZ (Weiss et al., 1989).

    5.3.6.2  Time trends

         Asthma morbidity in Indian children and adults in Saskatchewan
    was registered using hospitalization data from 1970 to 1989. Asthma
    hospitalization was higher among boys than girls at age 0-4 years,
    but this was reversed at the ages of 15-34 and 35-64 years.
    Hospitalization for asthma had significantly increased for the age
    groups 0-4 and 35-64 years. Increases of asthma morbidity in recent
    years were also observed (Senthilselvan & Habbick, 1995).

         The reported prevalence of ever having asthma increased among
    6- to 11-year-old children between the first (1971 to 1974) and second
    (1976 to 1980) US National Health and Nutrition Examination Surveys
    (NHANES) from 4.8 to 7.6% (Gergen et al., 1988). Asthma was more
    common in blacks than in whites, in boys than in girls, and in urban
    than in rural areas.

         Changes in the asthma prevalence between 1981 and 1988 were
    studied in the US National Health Interview Survey (NHIS) in 0- to 
    17-year-old children (Weitzman et al., 1992). The prevalence of 
    parent-reported childhood asthma increased from 3.1 to 4.3%. Trends 
    towards a lower rate of hospitalization and better overall health 
    status of the asthmatics from 1981 to 88 were reported. The overall 
    prevalence increased by 40%, which was mainly due to a prevalence 
    increase in white children. However, the prevalence was still higher 
    in blacks. There was no evidence of an increase in severity of asthma.

         An example of the uncertainties in collecting and interpreting
    epidemiological data is provided by consideration of the age-adjusted
    death rate for asthma in the USA, which increased by 40% between 1982
    and 1991. The increase was higher in females (59%) than in males
    (34%). The death rates were consistently higher in blacks than in
    whites. Prevalence rates for self-reported asthma increased also by
    42%, with a much greater increase in females (82%) than in males
    (29%). Death rate and self-reported asthma increased but
    hospitalization rates remained stable. This was possibly due to an

    improved outpatient treatment and changed billing practices reflecting
    changes in the classification of cases of asthma under other
    diagnostic categories. Ethnic differences in self-reported morbidity
    may potentially be explained by accessibility to health services and
    socioeconomic factors (CDC, 1995). This is an example of the
    difficulties in collecting and interpreting epidemiological data.

    5.3.7  The International Study of Asthma and Allergies in Childhood

         The International Study of Asthma and Allergies in Childhood
    (ISAAC) was initiated to maximized the value of epidemiological
    research into asthma and allergic disease by establishing a
    standardized methodology and facilitating international collaboration
    (Asher et al., 1995). Its specific aims are:

    a)   to describe the prevalence and severity of asthma, rhinitis and
         eczema in children living in different centres, and to make
         comparisons within and between countries;

    b)   to obtain baseline measures for assessment of future trends in
         the prevalence and severity of these diseases;

    c)   to provide a framework for further etiological research into
         genetic, lifestyle, environmental and medical care factors
         affecting these diseases.

         The first phase of the collaborative studies was completed by 155
    centres in 56 countries and included more than 720 000 participants
    (Strachan et al., 1997). The core questionnaires used were designed to
    assess the prevalence and severity of asthma, allergic rhinitis and
    eczema in children. A 20-fold difference in the 12-month-period
    prevalence of wheezing in 13- to 14-year-old school children (range
    1.6-36.8%) and an 8-fold variation between the 10th and 90th
    percentiles (3.9-30.6%) were observed (ISAAC Steering Committee, 1998)
    (Fig. 16). The 12-month-period prevalence of symptoms of allergic
    rhinitis in 13- to 14-year-olds varied from 1.4 to 39.7% with a
    four-fold variation seen between the 10th and 90th percentiles
    (4.9-21.0%) (Strachan et al., 1997; ISAAC Steering Committee, 1998).
    There was also a high variability in the 12-month-period prevalences
    of atopic eczema (defined as flexural dermatitis), which varied from
    0.3 to 20.5% (ISAAC Steering Committee, 1998).

    5.3.8  Conclusion

         Despite the differences in methodology between epidemiological
    studies conducted in the last 20-30 years, there is evidence that the
    prevalence of asthma and other atopic disorders is increasing in many
    countries (e.g., Burr et al., 1989; Burney et al., 1990; Ninan &
    Russell, 1992) (Fig. 17).

         A change in genetic susceptibility of populations to the
    manifestation of these diseases is unlikely to explain the observed
    time trend. However, although the studies suggest an increase in

    prevalence, this may partly reflect changes in diagnostic fashion and
    in public awareness of these diseases. The scientific evidence for
    increases in the prevalence of asthma in children and young adults
    since 1970, for example, is still weak because the measures used are
    susceptible to bias (Magnus & Jaakkola, 1997). Such methodological
    problems can only be minimized if a methodology using standardized
    questions on disease severity, including objective measurements, is
    applied (see also section 5.15).

         The ISAAC study in which such a standardized methodology was
    applied found a high worldwide variation in the prevalence and
    severity of asthma, rhinitis and eczema. In addition, countries such
    as Australia and New Zealand that have high prevalence rates of asthma
    in the ISAAC study are similar to those countries, including the
    United Kingom, with the highest prevalence rates for asthma symptoms
    in adults in the European Community Respiratory Health Survey (ECRHS)
    (Burney et al., 1996) and countries such as India or Algeria are in
    the lowest quartile for asthma prevalence in both studies. Together
    with the observed increases in the prevalences of these diseases in
    several countries during relatively short time periods of one or two
    decades, this variability in prevalence rates probably is a reflection
    of different lifestyle and environmental factors which seem to play an
    important role in the etiology of allergic diseases.

         There are worldwide differences in the distribution of these
    diseases. Among the exogenous factors that have been suggested are
    changes in allergen exposure, outdoor and indoor pollutants, cigarette
    smoke, work place exposure, personal hygiene and changes in diet.

    5.4  Age and gender distribution

         The natural history of asthma is not well understood (Martinez et
    al., 1995). Many infants have episodes of wheezing associated with
    viral infections soon after birth and during the first years of life.
    However, the majority of these children have transient conditions
    (Martinez et al., 1995). The period prevalence of wheezing has a peak
    before the age of 10 years (Anderson et al., 1992). During infancy
    boys are more often affected than girls, but this difference seems to
    disappear during and after adolescence (Barbee, 1987; Anderson et al.,
    1992). Several studies have shown a higher prevalence of asthma
    symptoms and bronchial hyperresponsiveness (BHR) in female than in
    male adolescents (Shaw et al., 1991; Anderson et al., 1992; Pearce et
    al., 1993; Riedler et al., 1994).

         Hay fever has a median age of onset of around 15 years. The
    prevalence has a peak in people aged 16-24 years. For perennial
    rhinitis the median age of onset is around 20 years and the prevalence
    reaches a peak in 20-30 year olds (Broder et al., 1974; Schachter &
    Higgins, 1976; Viner & Jackman, 1976; Sibbald & Rink, 1991a,b). A
    number of studies reported a slightly higher prevalence of allergic
    rhinitis in males than in females (Weeke, 1987). Atopic dermatitis

    FIGURE 16


    FIGURE 17


    starts typically during the first year of life (Hanifin, 1987). The
    onset of the disease is before the age of 7 in 60 to 90% of the cases
    (Schultz Larsen & Hanifin, 1992). The prevalence appears to be higher
    among females than among males (Schultz Larsen & Hanifin, 1992;
    Schultz Larsen, 1993).

    5.5  Migration

         Migration studies have suggested that the environment has a
    strong effect on the development of atopic disorders. Analyses of
    interregional migrants in the United Kingdom showed that the regional
    variation in cohorts, aged 5-7 years, was primarily related to the
    region of current residence, and not to the region of birth (Strachan
    et al., 1990). Ethnic group differences in the prevalence of atopic
    dermatitis were studied in London school children. It appeared that,
    compared to white children, London-born black Caribbean children were
    at increased risk of atopic dermatitis (Williams et al., 1995a).

         Striking differences in prevalence rates of asthma were found in
    young urban and rural Xhosa children (Van Niekerk et al., 1979). In
    this society, asthma was found mainly in urban communities and there
    appears to be a striking absence of asthma in the rural environment.
    Whether this is due to alterations in lifestyle or the possibility
    that the rural traditional way of life exerts a protective effect in
    the prevention of asthma is not clear.

         The prevalence of asthma in Tokelauan children has been studied
    in two environments, in Tokelau and in New Zealand (Waite et al.,
    1980). Prevalence of asthma assessed by an interview of the mothers
    was much higher among those children who were examined in New Zealand
    than among those examined in Tokelau. Furthermore, for the children
    examined in New Zealand, there was no significant difference in the
    asthma prevalence between those children born in New Zealand and those
    born in Tokelau.

         Asthma in children and adolescents living in the New Guinea
    Highlands was extremely uncommon in the sixties and early seventies.
    In 58% of the observed asthma cases the onset of the disease was not
    before the age of 30 years. Multiple sensitivities as assessed by
    skin-prick test were common and did not diminish with increasing age
    at onset. One possible explanation was that the degree of exposure to
    allergens is high enough to cause sensitization, but not high enough
    to cause lower respiratory symptoms until prolonged exposure in a
    perhaps especially sensitive person has taken place (Anderson, 1974).
    The prevalence of asthma among adults but not children living in the
    Eastern highlands of Papua New Guinea has risen drastically between
    1975 and 1985. Allergy to house dust mite appeared to be a significant
    feature in the disease pathogenesis, and it is likely that this is
    associated with modifications to traditional life style due to the
    introduction of blankets and changes in sleeping habits, which promote
    a more fertile environment for growth and multiplication of mites
    (Dowse et al., 1985).

    5.6  Viral infection

         Findings of different studies suggest that viral infections in
    early life may play a part in the prevention of allergic sensitization
    (Martinez, 1994). Interesting observations with regard to a possibly
    preventive role of viral infections in the development of asthma were
    made in the population of Tristan da Cunha. The prevalence of asthma
    among these islanders had earlier been reported to be one of the
    highest in the world (Mantle & Pepys, 1974). After the evacuation from
    Tristan da Cunha because of a volcanic eruption immunoassays in blood
    samples of the islanders revealed a low prevalence of serum antibodies
    against common viruses. During the years of evacuation, when the
    islanders lived in the United Kingdom, a high incidence of respiratory
    infection was observed. Thus, this population with a high prevalence
    of asthma and a high prevalence of allergic sensitization had a very
    low incidence of respiratory infections while living on the remote
    island, and became heavily infected after being exposed to respiratory
    viruses that they had probably rarely encountered before. Similar
    observations were made in the Western Carolina islands where the
    prevalence of asthmatic symptoms among children was very high, but
    viral infections presumably very uncommon (Martinez, 1994). A study
    from the United Kingdom found an inverse association between hay fever
    and the number of older siblings (Strachan, 1989; Strachan, 1995).
    Factors directly or indirectly related to the number of siblings may
    decrease the susceptibility of children to become atopic. Again,
    infections acquired during early childhood were proposed to be
    protective against allergic sensitization. It has been speculated that
    declining family size may in part contribute to the increasing
    prevalence of atopic diseases reported in Western countries over the
    past few decades, because of a lower chance of infection by older
    siblings. In children from eastern and western Germany, allergic
    sensitization as assessed by skin test reactivity was also inversely
    associated with the number of siblings (von Mutius et al., 1994b).

    5.7  Socioeconomic status

         Hay fever has been recognized as a complaint of the more affluent
    classes since the 19th century. A study of adults in south London
    found little difference in the prevalence of rhinitis symptoms or
    skin-prick reactions by social class, but a greater use of the label
    "hay fever" by doctors for patients of higher socioeconomic status
    (Sibbald & Rink, 1991a).

         Eczema is more prevalent in British school children of higher
    socioeconomic status than in those of lower status. Exposures
    associated with social class are probably at least as important as
    genetic factors in the expression of childhood eczema (Williams et
    al., 1994b). The authors suggest that most or even all of the reported
    changes in the prevalence of atopic dermatitis are due to a secular
    trend in diagnosis. Population studies have used a range of different
    methods and definitions for atopic dermatitis ranging from
    questionnaire-based recall of eczema as a child or parental recall to

    health visitors' and general practitioners' records. It is likely that
    the term eczema is heavily biased by social class (allergy being a
    more acceptable term in higher social classes) and with time (eczema
    was a less acceptable label in earlier years due to connotations of
    uncleanliness and arthropod infestation).

         Hospital admissions for eczema have fallen over the last 20
    years, but such in-patient data are misleading as much of this
    reduction is probably due to the success of treatment with topical
    corticosteroid preparations (Williams, 1992).

         The prevalence of wheeze varied little by socioeconomic group in
    an investigation of 5472 children, aged 5-17 years, in the United
    Kingdom (Strachan et al., 1994). However, marked trends of severity
    towards increased morbidity in poorer families were observed.
    Diagnostic labelling and drug treatment of wheezy children did not
    differ substantially with socioeconomic status. Thus, a degree of
    socioeconomic equality existed in the process of medical care for
    childhood asthma in the United Kingdom (Strachan et al., 1994;
    Strachan, 1996).

         No effect of socioeconomic status on the prevalence of asthma was
    noted in the first and second US National Health and Nutrition
    Examination Surveys (NHANES) (Gergen et al., 1988). In the Auckland
    region, 1050 children aged 8-9 years were examined by parental
    questionnaire and histamine inhalation challenge (Mitchell et al.,
    1989). There was no relationship between socioeconomic status and
    asthma diagnosis, bronchial hyperresponsiveness, or any combination of
    bronchial hyperresponsiveness with symptoms or diagnosis. The relative
    importance of socioeconomic status and several other factors in the
    etiology of wheezing illness in the first 5 years and on the
    persistence of this illness at the age of 16 years was studied in over
    15 000 children born in the United Kingdom during one week of April
    1970. Persistence of wheeze at age 16 years was related to high social
    status (Lewis et al., 1995).

         During a study of the prevalence of asthma and bronchitis in
    Sydney school children, some social and environmental factors were
    documented to ascertain if these affected the prevalence of either of
    these diseases (Peat et al., 1980). No consistent relationship was
    found between social class and lung disease with the exception of
    increased prevalence of asthma in boys and girls of higher
    socioeconomic status. Differential access and utilization of medical
    care by the poor and rich may contribute to differences in asthma
    prevalence. The relationship of socioeconomic status to various
    indicators of asthma was studied in Canada in the context of universal
    access to medical care. As compared with children from the most
    advantaged homes, children from the least advantaged homes were more
    likely to present exercise-induced bronchospasm, while there was no
    excess of reported wheeze or diagnosed asthma. This result was
    interpreted as indicating that there are unidentified environmental

    factors that contribute to the excess asthma morbidity in children
    (Ernst et al., 1995). In general, the association of socioeconomic
    status with hay fever and eczema seems to be more consistent than with
    asthma and respiratory symptoms.

    5.8  Occupational exposure

         Many prevalence studies have been conducted among workers in
    high-risk occupations. Asthmatic workers may differ with regard to the
    frequency of attacks, to the occurrence and to the onset of airway
    obstruction. Diagnostic guidelines for occupational asthma were
    internationally proposed in the USA and in Europe (Cartier et al.,
    1989; Burge, 1989; EAACI, 1992). The diagnosis of occupational asthma
    is based both on the clinical signs of the disease and the
    demonstration of the occurrence of a recognized allergen in the
    patient's workplace. Objective tests, such as skin testing, spirometry
    and serial measurements of the peak expiratory flow, should be
    performed to ascertain the occupational nature of the disorder (see
    also section 4.4.4). Other diagnostic tools in epidemiological surveys
    are standardized questionnaires inquiring information on work-related
    symptoms. Typical symptoms of occupational asthma are symptoms
    comprising difficulty in breathing, chest tightness, wheezing, a
    period of initial exposure of 2 weeks or longer before the first onset
    of symptoms, or evidence of airflow obstruction, and improvement of
    symptoms when the subject is not working for days or longer.

         Surveillance programs in the United Kingdom and Canada indicate
    that occupational asthma is the most frequently reported occupational
    lung disease accounting for 26 to 52% of the reports (Chan-Yeung &
    Malo, 1995a). The proportion of asthma attributable to occupational
    exposure is not known. Estimates range from 2 to 15%. The role of
    occupational exposure is difficult to ascertain, because: a)
    occupational asthma is still poorly recognized; b) affected workers
    are scattered through many small workplaces often employing few
    workers; c) methods of confirming work relationships are often rather
    crude; d) there is no good reporting system of occupational asthma in
    many countries; e) there are very few screening or surveillance
    programmes for this condition.

         Owing to such methodological problems it is often not possible to
    give unbiased estimates of the true prevalence and incidence of
    occupational asthma in populations if official government statistics
    such as disabling benefit awards or worker's compensation boards are
    used and comparisons within and between countries may be distorted
    (Meredith & Nordman, 1996).

         Occupational asthma falls into two categories: a) pre-existing
    asthma that is aggravated by irritant or physical stimuli in the
    workplace and b) asthma that is specifically induced by sensitization
    to a workplace chemical (Jarvis et al., 1996). About 250 agents that
    can give rise to occupational asthma are recognized (Chan-Yeung &
    Malo, 1995a). Isocyanates are responsible for the most common form of

    the disease, i.e., occupational asthma with latency. Occupational
    asthma without latency follows exposure to high concentration of
    irritant gases, fumes or chemicals; chlorine and ammonia are the most
    common agents (Chan-Yeung & Malo, 1995a,b). Some substances may give
    rise to asthma by inducing specific IgE antibodies. These allergens
    are mostly substances of high relative molecular mass (>5000) such as
    proteins in the urine of laboratory rats. Others are compounds of low
    relative molecular mass, such as complex halogenated platinum salts or
    acid anhydrides; these agents act as haptens and combine with a body
    protein to form a complete antigen (Venables & Chan-Yeung, 1997). It
    has been proposed that certain chemical allergens may cause
    sensitization of the respiratory tract via an IgE-independent
    mechanism. However, it is possible that new or technically refined IgE
    measurements may reveal a much greater association between IgE
    antibodies and chemical respiratory sensitization than is presently
    assumed (Kimber & Wilks, 1995). Intermittent exposure to high levels
    of an occupational agent is associated with a higher risk of
    development of work-related asthma than a steady exposure to lower
    concentrations, typical for isocyanate exposure.

         Not all subjects develop occupational asthma under the same
    exposure conditions. Various host markers (genetically determined) and
    factors (acquired) have been incriminated in occupational asthma. In
    general, atopy appears to be an important risk factor for occupational
    asthma due to compounds of high relative molecular mass, such as
    enzyme detergents or exposure to laboratory animals (Chang-Yeung,
    1990). In a survey of workers exposed to flour in bakeries or mills,
    the relation with symptoms was independent of atopic status (Cullinan
    et al., 1994). Atopy has little predictive value in occupational
    asthma due to chemicals of low relative molecular mass, and routine
    screening for atopy in high-risk workplaces may not be justified
    (Chan-Yeung & Malo, 1995b).

         The effect of smoking on occupational asthma is not clear and
    appears to be dependent on the type of occupational agent. When the
    agent induces asthma by producing specific IgE antibodies, cigarette
    smoking may enhance sensitization. However, cigarette smoking was not
    associated with increased work-related asthmatic symptoms in workers
    exposed to detergent enzymes, laboratory animals or colophony
    (Chan-Yeung, 1990; Chan-Yeung & Malo, 1995a,b).

         As part of the European Community Respiratory Health Survey
    (ECRHS), the risk of occupational asthma has been estimated in a
    random sample of 2646 young Spanish adults aged 20-44 years. Depending
    on the definition of asthma, 2.6-6.7% of asthma cases were
    attributable to occupational exposures (Kogevinas et al., 1996).
    Incidence rates for occupational asthma for 1992 were estimated to be
    approximately 153 cases per million workers in Finland (Meredith &
    Nordman, 1996). Incidence rates reported for the United Kingdom in the
    Surveillance of Work-related and Occupational Respiratory Disease
    Project (SWORD) between 1989 and 1991 showed a high risk of
    occupational asthma among paint sprayers, chemical and food

    processors, laboratory staff, plastics and metal treatment workers,
    and in welders and electronic assemblers (Meredith, 1993). For 1993
    the incidence was estimated to be 37 per million working persons per
    year (Meredith & Nordman, 1996). The most frequent agents causing
    asthma among the organic agents were flour, grain, hay, wood dust and
    laboratory animals; among the chemical agents were isocyanates and
    glutaraldehyde, and some miscellaneous compounds such as solder,
    colophony, glues and resins (Meredith & McDonald, 1994).

         In the UK SWORD project there is a high level of national
    coverage by chest physicians participating in the surveillance project
    and estimates of the working population at risk are available, so that
    incidence rates can be calculated by age, gender, region and
    occupation. However, there is underestimation, because some patients
    are seen only by general practitioners. Questions remain regarding
    diagnostic accuracy and etiology.

         In the following sections, studies on specific occupational
    exposures in relation to occupational atopic diseases, with emphasis
    on occupational asthma, are reviewed. The selection of exposures is
    based on findings of the SWORD project and the availability of
    epidemiological data.

    5.8.1  Chemicals with low relative molecular mass

         A total of 314 cases of occupational asthma were diagnosed at the
    Institute of Occupational Health in Helsinki, Finland during the
    period 1987 to 1990 (Savonius et al., 1993). By far the most common
    causes of occupational asthma were low relative molecular mass
    chemicals such as the isocyanates (76 cases), followed by formaldehyde
    (18 ases), epoxy resin and epoxy resin hardeners (17 cases), and
    cyanoacrylates (6 cases).

    5.8.1.1  Diisocyanates

         The main occupational hazards caused by polyurethane chemicals
    are asthma and rhinitis, but contact dermatitis and urticaria may also
    develop (Estlander et al., 1992). Exposures to toluene diisocyanate
    were studied for effects on respiratory health of workers in two
    plants manufacturing polyurethane foams (Jones et al., 1992).
    Intensive personal monitoring was performed to characterized job
    exposures. Initial questionnaire and spirometry data were obtained in
    386 workers. Multiple regression analyses showed significant adverse
    effects of cumulative toluene diisocyanate exposure on airway
    responsiveness. According to Diller (1987), the incidence of
    isocyanate asthma reported from different studies varies between 0 and
    25%. Reasons for differences in observed incidence are intensity of
    isocyanate exposure, criteria for diagnosis, mode of calculation,
    sensitizing capacity of different isocyanates, individual
    predisposition and confounding factors. No geographical or ethnic
    difference was observed.

    5.8.1.2  Acrylates

         Acrylate monomers are used in a variety of industrial fields.
    Their industrial use is increasing, since they have many features that
    make them superior to formerly used chemicals (Savonius et al., 1993).
    Contact sensitization is a well-known adverse health effect of
    exposure to acrylates, but they may also cause respiratory symptoms.
    The main acrylic compounds currently in use are acrylates,
    cyanoacrylates and methacrylates. While methacrylates are well-known
    contact sensitizers, cyanoacrylates have caused only few cases of
    contact allergy. Acrylates may also have other harmful health effects.
    Hand and finger symptoms and paraesthesiae have been reported among
    dental personnel preparing acrylates with their hands. The domestic
    use of acrylates is limited and the sensitizing problem is mainly
    occupational, but probably occurs in many different industries.
    Cyanoacrylates are used mostly as a component of a high strength glue
    used for joining different materials. Methacrylates are used as
    adhesives, as dental and orthopaedic fillings, as material for
    protheses and as an embedding material for different purposes,
    including histological preparations.

    5.8.1.3  Anhydrides

         Methylhexahydrophthalic anhydride (MHHPA) and
    methyltetrahydrophthalic anhydride (MTHPA) are dicarboxylic anhydrides
    used as hardeners for epoxy resins. MHHPA and MTHPA typically require
    an elevated curing temperature (50-200 °C), which facilitates escape
    of anhydride vapours. Anhydrides are low relative molecular mass
    chemicals that have been reported to cause immunologically mediated
    respiratory diseases (Tarvainen et al., 1995). Contact urticaria and
    other skin symptoms have also been described. Some anhydrides, e.g.,
    phthalic anhydride, have caused generalized urticaria, in connection
    with respiratory symptoms, after high exposure.

    5.8.1.4  Solder flux

         Questionnaires and lung function measurements were administered
    to 104 electronic workers in the USA who soldered printed circuits
    boards. Symptoms of eye, throat and nose irritation occurred in nearly
    half of the group. Lower respiratory tract symptoms, including cough,
    phlegm production and wheezing, also occurred with increased
    frequency, compared with reported rates among a general population
    sample (Greaves et al., 1984).

    5.8.2  Metals

    5.8.2.1  Cobalt

         Several clinical and experimental findings point to cobalt as a
    main sensitizer and causal agent of hard metal asthma (Nemery et al.,
    1992; Swennen et al., 1993; Cirla, 1994; Lauwerys & Lison, 1994).
    Clinical features have been clearly identified by bronchial

    provocation tests. IgE and IgG antibodies with cobalt specificity have
    been demonstrated. Clinically, the ability of cobalt to induce delayed
    hypersensitivity is well known for contact dermatitis (Cirla, 1994).
    Occupational exposure to cobalt occurs mainly by inhalation in various
    industries and occupations involved in the production and processing
    of metal and various cobalt-containing alloys and salts. The potential
    for exposure to cobalt is particularly important during the production
    of cobalt powder, the production and processing and use of hard
    metals, the polishing of diamonds with cobalt-containing disks, the
    use of pigments and dryers containing cobalt salts and the processing
    of cobalt alloys. In a cross-sectional survey of 194 workers from 10
    diamond polishing workshops and 59 workers from 3 other workshops, a
    questionnaire was administered and spirometry was performed to assess
    whether exposure to cobalt was associated with respiratory impairments
    (Nemery et al., 1992). Spirometry showed significantly lower indices
    of ventilatory function in the group with the highest exposure to
    cobalt. These differences were not due to differences in smoking
    habits.

    5.8.2.2  Metal-polishing industry

         A comparative study of spirometric measurements in 104 polishers
    and 90 unexposed controls was carried out in 25 brass and steel ware
    polishing industries in Moradabad, India (Rastogi et al., 1992). The
    polishing process generates dust, containing fine particles of emery
    and metal, which are mainly composed of copper and zinc and constantly
    inhaled by the polishers. Of the polishers 58.6% had one or more
    respiratory symptoms as compared to 25.5% of the controls.
    Occupational asthma was found to be confined to polishers, 4.8% being
    affected. The polishers exhibited significantly greater reduction in
    various lung function parameters over the work shift, which was larger
    in smokers than in non-smokers. The duration of exposure was directly
    correlated with acute fall in lung function.

    5.8.2.3  Aluminium

         Occupational asthma is a major respiratory health problem within
    the primary aluminium industry (O'Donnell, 1995). In Australia and New
    Zealand the incidence of occupational asthma in primary aluminium
    smelting varies between smelters, estimates ranging from 0 to 2%.
    Cases showed no association between the frequency of the symptoms or
    the severity of bronchial hyperresponsiveness and a family history of
    asthma, atopic skin test, tobacco smoking or age. Current evidence
    suggests that occupational asthma in the aluminium industry is
    irritant induced and caused by inhalation exposure to gaseous or
    particulate fluoride compounds.

    5.8.2.4  Platinum salts

         Nonspecific and specific bronchial hyperresponsiveness in
    immediate-type asthma caused by platinum salts did not cease after
    removal from exposure (Merget et al., 1994) (see also section
    4.3.4.1).

    5.8.3  Natural rubber latex

         Populations at increased risk of developing natural rubber latex
    hypersensitivity include health care workers, rubber industry workers
    and subjects undergoing multiple surgical procedures, especially
    children with spina bifida and urogenital abnormalities. Prevalence
    figures for natural rubber latex allergy in studies using skin-prick
    tests range from 2.9 to 17% among hospital employees and are around
    11% among glove-manufacturing workers (Vandenplas, 1995). Natural
    rubber latex allergy has been demonstrated in 32 to 50% of the
    children with spina bifida by skin-prick test or serological testing.
    The prevalence of sensitization to natural rubber latex in the general
    population ranged from 0 to 9% according to the atopic status of the
    populations under study (Turjanmaa, 1994; Vandenplas, 1995). The
    observed rise in incidence of sensitization to natural rubber latex
    during the last decade is probably related to the increased use of
    natural rubber latex devices as a protective barrier against
    infections. Other possible determinant factors include increased
    recognition of natural rubber latex allergy by exposed workers and
    clinicians, changes in manufacturing methods, and discontinuation of
    steam sterilization.

         Complications were thought to be limited to contact dermatitis
    due to irritation and sensitivity to certain rubber additives, and
    only few reports of phenomena consistent with natural rubber latex
    sensitivity were published before 1984. A possible explanation for the
    abrupt rise in the incidence of natural rubber latex sensitivity is a
    change in rubber manufacturing or rubber processing. There is a lack
    of epidemiological studies observing the longitudinal trends in
    prevalence and natural history of natural rubber latex allergy. The
    risk for natural rubber latex allergy among health care workers
    appears to vary with the frequency and intensity of exposure.
    Cross-reactivity between natural rubber latex and certain foods,
    particularly banana, avocado, chestnut and other fruits such as kiwi,
    papaya and passion fruit, have been described (Charous, 1994).

         A questionnaire and skin-prick tests with natural rubber latex
    and common inhalant allergens were administered to hospital personnel
    (201 nurses, 50 members of the cleaning staff, 38 laboratory
    technicians), of whom 4.7% showed skin reactivity to latex. Among
    those with a negative skin test to natural rubber latex no one had a
    history of occupational asthma. For the latex-sensitive subjects
    (N=12), a histamine challenge test was performed, which showed
    bronchial hyperresponsiveness in all subjects. Seven of the twelve
    developed significant bronchial response to a challenge test with
    latex gloves. The overall prevalence of occupational asthma due to
    natural rubber latex was estimated as 2.5% (Vandenplas et al., 1995).

         A skin-prick test was performed on 77 surgeons and nurses with an
    allergen solution made from latex gloves and rubber latex catheters.
    In addition, subjects answered a questionnaire on the history of hand
    dermatitis, atopic eczema, rhinitis or asthma, and symptoms when using

    natural rubber latex gloves. The prevalence of relevant immediate
    allergy assessed by skin-prick test was 5.2%. The reliability in
    detecting sensitized persons was limited. The agent used previously in
    skin-prick testing to demonstrate natural rubber latex sensitization
    gave unreliable results; the use of a standardized reagent is
    necessary (Cormio et al., 1993).

         Among 512 hospital employees 7.4% of the doctors and 5.6% of the
    nurses working in operational units were allergic to natural rubber
    latex. The frequency was lower in non-operating units and among
    laboratory personnel (Turjanmaa, 1987).

    5.8.4  Flour

         Occupational respiratory diseases are common among bakers
    (Keskinen et al., 1978; Meredith, 1993; Reijula & Patterson, 1994; De
    Zotti et al., 1994; Lauwerys & Lison, 1994). In Finland, for example,
    the mean annual incidence of occupational respiratory diseases was 31
    per 100 000 among the general work force compared to 374 per 100 000
    among bakery workers (Reijula & Patterson, 1994). The annual incidence
    of occupational asthma as reported to the SWORD project in the United
    Kingdom was 334 per million for bakery workers as compared to 658 per
    million for paint sprayers or 175 per million for electronic
    assemblers (Meredith, 1993). A substantial prevalence of wheat flour
    allergy was found in bakers and pastry cooks (Armentia et al., 1990).
    Not only wheat allergens, but also alpha-amylase must be considered as
    the causative agent (De Zotti et al., 1994). Other allergens such as
    storage mites are suspected to play a role in the development of
    occupational asthma (Tee et al., 1992a,b). Findings from the initial
    cross-sectional phase of a cohort study of employees exposed to flour
    in bakeries or mills showed that subjects without previous exposure to
    flour expressed related symptoms especially to flour aeroallergen
    (Cullinan et al., 1994). Forty-four male workers exposed to flour and
    164 unexposed controls were examined by using personal samplers
    measuring inspirable dust concentrations. The proportion of subjects
    with one or more symptoms and with bronchial hyperresponsiveness was
    significantly greater among workers exposed to flour than among
    controls. The conclusion was drawn that despite exposure to relatively
    low concentration levels of inspirable flour dust, subjects working in
    the baking industry are at risk of developing both respiratory
    symptoms and airway hyperresponsiveness (Bohadana et al., 1994).

    5.8.5  Animals

         The occurrence of respiratory disease was studied in 257 active
    veterinarians and 100 control subjects who had no occupational animal
    contact. Asthma was significantly more prevalent in veterinarians than
    in controls (Lutsky et al., 1985). Selected indicators of allergy and
    atopy were studied to determine predictors of laboratory animal
    allergy in a prospective study of laboratory technicians. Although the
    prevalence of atopy and allergic symptoms had increased in exposed
    technicians after the follow-up period of 2 years, this was also found

    in an unexposed matched control group, and there were no significant
    differences between the groups in any measured variable at follow-up
    (Renstrom et al., 1994).

    5.8.6  Other agents

         The association between occupational exposure to low levels of
    airway irritants and bronchial responsiveness to histamine was
    assessed in 688 male workers of synthetic fibre plants (Kremer et al.,
    1995). According to job titles, exposure status was grouped into 7
    categories: 1) reference group, 2) white collars, 3) SO2, HCl, 
    SO42-, 4) polyester vapour, 5) oil mist and oil vapour, 
    6) polyamide and polyester vapour, 7) multiple exposures. A higher 
    prevalence of airway responsiveness was associated with history of 
    allergy and respiratory symptoms. A slight trend was seen for subjects 
    with more than 5 years of exposure to polyester vapour and oil mist 
    and oil vapour towards a higher prevalence of bronchial 
    hyperresponsiveness. No overall association could be demonstrated.

    5.9  Allergic contact dermatitis

    5.9.1  Epidemiology of allergic contact dermatitis

         Only a few studies have investigated the frequency of allergic
    contact dermatitis in the general population (Coenraads & Smit, 1995).
    Nielsen & Menné (1992) studied the distribution of allergic contact
    sensitization in an unselected sample of 793 individuals. Of these,
    567 participants were patch tested with the standard series
    (TRUE-test). This series included the most common contact sensitizers,
    such as metals, fragrances, preservatives and medicaments. It has been
    found that 50-80% (depending on patient selection) of all cases of
    allergic contact dermatitis will be diagnosed using this series (Menné
    et al., 1992). Among the 567 volunteers were 15.2% who positively
    reacted to one or more of the substances included in the test series.
    Multiple contact sensitizations were thus observed more commonly than
    expected (Nielsen & Menné, 1992). Patients evaluated at patch test
    clinics often have multiple contact allergies. In experimental studies
    individuals with one contact allergy were found to be more easily
    sensitized to a second one (Friedmann, 1990). In the clinical
    situation the causes of multiple sensitivities are difficult to
    evaluate, because factors such as genetic susceptibility, broken skin
    barrier and multiple exposures are mixed up. Multiple contact
    sensitivities are typically seen in individuals with long-lasting
    chronic dermatitis.

         In the following paragraphs, specific exposures closely related
    to allergic contact dermatitis are discussed.

    5.9.1.1  Nickel

         Nickel-containing metal alloys and nickel-plated surfaces release
    free nickel ions when in direct contact with human sweat. The typical
    nickel dermatitis is therefore located beneath metal items, such as

    buttons, jewellery, suspenders, glasses and similar objects. Among the
    567 participants of an unselected sample of 793 subjects were 11.1%
    women and 2.2% men affected with a nickel allergy (Nielsen & Menné,
    1993) (see Table 18; section 4.1.5.1). Individuals primarily
    sensitized from consumer items might, at a later stage, develop
    occupational nickel dermatitis when occupationally exposed to this
    metal. Tools and equipment used in different jobs by workers such as
    carpenters, electricians, painters and plumbers were found to release
    nickel (Lidén et al., 1996).

    5.9.1.2  Chromates

         Chromium salts, but not metallic chromium, are sensitizing. The
    hexavalent salts are the main sensitizers as they penetrate the skin
    more easily than the trivalent chromate (Gammelgaard et al., 1992).
    Chromate was found to be a cause of occupational hand eczema among
    employees in the construction industry, mainly because of the presence
    of hexavalent chromate in wet cement (Irvine et al., 1994). The
    irritating capacity of cement (abrasive and high pH), combined with
    the potent allergen hexavalent chromate, caused occupational hand
    eczema in up to 10% of the workers. Addition of ferrosulfate to cement
    reduces hexavalent chromate to the trivalent state which has a minimal
    bioavailability. Introduction of ferrosulfate addition to cement in
    the Scandinavian countries has significantly reduced occupational hand
    eczema (Avnstorp, 1992).

    5.9.1.3  Fragrances

         Allergic contact dermatitis can also be caused by fragrances
    (Johansen & Menné, 1995; Johansen et al., 1996a, 1997; Scheinman,
    1996). In a study by Nielsen & Menné (1992), 1%-2% of the 567
    participants were affected by fragrance allergy (Table 18). Geier &
    Schnuch (1996) found a frequency of fragrance allergy of 8 to 17%
    among eczema patients.

    5.9.1.4  Preservatives

         Preservatives are widely used in water-based cosmetics,
    households, and industrial products. Preservatives have antimicrobial
    effect against a wide range of microorganisms. The most widely used
    preservatives in cosmetics are the parabens, formaldehyde and
    formaldehyde releasing substances (quaternium 15, diazolidinyl urea,
    imidazolidinyl urea) and the isothiazolones (Andersen & Rycroft,
    1991). Formaldehyde and isothiazolones are also frequently used in
    industrial products. In a British study, the frequency of contact
    allergy in dermatology patients varied between 1 and 3% for different
    preservatives (Jacobs et al., 1995). In another study the prevalence
    of contact allergy to formaldehyde in eczema patients was as high as
    8% (Flyvholm et al., 1997).

    5.9.1.5  Medicines

         The frequency of allergic contact dermatitis to topical medicines
    varies considerably from country to country, and even within regions,
    depending upon the availability and product preference by local
    prescribing doctors. Most cases of allergic contact dermatitis to
    medicines are caused by substances having doubtful documented effect
    or which can be replaced by other medicines (Angelini, 1995).
    Systematic studies have shown that 1-3% of eczema patients are
    contact-sensitized to topically used steroids (Dooms-Goossens &
    Morren, 1992; Lauerma, 1992). In addition, traditional remedies, such
    as balsam of Peru and Propolis, are well-known sensitizers (Li, 1995).

    5.9.1.6  Plants and woods

         Plants and woods contain a diversity of strong and weak allergens
    (Ducombs & Schmidt, 1995). Allergic contact dermatitis to plants and
    woods usually presents itself with acute oedematous bullous lesions
    that spread to skin areas distant from the primary contact. Airborne
    and dustborne patterns can be seen with severe facial dermatitis and
    flexural dermatitis. In North America poison ivy and poison oak are
    important plants because of their high content of urushiols, which are
    potent allergens (Kligman, 1958a,b). These allergens are also present
    in plants and trees in Asia and Australia. Allergic contact dermatitis
    from these plants represents an occupational health problem for
    outdoor workers.

         The  Compositae family comprises 13 000 species. It includes
    decorative flowers such as chrysanthemums and dahlias, but also a
    number of common weeds and salads. Some cases of  Compositae
    dermatitis present an airborne pattern often with a photo-aggravated
    facial dermatitis. This type of dermatitis is caused by different
    weeds in Europe, North America and Australia. Another variant of the
    disease has been recognized, giving a high morbidity and some
    mortality, in rural areas in India. The disease has been termed
    parthenium dermatitis after the offending plants ( Parthenium
     hysteropherous) (Lonkar et al., 1974). The main allergens in
     Compositae plants are different sesquiterpene lactones. In
    consecutive patch-tested eczema patients in Northern Europe 1-3%
    reacted to the sesquiterpene lactone mix (Ducombs et al., 1990). Most
    of these were hobby gardeners with severe hand eczema, but there were
    some cases among professional gardeners. The development of the
    sesquiterpene lactone mix illustrated how new diagnostic technologies
    can change the understanding of allergic skin diseases. The
    development of these techniques for routine testing has dramatically
    changed the prognosis for these patients, as the correlation between
    plant contact and their hand eczema was unrecognised earlier.

    5.9.2  Lack of a relationship between atopy and allergic contact
           sensitization

         Atopic hospital employees, who performed wet work, had a similar
    prevalence of contact allergy (22%) to that of their non-atopic
    colleagues (21%) (Lammintausta et al., 1982). In a hospital-based case
    series, the prevalence of contact allergy against ingredients of
    topical medications was common in subjects with atopic dermatitis.
    However, the sensitization rate to multiple other substances was low
    among these patients (Lammintausta et al., 1992). In another hospital
    based-study of patients with hand eczema, participants with past or
    present atopic disease showed a positive patch test reaction in a
    significantly lower proportion than non-atopic individuals.
    Furthermore, of all participants with a history of atopy, 22% had
    developed allergic contact dermatitis, while the corresponding figure
    for non-atopics was 45% (Rystedt, 1985). The reason for the apparently
    lower prevalence of allergic contact dermatitis in atopics might be an
    abnormal function of T-lymphocytes in atopic patients (Menne et al.,
    1987; Hanifin & Chan, 1995). However, a population-based study in
    Norwegian school children indicated a higher rate of contact allergies
    in atopic compared to non-atopic participants (Dotterud & Falk, 1994;
    Dotterud & Falk, 1995).

    5.10  Diet

         It has been hypothesized that changes in food habits are related
    to the increased prevalence of atopic diseases. Seaton et al. (1994)
    suggested that the increase in atopic disorders in the United Kingdom
    may in part be a result of a change in diet. Between 1961 and 1985 the
    average weekly consumption of fresh fruit, green vegetables, potatoes,
    fresh fish and red meat was reduced in Britain. The authors argue that
    a reduced dietary intake of natural antioxidants is related to a
    higher susceptibility to oxidant attack and airway inflammation. The
    food groups that were consumed less frequently between 1961 and 1985
    are main sources of antioxidants, such as vitamin C and beta-carotene,
    ubiquinone, and cofactors for antioxidant defence mechanisms, such as
    selenium, zinc and copper.

         The relationship between certain food groups and the risk of
    asthma was recently studied by Hodge et al. (1996). In this study,
    diet was assessed in 574 children by a detailed food frequency
    questionnaire including 200 food items, which was related to airway
    disease defined by respiratory symptoms or airway responsiveness to
    exercise. Children who ate fresh, oily fish had a significantly
    reduced risk of current asthma. No other food groups or nutrients were
    significantly associated with either an increased or reduced risk of
    current asthma (Hodge et al., 1996). The results of a cross-sectional
    study in Leipzig, Germany, indicated that a change towards a higher
    consumption of margarine was positively associated with hay fever; in
    turn, changes in the consumption of butter showed an inverse
    association with hay fever and atopic sensitization (von Mutius et
    al., 1998).

         Adverse reactions to food are a commonly encountered condition,
    especially in infancy and early childhood. The highest prevalence
    occurs between 1.5 and 3 years of age when in this age group as many
    as 25% have been reported to have adverse reactions to food (Björksten
    & Kjellman, 1987). However, only a minority of these reactions depend
    on immunological mechanisms (i.e., allergy) (Björksten & Kjellman,
    1987). Epidemiological studies on the most important dietary factors
    in relation to atopic diseases are reviewed below.

    5.10.1  Breast feeding

         The prophylaxis of atopy has been sought by elimination diets and
    by other preventive measures (Saarinen & Kajosaari, 1995). The role of
    breast feeding and/or avoidance of formulas based on cows' milk in
    early infancy has been the focus of much controversy (Anonymous,
    1982). Small amounts of protein ingested by the mother are secreted
    unchanged into breast milk. Potentially allergenic food ingested by
    the mother may thereby be transferred to the infant and cause
    sensitization. For this reason, maternal dietary restriction during
    lactation has been recommended (Hattevig et al., 1989). A prospective,
    long-term follow-up study from infancy to early adulthood indicated
    that breast feeding can protect against development of atopic disease
    (Saarinen & Kajosaari, 1995). Differences between the infant feeding
    groups were identified for atopic eczema, food allergy and respiratory
    allergy. Breast feeding for longer than 1 month without other milk
    supplements offers prophylaxis against food allergy at 3 years of age
    and also against respiratory allergy at age 17 years. Six months of
    breast feeding was required to prevent eczema during the first 3 years
    and possibly also to prevent substantial atopy in adolescence. Thus in
    this study, breast feeding seemed to confer long-term protection
    against allergic sensitization.

         In a trial to examine the effect of different feeding patterns on
    the incidence of atopic disease in newborns with a family history of
    atopy, the incidence of atopic symptoms in the control group of
    infants was 27% for those with a single relative with atopy and 50%
    for infants with a biparental history of atopy (Bardare et al., 1993).
    Breast feeding effectively reduced the incidence of atopic symptoms
    when the mothers complied with prescribed dietary restrictions. This
    may indicate that dietary restrictions are especially important in
    infants with biparental history of atopy.

         The stated reasons for discouraging the premature introduction of
    solid food include the possible risk of excessive weight gain,
    vulnerability of the gut to infection, and increased susceptibility to
    the development of allergic disease. The incidence of gastrointestinal
    illness, wheeze, and nappy dermatitis was not found to be related to
    early introduction of solid food feeding. There was a significant but
    rather small increase in respiratory illness at a particular age among

    infants given solids early. The incidence of eczema was increased in
    those infants who received solids at 8-12 weeks of age (Forsyth et
    al., 1993).

         Risk of atopy has been associated with a high cord-blood IgE
    (Kjellman & Croner, 1984). Therefore, many studies have focused on
    investigating those infants with high cord-blood IgE because
    development of an atopic disorder is more likely in these children. A
    dual approach of allergen avoidance, focusing on foods (breast milk
    and extensively hydrolysed formulas) and aero-allergens (treatment of
    bed- and living-room with acaricides), in comparison with controls who
    did not undergo any intervention, was beneficial in selected high-risk
    infants (Hide et al., 1994). Avoidance of potent food allergens in
    early life may increase the threshold for sensitization in those
    high-risk infants. Whether sensitization has been avoided or merely
    deferred has yet to be proved. Reduced exposure of infants to
    allergens in food and in house dust lowered the frequency of allergic
    disorders in the first year of life for high-risk children,
    pre-natally randomized to a prophylactic or control group (Arshad et
    al., 1992). A similar result was found in an outpatient follow-up of
    777 infants with a very low birth weight. Comparison after random
    assignment to early diet of human milk versus cows' milk-based
    pre-term or term formula revealed an increased risk of atopy for early
    exposure to cows' milk (Lucas et al., 1990).

    5.10.2  Sodium

         Asthma mortality is related geographically to sales of table salt
    and both epidemiological and experimental evidence suggest that a high
    dietary sodium intake may increase airway responsiveness. Studies have
    shown that a low-sodium diet contributes to a decrease in symptoms in
    children with severe asthma. Further investigations have presented
    ecological, observational and experimental evidence supporting a
    relation between salt intake and airway responsiveness (Tribe et al.,
    1994). Regional data from England and Wales showed a strong
    correlation between table salt purchases and asthma mortality for
    adult men and children of both sexes, but not for adult women. Asthma
    mortality in women was found to be more closely related to other
    factors (Burney, 1987).

         As part of a wider survey on asthma, 138 men living in two
    Hampshire villages in England underwent a bronchial histamine
    challenge test and had their 24-h urinary excretion of sodium
    measured. Bronchial reactivity was strongly related to 24-h excretion
    of sodium, suggesting that a high-sodium diet may enhance bronchial
    reactivity (Burney et al., 1986). A study on the effect of dietary
    sodium intake on the airway response to histamine supports the
    hypothesis that a high-sodium diet increases bronchial reactivity in
    men but not in women and suggests that moderate restriction of sodium
    intake in asthmatic men would reduce bronchial reactivity (Burney et
    al., 1989b).

         A controlled cross-over study of 14 asthmatics found that high
    salt intake worsened forced expiratory volume in one second (FEV1),
    peak expiratory flow rate (PEF) and symptoms (Medici & Vetter, 1991).
    This was interpreted as indicating a generalized dysfunction of
    cellular sodium regulation and providing an explanation for the
    salt-sensitivity of the asthmatics. In contrast, another study
    (Britton et al., 1994) found no support for the hypothesis that a high
    dietary sodium intake is a risk factor for airway hyperreactivity or
    atopic disease in the general adult population. No relationship either
    between Na+ and K+ intake assessed by a 7-day recall and bronchial
    hyperresponsiveness or chronic respiratory symptoms was found in a
    sample of 205 subjects (Zoia et al., 1995).

    5.10.3  Selenium

         There seems to be an association between reduced serum selenium
    concentration and lowered activity of the selenium-dependent enzyme
    glutathione peroxide (GSH-Px) (Hasselmark et al., 1993). The aim of a
    double-blind randomized study in 24 patients suffering from asthma was
    to investigate whether selenium (Se) supplementation in asthmatic
    patients increases GSH-Px activity and possibly brings about clinical
    improvement in the Se-supplemented group as compared to the placebo
    group. In the Se-supplemented group there were significant increases
    in serum Se and platelet GSH-Px activity after intervention, while no
    significant changes in these parameters could be observed in the
    placebo group. Although there were no significant changes in lung
    function measures in the Se-supplemented or placebo group, a
    statistically significant clinical improvement was observed in the
    Se-supplemented group. There are several possible mechanisms whereby a
    reduced selenium status with associated lower GSH-Px activity may
    contribute to the pathogenesis of asthma. Further research into the
    role of selenium and GSH-Px is required (Beasley et al., 1991).

         Selenium concentrations and GSH-Px activity were lower in 56
    asthmatic subjects compared to 59 control subjects (Flatt et al.,
    1990). In both the asthmatic and control groups, the mean whole blood
    selenium concentrations and GSH-Px activity were generally higher in
    those who reported having eczema or rhinitis or had positive responses
    to skin-prick tests. These findings are consistent with the hypothesis
    that low selenium concentrations may play a role in the pathogenesis
    of asthma in New Zealand.

    5.10.4  Vitamins and antioxidants

         Data from the Nurses Health Study (Troisi et al., 1995), a
    prospective investigation of major chronic diseases, suggests that
    anti-oxidant supplementation and intake of various fats during
    adulthood are not important determinants of asthma, although vitamin E
    from diet may have a modest protective effect. An effect of vitamin E
    on the inflammatory process seems plausible. The relation between lung

    function and dietary intake of the antioxidant vitamins C and E in the
    general population was investigated in a cross-sectional survey of a
    random sample of adults of Nottingham, England (Britton et al., 1995).
    After adjustment for gender, height, skin-prick test result and
    smoking, lung function parameters were significantly and independently
    related to the mean daily intake of vitamin C. These data support the
    hypothesis that lung function in the general population is related to
    antioxidant intake and that these vitamins may play a role in
    protecting against the development of chronic obstructive pulmonary
    disease. The generalizability of these study results, however, may be
    limited due to a low participation rate of less than 60%. In the
    Zutphen study, fruit intake was inversely related to the incidence of
    chronic nonspecific lung diseases (Miedema et al., 1993). No
    association was observed with intake of several antioxidants. The
    second US National Health and Nutrition Examination Survey (NHANES)
    reported an inverse association between wheezing and serum vitamin C
    and the serum zinc/copper ratio. These data suggest that several
    dietary constituents may influence the occurrence of respiratory
    symptoms in adults, independently of cigarette smoking (Schwartz &
    Weiss, 1990). The relationship between ventilatory function and winter
    fresh fruit consumption was studied in a random sample of British
    adults (Strachan et al., 1991). These findings suggest that
    antioxidant and other actions of vitamin C may protect against
    pulmonary emphysema and may reduce bronchorestrictor responses to
    environmental pollutants.

    5.11  Number of siblings and crowding

         Factors playing a role in the prevalence of allergic disease in
    industrialized countries might be associated with the generally
    improved standard of living (Williams et al., 1994b). Along with this
    change goes a smaller family size and lower number of children in
    families. Studies have observed a decrease in the prevalence of
    allergic rhinitis, eczema (Strachan, 1989, 1996)) and asthma symptoms
    (Shaw et al., 1994) with an increase in the number of older siblings.
    In addition, it was shown that the prevalence of atopic sensitization
    decreases with an increasing number of siblings (von Mutius et al.,
    1994a), and a strong inverse relation was found between atopic
    sensitization and the number of persons per room in households
    (Braback et al., 1995). Viral or bacterial cross-infections,
    especially in early life, which may occur more frequently in larger
    families, have been discussed to have a protective role against atopic
    disease by preventing proliferation of Th2-lymphocytes (Romagnani,
    1992b; Holt, 1994; Martinez, 1994). Helminth infestations have also
    been suggested to have a protective effect against allergy development
    (Williams, 1992) but studies in populations with a high prevalence of
    asthma indicated no such beneficial effect (Mantle & Pepys, 1974;
    Martinez, 1994).

    5.12  Indoor environment

         One of the most important changes in lifestyle during the past
    decades in industrialized countries is the change in indoor climate
    mainly due to modern building and furnishing materials, better
    insulation, wall-to-wall carpeting, increased indoor temperature and
    less ventilation. As a consequence of these factors and behavioural
    changes, such as indoor pet keeping, many pollutants and allergens can
    accumulate within dwellings. This may lead to higher sensitization
    rates to indoor allergens and consequently to an increased incidence
    of allergic diseases like asthma. This is especially relevant because
    people in industrialized countries spend up to 90% of their time
    indoors (Platts-Mills, 1994; Berglund et al., 1994). A variety of
    indoor and outdoor sources contribute to the indoor pollution burden
    and might play a role in the development or exacerbation of allergic
    disease. Important indoor sources are respirable particles from stoves
    and tobacco smoke; combustion products from cookers, ovens and
    heaters; formaldehyde from foam insulation, chipboards, furniture and
    fabrics; volatile organic compounds (VOCs) and other chemicals from
    paints, sprays fabrics, and combustion; biological material from
    animal sources such as dust mites, cats, etc., or from fungi, bacteria
    and pollen (Angle, 1988; Karol, 1991). Ventilation, humidity and
    temperature of dwellings also have an important influence on indoor
    concentrations of many pollutants and allergens (Brunekreef et al.,
    1989; Beggs & Curson, 1995). For many indoor factors, there is still a
    lack of good epidemiological studies.

    5.12.1  Tobacco smoke

         Passive exposure to tobacco smoke is an important risk factor for
    childhood asthma and wheezing, particularly when the child's mother
    smokes. Many studies have reported a higher risk of asthma or
    bronchial hyperresponsiveness among children exposed to tobacco smoke
    (Ware et al., 1984; Dekker et al., 1991; Martinez et al., 1992;
    Forastiere et al., 1992) and younger children are particularly prone
    to these side effects (Arshad et al., 1993; Stoddard & Miller, 1995;
    Platts-Mills et al., 1995). Children of smoking parents have also
    increased reactivity to allergens as assessed by skin-prick test
    (Martinez et al., 1988; Braback et al., 1995). Furthermore, children
    whose mothers smoked during pregnancy show increased IgE-levels in
    cord blood and increased risk of infant allergy (Magnusson, 1986).
    Therefore, the increasing prevalence of smoking in women of
    childbearing age may have contributed to the increase in atopic
    disease in children (Burney et al., 1990).

         Although a number of studies in adults have shown an association
    between active smoking and asthma, others have failed to find such an
    association. Thus, the evidence is not conclusive and the effect may
    be only small (Platts-Mills et al., 1995). Active smoking can increase
    total IgE serum concentration and ex-smokers show a decline in serum
    IgE concentration after cessation of smoking (Burrows et al., 1981).

    Studies on occupational allergies showed that the frequency of IgE
    sensitization and asthma is higher in cigarette smokers (Anonymous
    1985; Venables et al., 1985b). Little is known about the role of
    active and passive smoking on allergic rhinitis, and studies show, if
    any, only small effects (Ng & Tan, 1994a,b; Strachan, 1995; Tsunoda et
    al., 1995).

         In summary, smoking is known as a respiratory irritant and as an
    important source for indoor pollution. However, although many studies
    suggested that smoking plays a role in the etiology of asthma, there
    is no clear evidence from the available data to indict smoking as the
    driving force for the observed increases in asthma morbidity and
    mortality (Weiss et al., 1993). Nevertheless, even if the relative
    effect of active and passive smoking may be small, it can still have a
    major impact if large parts of populations are exposed (attributable
    risk) (Stoddard & Miller, 1995).

    5.12.2  Pets

         Many animals like cats, dogs, rodents, rabbits and cage birds
    live within or in close proximity to homes. About one third to one
    half of houses in the USA have a mammalian pet (Colloff et al., 1992;
    Ledford, 1994). There is strong evidence that exposure to a number of
    animal allergens can lead to primary sensitization and an increased
    risk of developing allergic disease (Colloff et al., 1992). The most
    important pet allergens are those from cats and dogs (Sears et al.,
    1989; von Mutius et al., 1994b; Ledford, 1994; Strachan & Carey,
    1995). Even after permanent removal of a cat from the home it may take
    several months before the concentration of allergens in domestic dust
    falls (Wood et al., 1992; Colloff et al., 1992; Munir et al., 1994b).
    Pet owners visiting a home with no pets can bring in pet allergens on
    their clothes, and rub it off during their visit, resulting in a
    considerable amount of pet allergens in the visited flat (Munir et
    al., 1993; Munir et al., 1994a,b).

    5.12.3  Biocontaminants

    5.12.3.1  House dust mites and insects

         House dust mites are an important source of indoor allergens.
    Most frequently detected species are  Dermatophagoides pteronyssinus,
     Dermatophagoides farina,  Euroglyphus maynei and  Blomia
     tropicalis (Colloff et al., 1992; Ledford, 1994). The mites have a
    narrow optimal temperature range for growth from 18°C to 27°C
    (Ledford, 1994). Because dust mites depend on ambient humidity they
    grow poorly in dry or high altitude climates. Mite allergens can be
    measured directly by ELISA. Indirect measurements of mite allergens
    rely on guanine quantification in house dust. However, this method has
    been shown to be of limited clinical value since a considerable amount
    of guanine may originate from non-house-dust mite sources (Hallas et
    al., 1993). Sensitization to dust mites seems to play an important

    role in the relation between damp homes and childhood respiratory
    symptoms (Verhoeff et al., 1995). High mite allergen exposure has been
    reported to increase the risk of sensitization in atopic children (Lau
    et al., 1989) and exposure in early childhood has been shown to be a
    determinant of subsequent asthma (Sporik et al., 1990). Sensitization
    to  Dermatophagoides pteronyssinus and other allergens was addressed
    in a study performed shortly after the fall of the Berlin wall on
    9- to 11-year-old children in eastern and western Germany. Skin-prick
    testing showed sensitization rates of 10.3% to  Dermatophagoides
     pteronyssinus (wheal reaction >3mm) in western Germany and of
    4.2% in eastern Germany, indicating that environmental and lifestyle
    factors may have an influence on sensitization to common allergens
    (von Mutius, 1994b).

         Cockroaches are also a substantial indoor allergen source.
    Sensitization has been shown to be a risk factor for emergency room
    visits in asthmatics (Pollart et al., 1989). Exposure to high levels
    of cockroach allergens and allergy against cockroaches was found to
    explain much of the asthma-related health problems in inner-city
    children from the USA (Rosenstreich et al., 1997). Exposure to
    cockroaches has also been associated with allergic rhinitis (Ng & Tan,
    1994b). Few data on sensitization to cockroaches exist for central
    Europe (Colloff et al., 1992), but the available data suggest that the
    prevalence of sensitization to cockroaches is low (Mosimann et al.,
    1992; Munir et al., 1994a). Other allergens from insects include
    moths, crickets, midges, locusts, beetles and various flies (Ledford,
    1994).

    5.12.3.2  Moulds

         The most common indoor moulds responsible for allergies are
     Aspergillus species,  Cladosporium, and  Penicillium (Ledford,
    1994). Moulds are responsive to temperature, humidity and substrate
    moisture level (Beggs & Curson, 1995). Damp housing has frequently
    been shown to be a risk factor for respiratory symptoms in children
    and adults (Brunekreef et al., 1989; Dekker et al., 1991; Brunekreef,
    1992) and may play a role in the sensitization to mould allergens
    (Verhoeff et al., 1995). However, it has been suggested that the
    observed association between damp housing and childhood asthma may be
    partly due to parental over-reporting (Strachan, 1988). Other possible
    indoor sources of fungal allergens include air conditioning equipment,
    humidifiers, degrading organic materials, and soil used for indoor
    plants; these sources can also be important for allergens from
    bacteria or protozoa (Dekker et al., 1991; Ledford, 1994).

    5.12.4  Other indoor factors

         A large variety of everyday rubber products (e.g., balloons, baby
    pacifiers, sports equipment, adhesives, etc.) contain natural latex,
    which may be a source of sensitization and a factor in allergic
    disease expression. Airborne leaf parts of the popular indoor plant

     Ficus benjamina (weeping fig) also contain latex particles (Axelsson
    et al., 1990; Bircher et al., 1993). It is not yet known whether
    natural latex, which is a well known occupational allergen
    (Vandenplas, 1995) (see also section 5.8.3) has any impact on allergic
    disease on a population level. Contrary to expectation, non-feather
    bedding, especially foam pillows, have been suggested to be a possible
    determinant for symptoms of asthma in adolescents (Strachan & Carey,
    1995).

    5.13  Indoor and outdoor environmental factors

    5.13.1  Nitrogen dioxide

         Nitrogen dioxide (NO2) is a strong oxidant. Indoor sources are
    cigarette smoke, gas and oil heaters or cookers, which can result in a
    high indoor concentration (Angle, 1988; Wardlaw, 1993). Main outdoor
    sources are combustion of fossil fuels in motor vehicles and power
    generation (Wardlaw, 1993). Combustion processes also generate a
    mixture of NO2 and nitric oxide (NO), and formation of indoor nitrous
    acid (HNO2) has also been demonstrated and associated with adverse
    respiratory effects (Samet et al., 1993). Experimental exposure and
    several epidemiological studies are inconclusive concerning the effect
    of NO2 on lung function (Ware et al., 1984; Angle, 1988; Neas et al.,
    1991; Gorski & Tarkowski, 1992; Samet et al., 1993). The current
    understanding of NO2 is that it can decrease pulmonary function in
    asthmatics, but its overall role in the development of allergic
    disease is not clear (Angle, 1988).

    5.13.2  Sulfur dioxide, acid aerosols and particulate matter

         The sources of these major outdoor air pollutants are mainly
    combustion of fossil fuels, like coal and oil, wood, and certain
    industrial processes. Sulfur dioxide (SO2) in water forms sulfurous
    acid (Gorski & Tarkowski, 1992). The indoor concentration of SO2 is
    raised if unvented combustion devices for kerosene are used or when
    cigarettes are smoked (Angle, 1988). Several studies on asthmatics
    have shown that SO2 can provoke bronchoconstriction and asthma-like
    symptoms even at low levels, especially during exercise and mouth
    breathing (Linn et al., 1983a,b). Animal studies indicated an
    enhancing effect of SO2 on sensitization to allergens (Riedel et al.,
    1988).

         Acid aerosols are comprised mainly of sulfuric acid and ammonium
    bisulfate (Wardlaw, 1993), which has also been shown to cause an
    increase in respiratory symptoms and a decrease in lung function
    (Koenig et al., 1983; Hackney et al., 1989). Acid aerosols can also
    comprise nitric acid, hydrochloric acid and hydroxymethansulfonic
    acid. Outdoor airborne acidity is associated with daily respiratory
    symptoms in asthmatics (Ostro et al., 1991).

         Particulate matter (e.g., dust, dirt and smoke) is a major source
    of air pollution and a complex and varying mixture of substances.
    Sources are motor vehicle emissions, such as exhaust fumes and abraded
    tyre fragments (Williams et al., 1995b), factory and utility
    smokestacks, residential wood burning, construction activity, mining,
    agricultural tilling, open burning, wind blown dust, fires, etc.
    Several studies have suggested an association between particulate
    matter (PM) exposure and asthmatic symptoms (Dockery et al., 1989; Xu
    & Wang, 1993; Dockery & Pope, 1994; Abbey et al., 1995; Brunekreef et
    al., 1995). Particles of a diameter <10 µm (PM10) are especially
    important for respiratory disease because they are readily inhaled
    deep into the lungs (CDC, 1994). Among them fine particles of a
    diameter <2.5 µm and ultrafine particles (<0.1 µm) may be most
    important (Peters et al., 1997). Animal studies indicated that diesel
    exhaust particles may increase the risk of sensitization against
    allergens (Muranaka et al., 1986; Takafuji et al., 1987; Fujimaki et
    al., 1994).

    5.13.3  Volatile organic compounds, formaldehyde and other chemicals

         Volatile organic compounds (VOCs) comprise a broad spectrum of
    substances including benzene, toluene, xylenes and aldehydes (Angle,
    1988; Becher et al., 1996). VOCs are emitted by a large number of
    materials and only few subgroups have been investigated for their
    potential role in allergy. The total indoor amount of VOCs and several
    sub-types like toluene and terpenes have been associated with
    asthmatic symptoms (Norback et al., 1995). Outdoor sources of VOCs
    have also been found to be related to increased rates of chronic
    respiratory symptoms characteristic of reactive airways (Ware et al.,
    1993). Another important VOC is formaldehyde, which is ubiquitous in
    the human environment. Important indoor sources are smoking,
    particle-boards, foam insulation and textiles (Imbus, 1985; Koenig,
    1988). Formaldehyde exposure has been related to asthma and other
    allergies (Imbus, 1985; Angle, 1988; Wjst et al., 1994; Norback et
    al., 1995), but evidence that formaldehyde exposure can cause allergic
    diseases in the airways is limited (Becher et al., 1996). Other
    chemicals such as ethylenediamine and isocyanate may also contribute
    to the allergic disease burden in the general community (Ledford,
    1994), but population-based data showing an association between
    allergies and indoor contamination by these pollutants are not
    available. The increasing use and diversity of household cleaning
    materials during the past decades have also been implicated in the
    expression of allergic diseases (Williams, 1992).

    5.14  Outdoor air pollution

         A link between outdoor air pollution and the increased prevalence
    of respiratory allergies such as asthma and allergic rhinitis has been
    suspected for some time. However, whether atmospheric air pollution
    can cause respiratory and other allergies is still not clear (Wardlaw,
    1993). Contamination of the air with plant pollen is a major natural
    source of air pollution. Important causes of outdoor air pollution are

    burning fuels such as coal, oil and wood, smelting of ores, and other
    industrial processes. Natural sources like volcanoes play only a small
    role. Motor vehicle tail-pipe emissions are a major contributor of
    several pollutants, such as diesel particles, oxides of nitrogen
    (NOX), carbon monoxide and other airborne particles (Bascom, 1996).

         The increase of allergic respiratory diseases coincided with a
    decrease in many outdoor air pollutants like SO2 and total suspended
    particles (TSP) (Weiss et al., 1993), whereas air pollution from motor
    vehicle emissions (e.g., O3, NOX) increased during the same period
    (Newman Taylor, 1995). Motor vehicles are also a major source of fine
    suspended particles (see section 2.5.3). Most outdoor air pollutants
    provoke more or less severe adverse effects in asthmatics, and
    exposure to multiple pollutants may cause synergistic effects (Koenig
    et al., 1990). A number of experimental animal studies indicate an
    association between allergy and air pollution (Osebold et al., 1980;
    Biagini et al., 1986; Muranaka et al., 1986; Takafuji et al., 1987;
    Riedel et al., 1988; Takafuji et al., 1989; Suzuki et al., 1993;
    Fujimaki et al., 1994). It is also assumed that pollutants can enhance
    the allergenicity of common allergens like pollen (Ishizaki et al.,
    1987; Behrendt et al., 1991; Gorski & Tarkowski, 1992). However, the
    observation that the sensitization rate to common allergens in cities
    in eastern Germany with previously high levels of traditional air
    pollutants such as SO2 or particulate matter was low in comparison to
    that of western Germany cities argues against the hypothesis that
    traditional air pollution from industrial production and household
    coal burning is the major factor driving the changes in morbidity
    patterns of allergic disease (von Mutius et al., 1992; von Mutius et
    al., 1994b). Similarly, a Swiss study reported effects of moderate
    average air pollution concentration from PM10, NO2 and SO2 on
    respiratory symptoms, such as chronic or nocturnal dry cough and
    bronchitis, but not between air pollution and asthmatic and allergic
    symptoms or diseases in children (Braun-Fahrländer et al., 1997).
    These results are in close agreement with findings from the US Six
    Cities Study (Dockery et al., 1989). In addition, the findings of the
    ISAAC study do not provide support for an association between air
    pollution and childhood wheezing; for example, countries with low
    degrees of ambient air pollution such as New Zealand were among those
    with the highest prevalence of asthma symptoms (ISAAC Steering
    Committee, 1998). Likewise, recent results from the Pollution Effects
    on Asthmatic Children in Europe (PEACE) project found only little
    overall adverse effect of ambient air pollutants (e.g., PM10, black
    smoke, SO2 and NO2) on respiratory health in children (PEACE, 1998).

    5.14.1  Pollen and dust

         Natural sources of air contamination are plant pollen from grass,
    ragweed, trees, etc. (Schutz-Kiss et al., 1995), moulds (Dawson &
    Mitchell, 1990), and natural particulate matter such as dust or dirt
    (CDC, 1994) which can cause symptoms of asthma and allergic rhinitis.
    Their role as a causal factor for the development of these diseases,

    however, is not clear. Air pollution due to natural factors like
    pollen or dust cannot easily explain the observed increase in allergic
    diseases in many regions because humans have always had intense
    outdoor contact with these substances. Several asthma outbreaks in
    populations have been shown to be related to man-made airborne
    allergen pollution. Examples are castor bean dust in USA, South Africa
    and Brazil (Anto, 1995), and soybean dust which was released during
    unloading of soybeans in the city harbour of Barcelona (Anto et al.,
    1989).

    5.14.2  Ozone

         A major source of ozone (O3) is motor vehicle exhaust. O3 is a
    main constituent of photochemical smog, and its formation requires
    NOX and reactive organic compounds as precursors and ultraviolet
    radiation (Gorski & Tarkowski, 1992; Wardlaw, 1993; Beggs & Curson,
    1995). It is the most ubiquitous air pollutant in the USA (Koenig,
    1995). Although O3 is mainly an outdoor pollutant, it is also present
    in low concentrations in the indoor environment (Koren, 1995). Many
    studies have shown that O3 has an aggravating effect in asthmatics
    and can reduce lung function (Krzyzanowski et al., 1992; Gorski &
    Tarkowski, 1992; Wardlaw, 1993; Koenig, 1995; Koren & Bromberg, 1995).
    Unlike SO2 there seems to be no marked difference in acute responses
    in asthmatics and non-asthmatics (Koenig et al., 1988). Animal models
    suggest that O3, like other air pollutants, may increase
    sensitization to allergens (Osebold et al., 1980; Sears et al., 1989).
    Ambient levels of O3 may also have a synergistic effect with pollen
    in the causation of allergic rhinitis (Bascom et al., 1990). However,
    only a few epidemiological investigations have been performed on a
    population level to evaluate the effect of ambient O3 on asthma and
    allergies (e.g., Braun-Fahrländer et al., 1997) and further
    investigations are needed to study the effect of O3 on these
    disorders under real-life exposure conditions (Magnussen et al.,
    1998).

    5.14.3  Motor vehicle emissions

         Motor vehicle traffic has increased dramatically in many
    countries (Utell et al., 1994) and it has been speculated that this
    increase may play a role in the observed changes of the prevalence of
    allergies. A number of occupational studies have shown an association
    between exposure to motor vehicle exhausts and adverse effects on
    respiratory symptoms and lung function (Gamble et al., 1987; Evans et
    al., 1988a; Ulfvarson & Alexandersson, 1990; Wade & Newman, 1993;
    Raaschou Nielsen et al., 1995). Others, however, have failed to find
    such an association (Speizer & Ferris, 1973; Tollerud et al., 1983;
    Ames et al., 1984). An association between allergic sensitization and
    components of motor vehicle exhaust fumes has been shown in various
    animal studies (Osebold et al., 1980; Muranaka et al., 1986; Takafuji
    et al., 1987; Riedel et al., 1988; Suzuki et al., 1993; Fujimaki et
    al., 1994; Lovik et al., 1997). A number of experimental studies
    suggested also an association between allergic disease and traffic

    pollution (Molfino et al., 1991; Braun Fahrländer et al., 1994;
    Devalia et al., 1994). Several epidemiological studies observed a
    relationship between exposure to motor vehicle traffic at residence
    and morbidity from respiratory and allergic disorders in adults
    (Yokoyama et al., 1985; Ishizaki et al., 1987; Nitta et al., 1993) and
    in children (Wjst et al., 1993; Edwards et al., 1994; Weiland et al.,
    1994; Keil et al., 1996; Oosterlee et al., 1996; Duhme et al., 1996;
    Brunekreef et al., 1997; Duhme et al., 1998a). Air pollution from
    motor vehicle traffic derives also from mechanical abrasion of tyres,
    which contain potentially allergenic latex particles (Williams et al.,
    1995b). However, evidence that exposure to motor vehicle traffic can
    cause asthma or allergies is not conclusive.

    5.15  Conclusions

         Asthma and allergic disorders represent a substantial burden not
    only on the affected individuals but also on health care resources in
    many countries. The costs of asthma are partly due to uncontrolled
    disease, and are likely to rise as its prevalence and severity
    increase (Barnes et al., 1996). One approach to reduce costs would be
    to improve disease control. Another approach would be to reduce the
    prevalence of asthma by preventive measures, and this would be
    accompanied by a reduction in the costs of treatment and care (Peat,
    1996). Environmental factors that have changed in the last decades
    appear to be largely responsible for the observed increase in the
    prevalence of asthma and allergic disease in many countries. The
    determinants of these changes need to be identified in order to design
    interventions that can reverse these trends. Such prevention
    strategies in the field of asthma and allergies can aim at high-risk
    groups or at populations as a whole (Rose, 1985). It is important to
    note that even if a preventive measure offers little to each
    individual it can bring large benefits to the community into which the
    preventive measure is introduced (prevention paradox) (Rose, 1985).

         It is often impossible to blame a single culprit within a complex
    mixture of behaviours and exposures (such as indoor or outdoor air
    pollution) for observed adverse health outcomes, and the studied risk
    factor might also have different effects in the presence of other
    factors (Greenland, 1993). Because randomized assignment of
    individuals to certain exposures is impractical, if not unethical, in
    environmental epidemiology, researchers rely mainly on data from
    non-experimental studies with well-known inherent methodological
    shortcomings (Rothman, 1993). Depending on the hypotheses being
    studied, a number of epidemiological research strategies are
    available. The applications, strengths and weaknesses of different
    studies have been described (e.g., Hennekens & Buring, 1987; Rothman,
    1993; Morgenstern & Thomas, 1993). Interpretation of (epidemiological)
    study results has to consider random errors of estimation (due to
    chance) and systematic errors or bias. Important sources of bias
    (selection bias, information bias and confounding) may hamper the
    validity of observed results. Therefore, possible systematic errors

    should be considered already at the planning stage of any study and
    measures should be implemented to minimize or control bias. It is also
    crucial at the planning stage of a study to perform statistical power
    calculations to evaluate how many study participants are needed to
    assure a given probability of detecting a true effect of a given
    magnitude.

         Epidemiological studies applying accurate exposure and disease
    measurements and taking into account important covariates, confounders
    and effect modifiers are needed (Hatch & Thomas, 1993; Prentice &
    Thomas, 1993). They can make an important contribution through
    regulatory decisions in public health in the difficult field of risk
    assessment for complex exposures.

         Besides other measures, occupational sentinel health events
    (Mullan & Murthy, 1991) and structure-activity research (Jarvis et
    al., 1996; Graham et al., 1997) may be useful tools to evaluate the
    sensitizing capacity of a chemical substance in populations. Lists of
    well-known allergens categorized according to potency and degree of
    exposure are available (Kayser & Schlede, 1995), and criteria for
    classification of sensitizing substances in the environment have been
    defined. Furthermore, by comparing different populations with
    different grades of exposures, major disease determinants can be
    uncovered that would otherwise remain undetected, if the suspected
    risk factors show only little variation within a single population
    (Rose, 1985). For comparison reasons, data should be collected and
    analysed in a standardized way wherever possible. Using such
    strategies, major health determinants have been successfully described
    and studied in other fields, such as cardiovascular diseases and
    cancer. It is an essential prerequisite of such internationally
    conducted studies to obtain the health outcome and exposure data in a
    standardized way. New epidemiological initiatives investigating
    determinants of asthma and allergies need to incorporate these
    principles, as is the case with the ongoing International Study of
    Asthma and Allergies in Childhood (ISAAC) (Asher et al., 1995; ISAAC
    Steering Committee, 1998).
    


    6.  HAZARD IDENTIFICATION: DEMONSTRATION OF ALLERGENICITY

    6.1  Hazard and risk; allergy and toxicity

         The conventional scheme for chemical risk assessment for human
    health protection follows the sequence:

    a)   Hazard identification; what is the potential of the substance to
         cause harm (sensitization and provocation of an allergic
         reaction), and what is the dose-response relationship?

    b)   Exposure assessment

    c)   Risk assessment: what is the likelihood of eliciting an allergic
         reaction in humans at the relevant level of exposure; are there
         any groups of increased susceptibility?

    d)   Risk characterization: non-scientific consideration of the risk
         weighed against the benefit of using the substance resulting in
         the decision to ban or limit exposure (NAS, 1983, 1993).

         Hazard identification, therefore, comprises procedures to
    determine the potential of a substance to induce allergy or elicit
    allergic reactions and the relationship between those properties and
    the circumstances of exposure.

         The prediction may be based on theoretical considerations of
    chemical structure, possibly on  in vitro experimental results, on
     in  vivo animal data, and on prior observations in humans. If it
    depends on the results of laboratory studies and not on clinical
    observations, the prediction, as is common to toxicity tests, must
    take account of species differences in metabolism, responsivity, dose
    (exposure), and the adequacy of validation of the experimental system.

         Testing allergenic potential requires study of selected
    immunological effects and differs from conventional toxicity testing
    in the nature and content of its procedures, which are focused on
    responses of the immune system and not on general screening for
    changes in all body systems. In both types of testing, however, there
    will be some form of relation between dose (exposure) and effect, as
    the capacity of a substance to produce effects, its potency, will be
    represented by the dose (exposure) required to produce sensitization
    (or toxicity). A strong sensitizer will require only a small dose,
    whereas a less potent compound will require a higher dose, or multiple
    exposures. Unlike conventional toxicity, further exposure of a
    sensitized animal (or man) will elicit a harmful allergic reaction
    after a much smaller dose than that required for sensitization,
    although there will still be a graduation of the severity and nature
    of the hypersensitivity reaction, for example ranging from slight
    bronchoconstriction to fatal bronchospasm or anaphylaxis after
    respiratory challenge.

         An additional important difference between conventional toxicity
    and allergy is that allergic sensitization (the induced state of
    hyperreactivity to a substance) normally persists for a long time,
    even for life, whereas for many toxic responses a state of lasting
    responsiveness is not induced. It is possible for different types of
    hypersensitization and provocation to be effective in the same organ
    but it is also possible for the route of sensitization and response to
    subsequent challenge to differ, e.g., sensitization via the skin and
    subsequent asthma on inhalation exposure.

    6.1.1  Testing allergic potential and toxicity testing

         Much testing to identify toxic hazard is done during industrial
    development of substances for purposes ranging from new medicines or
    consumer products to industrial intermediates or pesticides. There are
    well-developed regimes of accepted procedures done under controlled
    circumstances applicable to each intended use, and the results are
    used for regulatory purposes to control risk. Most of these procedures
    are not directed at revealing effects involving the immune system,
    although indirect indications may sometimes be obtained that can
    arouse suspicion that the immune system may have been affected.

         Certain specialized procedures are also conducted, based on
    consideration of the way in which humans may be exposed (e.g., in the
    skin or by inhalation), which experience has shown can reveal certain
    types of sensitizing and allergenic potential. The latter types of
    test are the most important in the present context. The procedures and
    their value and limitations are discussed here.

         Laboratory tests, especially  in vivo procedures, should be done
    in such a way as to minimize the need for experimental animals and
    scarce human and technical resources, and every attempt is made to
    extract as much information as possible from the work that is done.
    That includes considerable attention paid to quality assurance of
    tests.

         Accordingly, the testing of a new substance will follow a
    sequence, moving from theoretical to practical procedures. Only those
    techniques focused on the immune system are noted here, but it must be
    realized that product development and occupational safety needs
    require many other studies, too.

    6.1.2  Databases and prior experience

         A preliminary search should always be made for any information
    about experimental or clinical findings about the immunological
    consequences of exposure to the substance. This may include special
    consideration of any groups considered to be particularly susceptible,
    for example, because of pre-existing disease.

    6.2  Validation and quality assurance

         Obviously, the ideal situation would be that predictive tests
    yield easy-to-interpret outcomes and no false negatives or false
    positives, and that the tests will always give the same results,
    regardless of when or where they are carried out. In order to achieve
    this goal, validation of tests is required. Validation should occur at
    two levels: first at the level of technical quality and second at the
    level of specificity and sensitivity of the assay. It is important
    that potential methods are carefully evaluated for interlaboratory
    reproducibility and transferability, and for their ability to predict
    an  in vivo end-point. Owing to the complexity of the immune system,
    it is quite likely that predictive assays for the capacity of
    chemicals to induce skin, respiratory or food allergy, or autoimmunity
    will not always function adequately to show the absence or presence of
    such activity (Kammüller, 1996). Therefore, the outcome of these tests
    should always be evaluated with great care.

    6.3  Structure-activity relationships

         Structure-activity models are directed towards a fuller
    understanding of the relationship between chemical structure and
    physicochemical properties and skin-sensitizing activity, with the
    objective of deriving ideally quantitative structure-activity
    relationships (QSAR). In this context, parameters that appear to be of
    particular importance are protein reactivity and lipophilicity
    associated with the capacity to penetrate into the viable epidermis
    (Basketter & Roberts, 1990; Barratt et al., 1994a). The correlation of
    the protein reactivity of chemicals with their skin-sensitization
    potential is well established (e.g., Dupuis & Benezra, 1982), and it
    is accepted that if a chemical is capable of reacting with a protein,
    either directly or after appropriate (bio)-chemical transformation,
    then it has the potential to be a contact allergen, assuming of course
    that it can locate in the appropriate epidermal compartment.

         It is not within the scope of this monograph to give a
    comprehensive description of all the available models. A common
    feature of many of these models is that their development was based
    upon the mechanism of sensitization, i.e., the absorption of the
    chemical sensitizer through the skin, followed by its covalent
    modification of a skin-associated protein. Each of the existing
    structure-activity relationship (SAR) models proposes structural
    alerts, i.e., moieties associated with sensitizing activity. In all
    cases, the structural alerts comprise electrophilic moieties, or
    moieties that can be metabolized into electrophilic fragments
    (proelectrophiles). For example, Benezra et al. (1985) developed a
    hierarchical index of structures believed to be associated with
    allergic contact dermatitis that contained amines, ketones, metals,
    nitrogen-containing heterocycles, and oxygen-containing heterocycles,
    among others. Barratt et al. (1994a,b) developed a list of structural
    alerts based on the requirement for protein reactivity that included

    alkylating, acylating, and arylating agents, electrophiles, thiol
    exchange compounds, and free radical generators. Many of the existing
    SAR systems have incorporated physicochemical considerations (Roberts
    & Basketter, 1990a,b; Basketter et al., 1992; Ashby et al., 1995). The
    mathematical model described by Roberts & Basketter (1990a,b) for
    alkyl transfer agents has incorporated both a rate constant for
    reaction of a chemical with a nucleophile, and a lipophilicity factor
    (log P). Each of these models has been successful in predicting the
    activity of moderate to strong contact sensitizers. Barratt et al.
    (1994b) reported the greatest success in predicting active sensitizing
    potential (98%). A smaller database using similar alerts (Payne &
    Walsh, 1994) had a much poorer positive predictive ability of 57%. The
    classification model of allergic contact dermatitis used by Hostynek
    et al. (1996) made use of several parameters and multiple regression
    to predict 79% of the active sensitizers and 88% of the inactive
    chemicals. The relative alkylation index (RAI) is useful only in the
    prediction of a homologous series of chemicals (Roberts & Basketter,
    1990a,b). The model used by Benezra et al. (1985) did not specify
    whether a validation was attempted, therefore predictive ability is
    not known.

    6.3.1  Case-Multicase system

         The Case-Multicase system is not dependent upon a particular
    mechanism of sensitization and has shown ability to predict activity
    of weak sensitizers (Graham et al., 1996). The model operates by
    fragmenting chemicals in the database into substructures containing
    two or more heavy (non-hydrogen) atoms. It then identifies those
    fragments that are statistically associated with active chemicals and
    uses such fragments as structural alerts for prediction of test
    chemicals.

         The database for this model was derived from reports of animal
    and human studies and consists of more than 1000 chemicals (Graham et
    al., 1996). The model identified 49 structural alerts of allergic
    contact dermatitis. The major ones were: (a) a nitrogen double-bonded
    to a carbon or a nitrogen; (b) substituted aromatic structures; (c)
    thiol- and disulfide-containing fragments; and (d) electrophilic
    moieties. The model has been evaluated by testing its ability to
    predict correctly the activity of chemicals for which there is
    evidence of sensitization ability. The concordance between predictions
    by the model and established evidence of sensitization was 90% (Graham
    et al., 1996).

    6.3.2  DEREK skin sensitization rulebase

         A historical database (Cronin & Basketter, 1994) containing
    results of about 300 guinea-pig maximization tests (Magnusson &
    Kligman, 1970a,b), carried out over a number of years according to a
    single protocol on defined single substances, was used to derive a set
    of structural alerts for skin sensitization. The approach employed was

    to group the substances, where possible, according to their most
    likely mechanism of reaction with skin proteins. Where no mechanism
    could be clearly identified, structural alerts were derived for groups
    of chemicals with similar functional groups. This process initially
    resulted in the production of around 40 structure-activity rules
    (Barratt et al., 1994a), now increased to over 50. These were
    incorporated into the expert system Deductive Estimation of Risk from
    Existing Knowledge (DEREK) (Sanderson & Earnshaw, 1991; Ridings et
    al., 1996). DEREK contains both a controlling programme and a chemical
    rulebase. The chemical rulebase consists of descriptions of molecular
    structural alerts, which correlate with specific toxicological
    end-points.

         Whilst details of the biology of skin sensitization are only
    partly understood, it is now widely accepted that the ability to react
    with a nucleophile, either directly or after appropriate metabolism,
    is a prerequisite for the large majority of skin sensitizers. However,
    the potential of a chemical to act as a contact allergen is further
    modulated by its ability to penetrate the stratum corneum and
    partition into the epidermal compartment of skin; this is apparent
    from a number of QSAR studies (Roberts & Basketter, 1990a,b; Basketter
    et al., 1992) in which skin-sensitization potential was found to
    depend crucially on physicochemical parameters such as the log
    octanol/water partition coefficient (log Pow). These parameters have
    also been found to be equally important determinants of percutaneous
    absorption (Flynn, 1990), with higher log Pow values, i.e., greater
    lipophilicity, broadly leading to greater permeability. In QSAR
    studies of skin permeability, the  in vitro human skin permeability
    coefficient has also been shown to decrease with increasing relative
    molecular mass (Flynn, 1990) or molecular volume (Barratt, 1995). The
    skin-sensitization potential of a series of substituted phenyl
    benzoates was found to depend on log Pow and relative molecular
    volume in the same way (Barratt et al., 1994c). The logical
    consequence is that two chemicals may contain the same structural
    alert (i.e., be reactive, presumably by the same mechanism), but one
    will be a skin sensitizer because it can penetrate the skin whilst the
    other will not be a skin sensitizer because its skin permeability is
    too low, e.g.,  N-methyl- N-nitrosourea and streptozotocin (Ashby et
    al., 1995).

    6.3.3  SAR for respiratory hypersensitivity

         An initial SAR model for respiratory-sensitizing chemicals has
    been described (Karol et al., 1996). The model is based on the
    Case-Multicase system and the database was derived from a critical
    review of the published clinical literature. Criteria for inclusion of
    published data included a decrement in pulmonary function resulting
    from inhalation challenge with a non-irritating concentration of the
    chemical.

         In all, 39 respiratory chemical allergens were identified from
    the literature search, all being obtained from human studies. Among
    the chemicals were diisocyanates, acid anhydrides, antibiotics and
    dyes. Since the model requires a data set of inactive chemicals, and
    such chemicals could not be found in the literature, chemicals that
    were inactive as dermal sensitizers were assumed to be inactive as
    respiratory sensitizers as well, and were added to the respiratory
    model as "inactive" chemicals.

         The model identified structural alerts including the isocyanate
    functionality, amines and aromatic fragments. When respiratory
    sensitizers were compared with dermal sensitizers for both structural
    alerts and physicochemical characteristics, differences were noted.
    Among the physicochemical properties, respiratory chemicals had higher
    mean relative molecular mass and greater water solubility when
    compared with dermal sensitizers (Karol et al., 1996). However, the
    discrimination of dermal and respiratory sensitizers remains
    problematic.

    6.4  Predictive testing in vivo

    6.4.1  Testing for skin allergy

    6.4.1.1  Testing in guinea-pigs

         The guinea-pig was for many years the animal of choice for
    experimental studies of contact sensitization, and several test
    methods were developed in this species. The Draize test was developed
    over 50 years ago (Draize et al., 1944) and was widely used, but this
    is no longer the case and it has been superseded. Currently, the best
    known and most widely applied are the Buehler test (Buehler, 1965),
    the guinea-pig maximization test (Magnusson & Kligman, 1970a,b), and
    the guinea-pig optimization test (Maurer et al., 1975), and have
    formed the basis of hazard assessment for many years. Both the
    Magnusson and the Buehler test are recommended according to an OECD
    guideline (OECD, 1992). While these tests differ with respect to
    procedural details, they are in principle similar. Guinea-pigs are
    exposed to the test material or to the relevant vehicle. In the
    Buehler test, both induction and challenge exposures are done
    topically; in this test, false negatives are frequently observed. The
    test was improved by occluded application of the test compound. In the
    guinea-pig maximization test, induction is produced by intradermal and
    occluded epidermal exposure, and in the optimization test induction is
    done by intradermal exposure and challenge by intradermal and occluded
    epidermal exposure. Adjuvant is employed also to augment the induction
    of the immune responses. For induction, concentrations of up to 5% or
    a maximum non-irritant concentration are used for intradermal
    injections, and up to 25 % for epidermal application. Some time after
    induction exposure, test and control animals are challenged at a
    distant site with a sub-irritant concentration of the chemical, which
    is generally lower than the concentration used for induction.
    Challenge-induced inflammatory reactions, measured as a function of

    erythema and/or oedema, are recorded 24 and 48 h later. Classification
    of sensitizing activity is based usually upon the percentage of test
    animals that display macroscopically detectable challenge reactions.
    Any compound inducing at least 30% positive animals in an adjuvant
    test is labelled as a sensitizer; in the case of a non-adjuvant test,
    15% is sufficient for classification as a sensitizer.

         Of the available guinea-pig test methods, the guinea-pig
    maximization test is generally selected when the aim is to identify
    the weakest of skin sensitizers. However, the method is not well
    suited to the estimation of relative sensitizing potency, because of
    the requirement for the use of intradermal injections of test material
    and Freund's complete adjuvant (FCA) (Basketter et al., 1996). The
    Buehler test is easier to use because the mode of application is
    epicutaneous occlusive treatment for both induction and elicitation
    (Chan et al., 1983).

         Although guinea-pig test methods, such as the Buehler test and
    the guinea-pig maximization test, have been in use for more than 25
    years, there has not been extensive examination of their sensitivity
    and specificity in comparison to what is known about human skin
    sensitization. Both the tests, when conducted properly, offer
    sufficient sensitivity to detect many known human skin sensitizers
    (Wahlberg & Boman, 1985; Basketter et al., 1996). However, there has
    been very little assessment on the specificity of these methods.

         It should be noted that these guinea-pig methods are not well
    suited to the identification of protein allergens. In general, the
    maximization of exposure involved in these skin sensitization tests
    will result simply in a large immune response. This will mask any
    differential allergic response.

    6.4.1.2  Testing in mice

         Increased understanding of the cellular and molecular mechanisms
    associated with contact allergy have derived largely from experimental
    investigations in the mouse. Two different types of tests to predict
    the capacity of chemicals to induce skin allergy have been developed.
    One is the mouse ear swelling test (MEST), which, like the guinea-pig
    methods described above, is based upon the evaluation of
    challenge-induced reactions in previously sensitized animals (Gad et
    al., 1986). In this test, mice sensitized by a comparatively rigorous
    regime (the intradermal injection of adjuvant followed by the daily
    application of the test material, for 4 consecutive days, to
    tape-stripped skin) are challenged on one ear with the test compound
    and on the contralateral ear with vehicle alone. Sensitizing potential
    is evaluated by consideration of both the degree of oedema (ear
    swelling) induced and the percentage of animals displaying a reaction
    (Thorne et al., 1991).

         The second test developed in mice is the local lymph node assay
    (Kimber et al., 1989). In contrast to the mouse ear swelling test and
    guinea-pig assays, activity here is measured as a function of events
    occurring during the induction, rather than elicitation, phase of
    contact sensitization. Mice are treated daily, for 3 consecutive days,
    on the dorsum of both ears with the test material or with an equal
    volume of vehicle alone. Proliferative activity in draining lymph
    nodes (measured by the incorporation  in situ of radiolabelled
    thymidine) is evaluated 5 days following the initiation of exposure.

         The local lymph node assay has been the subject of extensive
    comparisons with guinea-pig methods (Kimber et al., 1994; Basketter et
    al., 1996), and offers significant advantages compared with the
    available guinea-pig test methods. Important among these is the fact
    that there is an objective read-out. The assay has been evaluated by
    collaborative trials, and an OECD (Organisation for Economic
    Co-operation and Development) test guideline issued in 1992 states
    that the local lymph node assay (or the MEST) can now be used as a
    first stage in an assessment of skin-sensitizing activity (OECD, 1992)
    If a positive result is seen, then a test substance may be designated
    a potential sensitizer and it may not be necessary to conduct a
    further guinea-pig test. However, if a negative result is seen, a
    guinea-pig test must be conducted subsequently.

    6.4.1.3  Predictive testing for skin allergy in humans

         There are various skin test procedures for the diagnosis of
    several types of contact dermatitis (see also chapter 4). Basically,
    predictive tests in humans for skin allergy are similar to diagnostic
    tests for contact sensitization, but the aims are different. For
    diagnostic tests, the aim is determining sensitization to chemicals to
    which there was a prior exposure, and avoiding new sensitizations
    because of the procedure. For predictive testing in humans the aim is
    to show the sensitizing capacity in individuals that have not been
    exposed to the chemical previously.

         Predictive testing in humans generally requires multiple
    occlusive patches for induction of sensitization (10 patches, 48 h
    each, same site) followed by a 2-week rest period and then challenge
    (48 h) with a patch at a new skin site (Marzulli & Maibach, 1973,
    1996). There are a number of variations in these procedures, including
    the use of provocative chemical agents such as sodium lauryl sulfate
    (Kligman, 1966c), special skin preparation such as stripping (Spier &
    Sixt, 1955) or freezing (Epstein et al., 1963), special patches
    (Magnusson & Hersle, 1965), high concentrations at induction (Marzulli
    & Maibach, 1974), and 25-200 test subjects (Draize et al., 1944;
    Kligman, 1966b). It is not entirely clear, however, how useful these
    variations are and what the limitations are under the use conditions
    because validation has not kept pace with the use of the different
    approaches. Furthermore, predictive tests are often performed on a
    single chemical entity, whereas ultimate use may occur as part of a

    multicomponent formulation in a marketed product, where the vehicle
    and associated ingredients may influence the outcome. It is essential
    that the test has sufficient statistical power to provide appropriate
    protection for the population at risk. This is illustrated by the
    mathematical considerations of Henderson & Riley (1945) in their
    classical paper on extrapolating data from a small test population to
    large numbers of users. Briefly stated, there may be no skin reactions
    in a test population of 200 random subjects, yet as many as 15 of
    every 1000 of the general population (95% confidence), or up to 22 of
    every 1000 may react (99% confidence). If the test group is reduced to
    100 subjects, up to 30 of every 1000 of the general population may
    react (95% confidence). Conversely, when 1 of 200 subjects in a test
    population becomes sensitized, a test population of 10 000 subjects
    might show from 1 to 275 sensitized, with 95% confidence.

         Prospective tests of skin sensitization using human volunteers
    should always be conducted in accordance with ethical principles.

    6.4.2  Testing for respiratory allergy

         Most of the animal models that are used for studying specific
    respiratory hypersensitivity were developed using allergens with high
    relative molecular mass, notably proteins. Very few animal models have
    been developed as predictive tests for hazard identification and risk
    assessment in the area of chemical-induced respiratory allergy. The
    majority of these models are based upon antibody-mediated events. The
    models differ with regard to the following aspects: the animal species
    utilized, the route of administration of the agent, the protocol for
    both induction and elicitation of responses, type of response
    measured, and judgment of significant response.

    6.4.2.1  Guinea-pig model

         The guinea-pig has been used for decades for the study of
    anaphylactic shock and pulmonary hypersensitivity (Sarlo & Karol,
    1994). The guinea-pig is similar to humans in that the lung is a major
    shock organ for anaphylactic responses to antigens. The guinea-pig
    responds to histamine and can experience both immediate-onset and
    late-onset responses. Airway hyperreactivity and eosinophil influx and
    inflammation can also be demonstrated in this animal species.
    Mechanistic studies have been hampered by the lack of reagents needed
    to identify cells and mediators in respiratory allergy. In addition,
    the major anaphylactic antibody is IgG1a, whereas it is IgE in other
    rodent species and in humans. The  in vivo passive cutaneous
    anaphylaxis (PCA) assay was used to measure IgG1a antibody responses,
    but ELISA methods have now been developed that have eliminated some of
    the variability seen with the PCA.

         The guinea-pig inhalation model focuses on identifying chemical
    sensitizers by measurement of the response, or elicitation phase, of
    sensitization. In contrast with the mouse IgE test, the model does not

    depend upon a preconceived mechanism of sensitization. Rather, it
    functions by reproducing the characteristics that typify the
    hypersensitivity reactions, i.e., the physiological response of the
    airways and the pulmonary inflammation. Measurement of specific
    antibody formation provides ancillary evidence of the response. The
    method has been successfully used to distinguish low relative
    molecular mass contact sensitizers from respiratory sensitizers. The
    model utilizes inhalation as the route of exposure for both the
    sensitization phase and the elicitation phase of the response. A
    variation of the method is the use of intratracheal administration of
    the agent (Sarlo & Karol, 1994). The model has the capacity to assess
    immediate-onset responses (IAR), as well as late-onset responses
    (LAR). The latter is possible since minimal restraint of animals is
    used. A dynamic air supply and passive detection devices allow
    continuous 24-h monitoring of respiratory function of animals. The
    ability to detect late-onset responses makes the model particularly
    appropriate for evaluation of chemical allergy, where late-onset
    responses are a frequent occurrence.

         The advantages of the guinea-pig model for chemical sensitization
    include: use of inhalation as the relevant route of exposure;
    generation of atmospheres of reactive chemicals; measurement of
    physiological responses including immediate-onset responses,
    late-onset responses, fever and hyperreactive airways; measurement of
    specific antibody production; and histopathological evaluation of
    pulmonary tissue. Disadvantages of the model are the cost, the time
    involved, the need for specialized facilities, and the employment of
    guinea-pigs. The latter is a disadvantage in that IgGl rather than IgE
    is the major class of cytophilic hypersensitivity antibody in
    guinea-pigs.

         Variations of the guinea-pig model have been developed to
    optimize the response, to monitor additional respiratory parameters,
    or to simplify the procedure (Briatico-Vangosa et al., 1994). Such
    variations include single or repeated intradermal administration of
    free chemical, elicitation with a multivalent chemical-adducted
    protein, and measurement of flow-volume loops, respiratory minute
    volume, inspiratory and expiratory time, and peak respiratory flow
    rates. Using chemical-protein adducts for elicitation avoids the
    possible development of airway hyperreactivity due to chemical
    irritation (analogous to the reactive airways syndrome in humans).

         Using the guinea-pig model, differences were readily apparent
    between sensitization to allergens of high relative molecular mass
    (HRMM) versus those of low relative molecular mass (LRMM). Responses
    to ovalbumin and to bacterial subtilisin (HRMM allergens) consisted of
    severe immediate-onset responses in 90-100% of animals and late-onset
    responses in 50% of animals (Thorne & Karol, 1989). By contrast,
    sensitization to diphenylmethane-4,4-diisocyanate (MDI), a LRMM
    allergen, consisted predominantly of late-onset responses (Karol &
    Thorne, 1988). This finding reflects the human experience where
    late-onset responses are the most frequently observed responses to
    LRMM allergens.

         The model has been validated in two ways. Firstly, it has been
    established in several laboratories (Pauluhn & Eben, 1991; Sarlo &
    Clark, 1992; Stadler & Loveless, 1992; Warren et al., 1993; Sarlo &
    Karol, 1994) and responses of animals to inhaled toluene diisocyanate
    have been reproduced. This confirms the robustness of the model.
    Secondly, the model has been found to distinguish pulmonary from
    dermal chemical sensitizers, and from non-sensitizers. For example,
    inhalation of toluene diisocyanate (Karol, 1983) and
    diphenylmethane-4,4-diisocyanate (Karol & Thorne, 1988) by animals
    resulted in pulmonary sensitization, whereas similar exposure to
    formaldehyde (Lee et al., 1984) and hydrogenated
    diphenylmethane-4,4-diisocyanate (Karol & Thorne, 1988), two
    recognized contact sensitizers, resulted in dermal sensitivity.
    Further validation with additional classes of chemicals is needed to
    generate confidence that information will be applicable to human
    disease.

         It should be emphasized that the measurement of respiratory
    responses induced by chemical respiratory allergens is technically
    demanding, and consequently conflicting results have been reported.
    For example, it has proven difficult to induce robust respiratory
    responses in animals sensitized to the potent human respiratory
    allergen diphenylmethane-4,4-diisocyanate. Pauluhn & Mohr (1994)
    reported that a proportion of animals sensitized to
    diphenylmethane-4,4-diisocyanate by inhalation responded to challenge
    with inhaled diphenylmethane-4,4-diisocyanate, while those sensitized
    by intradermal injection did not. The converse finding was reported by
    Rattray et al. (1994).

         Although the guinea-pig can be used for testing the capacity of
    chemicals to induce respiratory allergy, it needs further validation
    in terms of predictive value. However, it should be noted that it can
    also be used to test proteins for their ability to stimulate the
    production of anaphylactic antibody (Blaikie et al., 1995; Sarlo et
    al., 1997). Such information may be of value in assessing the relative
    allergenic potency of proteins.

    6.4.2.2  Mouse IgE model

         A mouse inhalation model to study airway responses to sensitizing
    chemicals has been developed (Garssen et al., 1991), but has not yet
    been used with a wide range of chemicals associated with respiratory
    allergy. The mouse IgE test currently represents the furthest
    developed systematic approach to the prediction of respiratory allergy
    in the mouse. It does not evaluate actual airway responses, but is
    based on the notion of the nature of immune responses elicited in mice
    by chemical allergens and of the qualitative differences in immune
    responses provoked by contact and respiratory sensitizers as they are
    generally found.

         Topical administration to mice of chemical respiratory allergens
    stimulated a substantial increase in the serum concentration of total
    IgE, a response not seen with contact allergens considered to lack the
    ability to cause sensitization of the respiratory tract (Dearman &
    Kimber, 1991, 1992). Observations suggested that it might be possible
    to identify chemical respiratory sensitizers as a function of induced
    changes in serum IgE concentration; the advantage of this approach
    being that measurement of a serum protein, rather than of
    hapten-specific antibody, is required. This forms the basis of the
    mouse IgE test.

         Investigations have suggested that the mouse IgE test may provide
    a useful method for the prospective identification of chemical
    respiratory allergens, a conclusion that is supported by recent
    studies in an independent laboratory (Potter & Wederbrand, 1995). It
    must be emphasized, however, that to date the assay has been evaluated
    only with a limited number of chemicals, most of the analyses have
    been performed in a single laboratory, and the mechanistic basis of
    the model has not been established.

         Respiratory allergic responses, associated with increased
    reactivity of airways, were observed in mice that were topically
    sensitized to and intranasally challenged with picryl chloride, a
    Th1-type immune-response-inducing chemical (Garssen et al., 1991). In
    addition, it has been shown that IgE-deficient mice undergo
    anaphylaxis (Oettigen et al., 1994). Other investigators have noted a
    decrease, rather than an increase, in serum IgE following exposure of
    mice to toluene diisocyanate (TDI) (Satoh et al., 1995). For this
    reason, actual testing of lung function  in vivo after sensitization
    and challenge with chemicals known to sensitize, yet unable to produce
    IgE responses, as can be done in mice (Garssen et al., 1991) as well
    as in the guinea-pig (section 6.4.2.1), seems prudent. Moreover,
    standardization of the methodology is necessary, including dosages
    appropriate for testing, optimal time for repeated administration of
    the chemical and for obtaining sera, and clarification of the meaning
    of "elevated" IgE titre. Validation of the method is still needed.

    6.4.2.3  Rat model

         A rat bioassay has been developed by Pauluhn (1996). Nose-only
    exposure for 1 or 2 weeks, using non-irritant concentrations as judged
    in short-term pilot experiments, is carried out, upon which lung
    functions are tested and biochemical and morphological signs of
    effects in the airways are determined. The model needs further
    validation to evaluate whether it is suitable for the prediction of
    respiratory sensitization. A rat model to study respiratory syndromes,
    including the IgE-mediated allergic responses, in addition to anaemia
    and haemolysis in workers exposed to trimellitic anhydride (Zeiss et
    al., 1977), was used by Leach et al. (1987). In this model, animals
    are subjected to single or multiple exposures at several

    concentrations of trimellitic anhydride dust and at selected time
    points are challenged with a single exposure of trimellitic anhydride
    dust. The lungs are evaluated for haemorrhage and serum is tested for
    IgG antibody specific for trimellitic anhydride. The model has also
    been applied for other types of anhydrides.

    6.4.2.4  Predictive testing for respiratory allergy in humans

         For obvious reasons, predictive testing for respiratory
    sensitization is not done in humans. Occasionally, case reports may
    serve as an adequate hazard identification but not as a risk estimate,
    because data on route and extent of exposure, and on the "population
    at risk" are usually lacking. In the absence of case reports, it is
    not possible to conclude that there exists no potential for
    sensitization.

    6.4.2.5  Cytokine fingerprinting

         Contact and respiratory chemical allergens provoke in mice
    qualitatively different immune responses suggestive of divergent
    Th-cell activation and characterized by different patterns of cytokine
    production. Chronic exposure of mice, over a 13-day period, to
    trimellitic anhydride was found to result in the production of high
    levels of mitogen-inducible IL-4 and another Th2-cell cytokine
    interleukin 10 (IL-10) by draining lymph node cells, but only low
    levels of IFN-gamma. In contrast, treatment of mice under the same
    conditions of exposure with oxazolone (a potent contact allergen)
    caused the production by draining lymph node cells of only
    comparatively low levels of IL-4 and IL-10, but high concentrations of
    IFN-gamma (Dearman et al., 1995). Similar selective cytokine secretion
    profiles have been recorded following exposure of mice to other
    contact and respiratory chemical allergens (Dearman, 1996). These data
    raise the question of whether it might be possible to monitor the
    sensitizing properties of chemicals as a function of induced cytokine
    production profiles. Evidence suggests that this is the case, and the
    value of cytokine fingerprinting in the routine identification and
    classification of chemical allergens is being explored currently.

    6.5  Testing for food allergy

         Despite the availability of several methods to study antigenic
    and allergenic properties of protein products, current testing
    possibilities to investigate the allergenicity of food proteins at a
    pre-market stage are very poor. Validated methods with a high
    sensitivity have not yet been developed. However, some strategies can
    be followed to obtain additional relevant information apart from the
    information obtained from the currently applied assays; for instance,
    taking account of the role of the gastrointestinal tract physiology.

         The major adverse reactions to food constituents that involve the
    immune system are to proteins; non-proteins may cause food intolerance
    that does not involve the immune system, and hence are non-allergic
    reactions that do not fall within the scope of this monograph.

         Most proteins encountered by the immune system can produce immune
    responses. To determine the antigenicity of proteins, several assays
    are operational. However, these assays are based on parenteral
    application of the test proteins to laboratory animals. For food
    allergy research, three rodent species have frequently been used: the
    mouse, the guinea-pig and the rat. In many studies, sensitization was
    performed parenterally or passively and effects of enteral challenges
    were subsequently studied (Bloch & Walker, 1981; Freier et al., 1985;
    Granato & Piguet, 1986; Pahud et al., 1988; Miller & Nicklin, 1988;
    Turner et al., 1988, 1990; Curtis et al., 1990). In addition, effects
    of challenges have frequently been investigated in  in vitro studies
    with intestinal tissue or with, for instance, ligated gut (Roberts et
    al., 1981; Baird et al., 1984; Lake et al., 1984; Catto-Smith et al.,
    1989a,b). The guinea-pig is the most regular test species. In general,
    any protein that may be recognized as an antigen (foreign protein)
    will induce a humoral immune response upon injection and will most
    likely give a positive testing result in rodent assays. Although the
    information from antigenicity assays may be of major relevance, in
    that they will provide information on the quality and vigour of the
    response, it must always be recognized that such assays only provide
    information in the species examined. Whether a protein has a high or
    low potency of inducing food allergic reactions in (susceptible)
    humans cannot be concluded or predicted based only on the results of
    parenteral antigenicity assays.

         In addition to  in vivo antigenicity assays, several
    (combinations of) physicochemical and immunochemical analyses are used
    routinely to detect antigenicity.

         Determination of allergenic proteins or fragments that are able
    to cause activation of mast cells and basophils is possible using  in
     vitro  mast cell or basophil degranulation tests. For these assays,
    mast cells or basophils are loaded with antigen-specific cytophilic
    antibodies using serum from sensitized humans or test animals. The
    cells are subsequently incubated with the antigen or test product, and
    degranulation of the cells can then be determined. A well-validated
     in vivo counterpart for the detection of mast cell activation is the
    Passive Cutaneous Anaphylaxis (PCA) test, in which sera from
    sensitized animals are injected subcutaneously in unsensitized
    animals. Possible cytophilic antibodies present in the sera are bound
    by the receptors on the mast cells in the skin, and bridging of the
    antigen-specific antibodies on the mast cells by injected antigen
    induces mast cell activation -- the resulting reaction is detectable
    by various methods. In the Active Systemic Anaphylaxis (ASA) test,
    which is also a validated anaphylaxis test, actively sensitized
    animals are injected with the test substance intravenously and several
    parameters are recorded to determine the systemic anaphylactic
    response as a measure for the degree of sensitization.

         In the evaluation of the potential allergenicity of food
    products, clinical assays such as skin-prick tests or challenge
    procedures may also be used. For instance, these assays are applicable
    in the evaluation of the residual allergenicity of hypoallergenic
    products, in the evaluation of cross-allergenicity, or in the
    evaluation of the possible allergenicity of food products derived from
    biotechnologically derived crops in which a gene from a known
    allergenic source species has been introduced. However, the use of
    patients in such assays for non-diagnostic purposes requires careful
    ethical consideration. Since many of the questions may also be
    addressed by performing  in vitro assays such as immunochemical
    analyses, the use of sera from patients is always preferable to
    intentional exposure of humans.

    6.6  In vitro approaches

         Another approach for the detection of potential sensitizing
    capacity that would not rely on  in vivo animal or human testing is
    directed towards the construction of  in vitro experimental models
    that reflect accurately some pivotal event during skin sensitization.
    Accurate modelling of the immune system  in vitro is not possible
    without considerable difficulty. Nevertheless advances have been made.

         Wass & Belin (1990) and Gauggel et al. (1993) used biochemical
    techniques to examine the ability of low relative molecular mass
    chemicals to act as haptens by combining them with a model protein or
    a polypeptide. Gauggel et al. (1993) showed that their test correctly
    identified 12 out of 14 known human allergenic haptens and 23 out of
    24 non-allergenic low relative molecular mass chemicals. Neither
    method can detect sensitizers that must be metabolized to form a
    hapten.

    6.7  Testing for autoimmunity

         Although there are currently no predictive assays developed and
    validated to identify in the early phases of toxicity testing the
    potential of chemicals to induce autoimmune responses, it should be
    noted that assays to identify contact sensitizers (Kimber et al.,
    1994; Vial & Descotes, 1994) might be helpful to identify systemic
    sensitizers. Clinical signs of systemic immune-mediated side-effects
    usually become manifest only during advanced clinical development of
    drugs. The conditions used in routine preclinical toxicological
    screening are obviously not optimal for the detection of the
    immune-dysregulating potential of drugs and chemicals (e.g., small
    animal number, use of outbred animal strains, dynamics of disease
    development versus snapshot determinations, lack of predictive
    parameters). An economically and practically relevant question
    concerning screening studies is whether actual evidence of an agent's
    ability to induce manifest hypersensitivity or autoimmune disease
    should be and can be obtained, or whether (preferably short-term)
    assays not measuring the actual clinical end-points can be
    sufficiently predictive in this respect.

          In vitro tests for biological effects of sensitizers or
    chemicals able to induce autoimmunity are at present in their infancy.
    It should be emphasized that there are two major limitations: (a) it
    is so far impossible to completely reproduce  in vitro the complex
    microenvironment in which immune responses are initiated  in vivo;
    and (b) some immune reactions are elicited not by native xenobiotics
    but by their metabolic products generated  in vivo.

    6.7.1  Popliteal lymph node assay

         At present this appears to be the only method available to
    examine the ability of chemicals to induce an autoimmune response, but
    the usefulness of the results in risk assessment remains to be
    demonstrated. Based on the hypothesis that chemicals may elicit
    autoimmune disorders by a mechanism resembling graft-versus-host
    reactions, an existing graft-versus-host assay, the popliteal lymph
    node assay, has been modified to study chemical-induced immune
    reactions (Bloksma et al., 1995). Using the popliteal lymph node
    assay, many drugs known to occasionally induce immune-mediated
    systemic side-effects in humans were shown to trigger significant
    reactions in mice and rats (Gleichmann, 1981; Gleichmann et al., 1983,
    1989; Hurtenbach et al., 1987; Kammüller & Seinen, 1988;
    Stiller-Winkler et al., 1988; Kammüller, 1989a; Thomas et al., 1989,
    1991; de Backer et al., 1990; Verdier et al., 1990; Katsutani &
    Shionoya, 1992; Krzystyniak et al., 1992; Bloksma et al., 1995).
    Effects observed were very similar to those induced during a local
    graft-versus-host reaction in the popliteal lymph node assay (de
    Bakker et al., 1990). Immunogenetic studies in mice with
    diphenylhydantoin (Bloksma et al., 1988) and D-penicillamine
    (Hurtenbach et al., 1987) have indicated that the extent of popliteal
    lymph node enlargement is controlled by major histocompatibility
    complex (MHC (H-2)) as well as non-major histocompatibility complex
    genes. Furthermore, the popliteal lymph node assay was able to
    discriminate between structurally closely related compounds, for
    example chemical congeners of D-penicillamine (Hurtenbach et al.,
    1987), diphenylhydantoin (Kammüller & Seinen, 1988) and zimeldine
    (Thomas et al., 1989, 1991). Thus, the direct popliteal lymph node
    assay seems to be a versatile tool for recognizing T-cell-activating
    drugs and chemicals, including autoimmunogenic chemicals, but it also
    produces false-negative results (Bloksma et al., 1995). With the
    adoptive transfer popliteal lymph node assay, sensitized cells are
    used as probes to detect the formation  in vivo of immunogenic
    metabolites of low relative molecular mass chemicals (Kubicka-Muranyi
    et al., 1993; Bloksma et al., 1995). However, further mechanistic
    studies and interlaboratory validation is required before either
    variant of the assay can be recommended for routine use in the
    preclinical toxicity screening (Verdier et al., 1997).

    6.7.2  Animal models of autoimmune disease

         Three basic types of animal models may be employed to identify
    the potential of drugs or chemicals to induce systemic
    hypersensitivity or autoimmune responses: (a) genetically predisposed
    animals; (b) autoimmunization; and (c) organic or chemically induced
    (Table 26). In each type of model the development and severity of
    symptoms is multifactorial, in that the disease state can be
    influenced by age, hormonal and/or environmental factors. In addition
    there is a tendency for more than one autoimmune disorder to occur in
    a number of individual models. Nevertheless, a number of syndromes
    similar to that observed in humans can be mimicked in animal models.

         The genetically predisposed models, whether naturally occurring,
    transgenic or knockout based, tend to be the most reliable and
    therefore have been more commonly employed in autoimmunity research
    (Lo, 1996). In this type of model, mild to severe syndromes
    spontaneously develop, usually due to specific MHC allele mutations
    encoding class II molecules and often inducing function abnormalities
    of the CD4+ Th-cell (Theofilopoulos, 1995a,b). The most common
    genetic models include several MRL mouse variants for autoimmune
    thyroiditis, arthritis and lupus, several variants of New Zealand
    Brown mice (NZB), New Zealand White mice (NZW) and BXSB mice for
    lupus, and the non-obese diabetic (NOD) mouse and BioBreeding (BB) rat
    for insulin-dependent diabetes mellitus (Cohen & Miller, 1994).

         Autoimmunization with purified self-antigens can elicit a
    specific autoimmune response, particular when adjuvants are
    administered in conjunction with self-proteins. A frequently used
    model of this type, experimental autoimmune encephalomyelitis, is
    induced by immunization of rodents with myelin basic protein. The
    resulting pathology is a CD4+ T-cell-mediated autoimmune disease
    characterized by central nervous system perivascular lymphocyte
    infiltration and destruction of the myelin nerve sheath with resultant
    paralysis, similar to that observed in patients with multiple
    sclerosis (Constantinescu et al., 1998). Additional models included
    immunization with thyroglobulin to simulate Hashimoto's thyroiditis,
    and the injection of type II collagen to induce rheumatoid arthritis
    (Wick et al., 1974; Durie et al., 1994).

         In experimental models, foreign substances are used to induce the
    autoimmune disease state. These include chemicals, drugs and
    biological substances such as bacterial or viral antigens. Brown
    Norway (BN) rats injected with non-toxic amounts of mercuric chloride
    which produce no signs of overt toxicity develop an immunologically
    mediated disease characterized by a T-cell-dependent polyclonal B-cell
    activation (Pelletier et al., 1994). These animals demonstrate
    increases in serum IgE and the production of autoantibodies to a
    number of proteins including DNA, laminin, collagen IV and other
    components of the glomerular basement membrane. Proteinuria and


        Table 26.  Experimental models for autoimmune diseasesa
                                                                                                                                      

    Autoimmune disease                                                        Classification
                                                                                                                                      
                                Genetically predisposed strainsb     Autoimmunizationc             Biological/chemical induction
                                                                                                                                      

    Autoimmune thyroiditis      Murphy-Roths lymphoma (MRL) mouse,   Thyroglobulin - EAT
    (Hashimoto's and Graves')   BioBreeding (BB) rat,                (mouse, rat)
                                Obese strain (OS) chicken

    Insulin-dependent           Non-obese diabetic (NOD) mouse,                                    Streptozotocin (STZ) (mouse)
    Diabetes mellitus           BioBreeding (BB) rat,
                                Diseases resistant BioBreeding
                                (DRBB) rat,
                                Brown Norway rat

    Myasthenia gravis                                                Acetylcholine receptor        Penicillamine (mouse, rat)
                                                                     - EAMG (mouse, rat)

    Multiple sclerosis                                               Myelin basic protein
                                                                     EAE (mouse, rat, chicken)

    Rheumatoid arthritis        Murphy-Roths lymphoma                CFA + Type II collagen        Streptococcal cell-wall (rat)
                                (MRL)/lpr mouse,                     (mouse, rat, monkey)
                                Severe combined immunodeficient      CFA + TB hsp (mouse, rat)
                                (SCID) mouse,
                                Human leukocyte antigen
                                B27 (HLA B27) transgenic rat

    Systemic lupus              Murphy-Roths lymphoma                CFA + antiDNA antibodies      Mercury (mouse, rat, monkey)
    erythematosus               MRL +/+ mouse,                       (mouse, rat)                  Penicillamine (mouse, rat)
                                Murphy-Roths lymphoma MRL/lpr                                      Procainamide (mouse, rat)
                                mouse,
                                Murphy-Roths lymphoma
                                (MRL)-mp-lpr/lpr mouse,
                                New Zealand White (NZW) 2410 mouse,
                                New Zealand Black (NZB) mouse/
                                New Zealand White (NZW) mouse

    Table 26.  (continued)
                                                                                                                                      

    Autoimmune disease                                                        Classification
                                                                                                                                      
                                Genetically predisposed strainsb     Autoimmunizationc             Biological/chemical induction
                                                                                                                                      

    Systemic sclerosis          Tight skin (TSK) mouse,
    (Scleroderma)
                                                                                                                                      

    a  Adapted from Cohen & Miller (1994); Farine (1997); Bigazzi (1997)
    b  lpr = lymphoproliferation
    c  CFA = Complete Freunds Adjuvant
    

    nephrotic syndrome similar to that observed in humans are also
    observed. The disease is characterized by a glomerulonephropathy
    evolving in two phases: (a) a linear anti-GBM antibody deposition
    along the glomerular capillary pattern, and (b) a change to a granular
    pattern of immunofluorescence with the appearance of immune complex
    deposits (Pelletier et al., 1994).

    6.8  Clues from general toxicity tests

         The conditions used in conventional systemic toxicological
    screening are not designed for the detection of the potential of drugs
    and chemicals to induce allergy or autoimmunity, as general toxicity
    screening does not include specialized tests required for this
    purpose. In more recent guidelines for general toxicity screening,
    attention is given to the immune system, especially with the purpose
    of detecting direct toxicity to components of the immune system. This
    pertains for instance to the updated OECD guideline for 28-day oral
    toxicity testing (Koeter, 1995). According to this guideline, lymphoid
    organs are weighed and lymphoid tissues are examined microscopically.
    In addition to morphological examinations of lymphoid organs, a
    selection of non-lymphoid tissues (e.g., blood vessels, renal
    glomeruli, synovial membranes, thyroid, skin, liver and lungs) should
    be investigated. Tissue damage, protein (immune) complex deposits,
    and/or inflammatory cell infiltrates in these tissues may indicate the
    induction of hypersensitivity or autoimmune phenomena.

         It is common practice in toxicological pathology to rely most on
    statistically significant differences in incidence between test groups
    and controls. However, individual variability can be high, especially
    regarding hypersensitivity and autoimmune phenomena (Holt & Sedgwick,
    1987; Kammüller et al., 1989). Therefore, in studies with a limited
    number of animals per group, a change in only a single or a few test
    animals may have considerable biological relevance. This is
    particularly true in outbred animals.

         In addition to morphological examinations in routine
    toxicological studies, measurements of some immunologically relevant
    serum parameters can provide important information about
    antibody-mediated responses. Parameters may comprise levels of total
    immunoglobulins and of various immunoglobulin (sub)classes, immune
    complexes, and some commonly observed autoantibodies, e.g.,
    anti-nuclear antibodies (ANAs), anti-histone, and anti-single-stranded
    deoxyribonucleic acid (denatured; ssDNA) autoantibodies. Some of these
    autoantibodies proved to be useful in the diagnosis of
    procainamide-induced systemic lupus erythematosus in humans as shown
    by Rubin et al. (1995). Measurement of these parameters at suitably
    chosen intervals, especially during subchronic and chronic exposure,
    may obviate the snapshot nature of the histopathological examinations
    and will give a reflection of cellular and humoral immune function in
    time. With regard to autoantibody measurements it is important to
    measure both total immunoglobulin classes and subclasses. This can be

    illustrated by murine models of spontaneous systemic lupus
    erythematosus in which a switch from IgM to IgG autoantibodies could
    be associated with development of overt disease. Also, data on
    drug-induced systemic lupus erythematosus in humans point in this
    direction. Anti-nuclear antibodies in patients with
    procainamide-induced systemic lupus erythematosus appeared to be of
    the IgG class (in particular IgG1 and IgG3), whereas IgM anti-nuclear
    antibodies are predominant in asymptomatic users of the drug.

         It has been suggested that parameters of the immune system are
    included in conventional chronic toxicity testing and also in
    reproductive toxicity testing and evaluation, since the developing
    immune system may be particularly vulnerable to immune dysregulating
    effects (Hollady & Luster, 1996). For example, a newly developed
    immunomodulating agent was found to induce thyroiditis and significant
    antithyroglobulin autoantibodies in a 6-month and a 2-year rat study
    (Verdier et al., 1997).
    

    7.  RISK ASSESSMENT

    7.1  Introduction

         Risk assessment for human health protection is the final stage in
    evaluating the likelihood that potential adverse health effects will
    manifest themselves in humans.

    7.2  Risk assessment of allergy

         Effective risk assessment requires an appreciation of the
    potential of a chemical or drug to induce an allergic response, or
    elicit an allergic reaction, and its potency. Potency here is
    considered as the amount of chemical necessary to induce an allergic
    response or to elicit an allergic reaction. Risk assessment demands
    also an understanding of the conditions of human exposure, i.e., the
    extent, duration and route(s) of likely exposure.

         With the assembly of the available data completed, risk assessors
    must decide whether sufficient data are available to proceed. It may
    be necessary to produce a preliminary or provisional risk assessment
    irrespective of the availability of full documentation. However, at
    this stage, following comparison of what is available with minimal
    data sets, consideration is normally given to further searching,
    further requests for information to industry, and the use of modelling
    or default values to fill data gaps. A decision is then made on
    whether to proceed.

         Risk assessment of allergy is complicated by the fact that there
    exist two related, but nevertheless independent considerations.
    Allergic disease almost always occurs in two phases. During the first
    phase the previously non-sensitized individual is exposed to
    sufficient allergen in such a way that an allergic response is induced
    which results in sensitization. This is the induction phase. An
    allergic reaction will be elicited if this now sensitized individual
    is exposed again to the same allergen under conditions where a
    secondary, more aggressive allergic response is provoked. This is
    known as the elicitation phase. It is important to appreciate that
    these phases usually differ with respect to the exposure conditions
    required. The induction of allergic sensitization in a previously
    naive individual almost invariably requires exposure to concentrations
    of the allergen that are greater than those necessary to elicit a
    response in a sensitized subject. Allergic reactions can be provoked
    in sensitized individuals with small amounts of the inducing allergen
    that are without effect in non-sensitized populations. It is possible
    also that sensitization may be achieved from exposure via a route that
    is different from that needed to elicit a response. Thus, there is
    some evidence that dermal exposure to certain chemical allergens may
    stimulate the quality of immune response necessary to cause
    sensitization of the respiratory tract. The elicitation of pulmonary
    responses will subsequently be provoked following inhalation exposure
    of the now sensitized individual to the relevant chemical.

         Taken together it is clear that risk assessment for chemically
    induced allergic disease has two components: (a) the likelihood that a
    chemical will induce sensitization in a previously unsensitized
    individual; and (b) the conditions under which a chemical will provoke
    allergic reactions in those who are already sensitized.

         Information on risk, plus suggestions about any group of
    increased susceptibility, is used for risk evaluation, that is the
    process of deciding for medical, ethical, economic and legal reasons
    whether the identified risk will be "acceptable" and under what
    conditions, e.g., restricted availability of the substance in special
    circumstances or wider availability with a warning label and advice
    about protective measures. The decision may also be made to ban the
    use of a material deemed to carry too large a risk in relation to
    anticipated benefit from its use. These decisions reflect
    socioeconomic and political factors as well as medical and scientific
    conclusions, and so lie outside the purpose of this monograph.

    7.3 Factors in risk assessment of allergy

         Risk assessment requires answers to the following questions:

    *     Who will be exposed?

         How many people: Is there any information to suggest particular
    susceptibility (or resistance) to allergic hypersensitivity, e.g.,
    genetic, nutritional or other factors?

    *     Circumstances of the exposure

         Is it an initial sensitizing exposure or a subsequent provocative
    exposure that may elicit an allergic reaction?

    *     Extent and duration of exposure

         This defines the "load" of the allergen. Most of the available
    evidence suggests that it is the peak concentration of exposure rather
    than the cumulative delivered dose that is critical in determining the
    extent to which sensitization will develop. The same principle
    probably applies also to the elicitation of allergic reactions in
    sensitized individuals.

    *     Nature of exposure

         Exposure to chemical allergens occurs frequently in the context
    of mixtures and formulations. An important consideration is whether
    other components of the mixture will influence the ability of the
    chemical allergen to induce sensitization or elicit a reaction.

    *     Route

         If it is the first exposure, is it by a route that may sensitize
    people, usually via inhalation or skin contact, or by ingestion, which
    is much less likely to do so? If an individual is already sensitized,
    is it via the route by which he has already been sensitized, in which
    case an allergic response may occur? Or, will it be by another route,
    which may not necessarily evoke an immunological response?

    *      Evidence of hazard

         What is the nature and quality, i.e., the "strength", of the
    laboratory information showing the sensitizing potential of the
    substance? Has it come from laboratory tests of proven value, from
    chance observations during some other type of experiment, or is it
    based on prediction from structure-activity relationships? In each of
    these, are the data qualitative or quantitative?

    *      Prior evidence of risk

         Are there already results from humans, either as tests in a
    clinical laboratory setting, or following exposure in the real world
    of work or home, showing that people have been sensitized and how, and
    the consequences of subsequent exposure by any route?

         Taking all this information together, each aspect of which has
    already been discussed in this monograph, should suggest the totality
    of the  risk under a given set of circumstances, in other words, the
    likelihood that a given exposure to a substance will sensitize people,
    or evoke an allergic response in them, perhaps the proportion
    affected, and the nature of that response (including clinical feature
    of the disorder and their severity). That represents the ideal. In
    practice, the available information may be incomplete, and skilled
    judgement is required to extrapolate from whatever is known to suggest
    what may happen in practice.

    7.4  Information aspects

         The three commonest deficiencies in knowledge are discussed
    below.

    7.4.1  No information about hazard

         For a novel substance, a novel use, or because hypersensitivity
    may not previously have been recognised, there will be no information
    from humans. In that case, prediction can only be based upon the
    chemical structure of the substance, the results of appropriate
    laboratory tests, and assumptions about exposure of people.

         Such an extrapolation based solely on laboratory-defined hazard
    is the usual situation faced by the toxicologist, occupational health
    specialist and regulator in dealing with a new substance.

         As discussed earlier in this monograph, many of the laboratory
    procedures available to identify hazard are well proven and some are
    valuable in assessing relative potency for skin sensitizers. Their
    quantitative predictivity, sensitivity and specificity are difficult
    to state, but they probably give about 80% correct negative results,
    i.e., sensitization is unlikely (Kimber & Maurer, 1996) and about 60%
    correct positive predictions, i.e., skin sensitization is likely to
    occur.

         For respiratory tract sensitization, far fewer results are
    available, but a positive finding in any of the types of experiment
    considered here should be regarded as a powerful indicator of this
    type of hazard. The strength of a negative finding is less certain,
    because there is little published information.

         True hypersensitivity to oral antigens is still poorly
    understood, and the more common but more fickle "food intolerance"
    remains clouded by uncertainties. For a novel substance in a food, it
    is not yet possible to predict the likelihood that it will cause true
    allergy or intolerance if consumed. Known allergens and the other
    materials that sometimes cause disorders when eaten can be indicated
    by reference to published clinical information, which should also
    point both to the likely frequency of the harmful response, and
    whether it is affected by ethnic or by other physiological factors,
    the methods of preparation and cooking, and the matrix in which a
    putative food allergen is presented, because the complex nature of the
    mixtures that are foods does affect the risk of sensitization and
    elicitation of reactions to materials in the diet.

         The best predictor at present of the risk of true immunologically
    mediated hypersensitivity to a food component is probably
    demonstration of the presence of antibodies to known potent allergens,
    such as peanut (groundnut) protein.

         Predicting the risk of autoimmunity, too, can only be done by
    demonstrating the presence of a known antigen in an appropriate
    matrix, or, with less certainty, by demonstrating a very close
    structural analogy to a known cause of an autoimmune response.

    7.4.2  Scanty or no information about exposure

         It is sometimes difficult to envisage exactly how a substance
    will be used in the workplace or at home, or how it may be
    disseminated via the environmental media. In both of these instances
    the prediction of risk must carry some uncertainty.

         The problem of predicting exposure is well shown by the
    demonstration of an increased incidence of asthmatic attacks in the
    population of the city of Barcelona during large-scale transhipment of
    soy beans in the docks. That appears to have released a sufficient
    quantity of an allergen into the air for it to be disseminated and to
    expose the entire population under certain meteorological conditions,
    resulting in allergic asthma in so many people as to have a serious
    effect on the health of the community.

    7.4.3  Unreliable or scanty information about risk

         This is a particular problem because of the propensity of
    clinicians to report unusual or unique examples of disorders and not
    to describe negative findings. It may be difficult, therefore, to gain
    a balanced or critical view of the allergenic effects of substances
    from much of the clinical literature. It can provide an indication of
    suspicion, but large, carefully constructed series of patients or
    cautious surveillance results are rare.

         At a different level, but well able to provide helpful
    information, are national occupational health statistics on
    appropriate industrial diseases, particularly those for which a
    pension has been awarded. For many reasons those series are unlikely
    to provide unbiased information about incidence or prevalence, because
    of the inevitable bias when legal liability may be involved, but they
    can act as broad indicators of risk, and even of the allergenic
    potential of particular substances (Drever, 1995).

         Databases of consumer products used by the general population are
    very limited and may be close to capricious in their content, but they
    should be consulted when possible.

         Overall, therefore, the risk assessor has a very difficult task
    in the general field of allergic hypersensitivity. At present,
    laboratory tests may indicate the potential for a hazard, and
    searching consideration may suggest many of the features about
    exposure. From that a risk to humans may be predicted in qualitative
    terms, at least for the common route of domestic and occupational
    exposure of the skin. The prediction of respiratory tract allergy
    remains less certain, and the likelihood of effects following
    ingestion, or of an autoimmune disorder, can very rarely be predicted.

         Many predictive test methods serve simply to identify the
    inherent potential of a chemical to induce allergy but provide no
    indication of the potency with which it will do so. One problem is
    that some methods do not incorporate a dose-response analysis. The
    other issue is that some tests measure activity as the frequency of
    responses rather than the severity of responses. The need is to have
    available information on potency defined as the quantity of chemical

    necessary to induce sensitization (or to elicit a reaction). Some
    newer test methods are beginning to address this issue and are
    providing information about allergic potency relative to index
    allergens. Such comparisons have been of value in setting safe
    occupational exposure levels and minimizing the risk of allergic
    disease.

         As in any form of toxic reaction, "dose" is important, in that
    initial sensitization requires at least a certain minimum exposure
    (concentration of allergen, its local availability at the site of
    administration, and the duration of contact). In someone already
    sensitized, the likelihood of producing a clinical disorder and its
    severity are also related to dose, although, by definition, the
    quantity of allergen required to produce an effect is very much
    smaller than that associated with a conventional toxic action. This
    aspect of the extent or intensity of dose (=exposure) is more
    important at the practical level of preventing sensitization or
    protecting the sensitized individual, i.e., risk management, but it
    should not ignored at the risk assessment stage.

         A further difficulty experienced by the risk assessor in this
    area is that of individual susceptibility. Genotype does play some
    role in the propensity of people to develop clinical disorders due to
    hypersensitivity, as shown, for example, by the familial nature of
    atopy. There is also good clinical evidence, however, to show that the
    inheritance of eczematous skin hypersensitivity, or of asthmatic
    hypersensitivity to common domestic allergens, involves many powerful
    factors in addition to genetic constitution. Although there is no  a 
     priori reason to believe that humans lack "immune response" genes
    for hypersensitivity as pragmatically defined in animals, their
    importance remains uncertain. Some chemically related autoimmune
    diseases though are associated with certain "immune response"
    genotypes. For example, HLA DR3 and DR3/B8 haplotypes are associated
    with vinyl chloride disease (Black et al., 1983).

         Skin sensitization risk assessment is not a highly prescriptive
    process that should always be followed in the same way (Calvin, 1992).
    On the contrary, what is necessary is that it is carried out
    thoroughly to the point where the risk has been adequately assessed.
    In some circumstances, this point may be reached quite quickly and
    with minimal expenditure of time and effort. In other cases,
    substantial and sustained effort is required. For example, in the
    situation where exposure to the contact allergen is essentially zero,
    then even for highly potent contact allergens, there is no need to go
    further with a risk assessment because allergic contact dermatitis
    will not occur. Furthermore, if the exposure is sufficiently low, it
    may not be necessary to know precisely the potency of a contact
    allergen. Simply the knowledge that it is not a very strong allergen
    may be sufficient to permit a proper conclusion of the risk
    assessment. Another situation where risk assessment may be relatively
    simple is the replacement of an ingredient with another of the same or

    similar type (e.g., an alternative supplier of a raw material). In
    such a case, and where the risk is already known to be very low, all
    that may be necessary is to confirm that the specification of the new
    source of raw material is the same. Alternatively, data that provide
    evidence that the relative sensitization potential of the old and new
    materials is similar may suffice. In contrast, even where the
    intrinsic sensitization potential is very low, if skin contact is
    sufficiently intense and prolonged, then sensitization may occur. An
    example of this is the situation where medicines are applied
    continuously to skin, often damaged and/or inflamed skin, under
    occlusion. A prime example is found with stasis ulcers, where a
    variety of medicines and chemicals with negligible sensitization
    potential, such as cetostearyl alcohol and paraben esters, quite
    frequently cause allergic contact dermatitis.

         The comments above on skin sensitization all presuppose a good
    knowledge of the relative sensitizing potency of the substance under
    consideration. Such data may be obtained by examination of the skin
    sensitization potential of the substance in guinea-pig methods
    referred to earlier in this monograph. Critical to this type of
    analysis is data from the same method at the same testing institution
    with suitable benchmark substances. These may well need to be selected
    on a case by case basis in the light of the risk assessment that needs
    to be made. Alternatively, use made be made of the local lymph node
    assay, the results of which can be interpreted to yield an objective
    estimate of relative sensitizing potency (Kimber & Basketter, 1997).

         To determine what represents a safe threshold for a sensitizer is
    a complex matter (Basketter et al., 1997). The key point is that the
    threshold depends on the method used to determine it. Thresholds in
    animal models my differ substantially from those for humans even in
    situations where the pattern of exposure is similar. However, where it
    is possible to match the type of exposure in the test in humans to
    that which is expected in practice, then such data may be interpreted
    directly to humans (Johansen et al., 1996b).

    7.5  Conclusions

         The scientific, practical and clinical uncertainties that affect
    allergic hypersensitization make the task of the risk assessor
    particularly difficult. In practice, it may be reasonable in such
    assessments to favour the fail-safe approach by emphasising the
    "precautionary principle", namely that if there is even a suspicion of
    a risk, exposure should be minimized and preferably be entirely
    prevented. That will have a considerable influence on the development
    of new substances, on devising new uses for existing materials, and on
    instituting controls over exposure in the home and safe working
    practices, because an assessment that suggests a risk of allergic
    sensitization must lead at once to an appropriate decision about risk
    management.
    

    8.  TERMINOLOGY

    Adhesion molecules.  Molecules, belonging mainly to the 
    immuno-globulin or integrin superfamily of molecules (e.g., LFA-1, 
    ICAM-1), expressed on the membrane of various cells of the immune 
    system. Interactions with each other as receptors and corresponding 
    ligands facilitate cooperation (cross-talk) of cells, signal 
    transduction and information transfer between cells.

    Adjuvant.  A material that enhances immune response to substances in
    a non-antigen-specific manner.

    Allergen.  An antigen that provokes allergy.

    Allergic contact dermatitis.  An inflammatory skin disease resulting
    from allergic sensitization.

    Allergy.  Hypersensitivity caused by exposure to an exogenous
    antigen (allergen) resulting in a marked increase in reactivity and
    responsiveness to that antigen on subsequent exposure, resulting in
    adverse health effects.

    Allogenic.  Term describing genetically different phenotypes in
    different (non-inbred) individuals of the same species.

    Anaphylaxis/anaphylactic reaction.  A local or systemic immediate
    hypersensitivity reaction initiated by mediators released after
    immunological stimulation. Symptoms can be a drop in blood pressure
    related to vascular permeability and vascular dilatation, and
    obstruction of airways related to smooth muscle
    contraction/bronchoconstriction.

    Anergy.  Lack of immune responsiveness (usually defined as lack of
    response to common recall antigens).

    Antibody.  An immunoglobulin produced by activated B-cells and
    plasma cells after exposure to an antigen with specificity for the
    inducing antigen.

    Antibody-dependent cell-mediated cytotoxicity (ADCC).  Lysis of
    various target cells coated with antibody by Fc receptor-bearing
    killer cells, including large granular lymphocytes (NK cells),
    neutro-phils, eosinophils and mononuclear phagocytes.

    Antigen.  Any compound recognized by antigen-receptor-bearing
    lymphocytes. Antigens induce immune responses or tolerance. Antigens
    inducing immune responses only with the help of T-cells are
    T-dependent antigens, while those that do not need T-help are
    T-independent antigens. All immunogens are antigens but not all
    antigens are necessarily immunogens (see also immunogens).

    Antigen-presenting cells.  Cells expressing MHC gene products, with
    the capacity to process and present antigen. Macrophages, dendritic
    cells, B-lymphocytes and Langerhans cells are termed professional or
    constitutive antigen-presenting cells. However, other cells (such as
    endothelial cells) can acquire the ability to present antigen in
    certain pathological conditions.

    Antigen processing and presentation.  Protein antigens are processed
    (cleaved by enzymes) in various compartments of antigen-presenting
    cells. The immunogenic peptides interact with the binding sites of MHC
    class II products (exogenous antigens) or with those in MHC class I
    products (endogenous antigens, including viruses). The processed
    antigen-MHC complex is recognized by the antigen receptor complex of
    T-lymphocytes.

    Antigenic determinant.  A single antigenic site (epitope) usually
    exposed on the surface of a complex antigen. Epitopes are recognized
    by antigen-receptors on T- or B-cells (T-cell epitopes or B-cell
    epitopes).

    Anti-nuclear antibody.  Antibody directed to nuclear antigen. These
    antibodies can have various specificities (e.g., to single- or
    double-stranded DNA, or to histone proteins). These antibodies are
    frequently observed in patients with rheumatoid arthritis,
    scleroderma, Sjögren's syndrome, systemic lupus erythematosus, and
    mixed connective tissue disease. Also called anti-nuclear factor
    (ANF).

    Arthus reaction (Gell and Coombs Type III reaction). Inflammatory
    response, generally evoked in skin, that is induced by immune
    complexes formed after injection of antigen in an individual
    containing antibodies.

    Asthma.  A respiratory disorder characterized by variable air flow
    limitation. Most cases are associated with bronchial
    hyperresponsiveness and chronic inflammatory changes in the airways.

    Atopic dermatitis.  Inflammation of the skin in atopic individuals.
    The term is broader than atopic eczema (see also atopic eczema).

    Atopic eczema.  Chronic skin disease, often localized on flexural
    surfaces, in individuals with propensity to develop IgE-mediated
    allergy. The term describes eczema occurring in atopic individuals and
    does not imply mechanisms (see also eczema).

    Atopy.  A genetic predisposition toward development of IgE-mediated
    immediate hypersensitivity reactions against common environmental
    antigens.

    Autoimmune disease.  A disease involving immune responses against
    self antigens, resulting in pathological change.

    Autoimmunity.  Responses against self (autologous) antigens.

    B-lymphocytes.  Bone-marrow-derived lymphocytes, expressing an
    antigen-receptor complex composed of membrane-bound immunoglobulin
    (mIg) and associated molecular chains. B-cell receptors interact with
    epitopes directly (no MHC restriction). Activated B-lymphocytes
    produce antibody and are efficient antigen-presenting cells. They are
    the precursors of plasma cells.

    Basophil.  A circulating granular leukocyte having prominent
    cytoplasmic granules when stained with dyes that indicate a basic pH.
    The granules contain histamine and sulfated mucopolysaccharides. After
    binding of antigen to membrane-bound IgE via Fc-epsilon-RI receptors, 
    they release histamine, platelet activating factor (PAF) and 
    leukotrienes, and other inflammatory mediators.

    Bronchoalveolar lavage.  Harvesting of cells and fluid from the
    lung, commonly by bronchoscopy and lavage.

    Bronchoprovocation.  Use of inhaled triggers (cold air, histamine,
    methacholine, allergen, etc.) to assess the responsivity of the
    airways.

    Carrier.  An immunogenic macromolecule (usually protein) to which a
    hapten is attached, allowing the hapten to be immunogenic.

    CD.  A molecular marker on a cell surface that may be used
    operationally to define phenotype, origin and activation state of the
    cell.

    CD3.  A molecule composed of five polypeptide chains associated with
    the heterodimer T-cell receptor (TCR), forming the T-cell receptor
    complex (TCR/ CD3); CD3 transduces the activating signals when antigen
    binds to the TCR.

    CD4.  A cell surface antigen belonging to the immunoglobulin
    superfamily of molecules. Marker of T helper cells. As an adhesion
    molecule, it interacts with the non-polymorphic part of MHC class II
    gene product.

    CD8.  A cell surface molecule belonging to the immunoglobulin
    superfamily of molecules. Marker of suppressor and cytotoxic T-cells.
    As an adhesion molecule, it interacts with the MHC class I gene
    product.

    CD16.  Low-affinity Fc-gamma receptor (Fc-gamma-RIII) expressed 
    mainly on NK cells, granulocytes and macrophages, mediating ADCC.

    CD23.  Low-affinity Fc-epsilon receptor induced by IL-4 and 
    expressed on activated B-cells and macrophages.

    Cell-mediated or cellular response.  A specific immune response in
    which T-lymphocytes mediate the effects, either through the release of
    cytokines or through cytotoxicity.

    Class I MHC gene products.  Antigens encoded by the MHC class I
    genes are expressed on all nucleated cells. They present 
    antigen-derived peptides of endogenous origin.

    Class II MHC gene products.  Antigens encoded by the MHC class II
    genes are expressed on antigen-presenting cells. They present
    antigen-derived peptides of endogenous origin.

    Clonal anergy.  A form of self tolerance developing as a consequence
    of negative selection during the thymic selection processes. Clones of
    thymocytes whose antigen receptors (TCR) bind with high affinity to
    self antigens in association with MHC molecules are inactivated.

    Complement system.  A group of serum proteins with the capacity to
    interact with each other when activated. The chain reaction of the
    activated complement components results in formation of a lytic
    complex and several biologically active peptides of low relative
    molecular mass (anaphylatoxins). The system can be activated by
    antigen-antibody complexes (classical pathway) and by other
    components, e.g., bacteria (alternative pathway). As an effector
    mechanism of the humoral immune response, the activated complement
    system facilitates opsonization, phagocytosis and lysis of cellular
    antigens.

    Contact sensitivity.  A state of immunological sensitization in
    which an eczematous epidermal reaction may occur when a hapten is
    applied to the skin of a sensitized individual. (see allergic contact
    dermatitis).

    Contact urticaria.  Urticaria provoked by contact with inducing
    agents (see urticaria).

    Cross-reactivity.  The ability of an antibody or a T-cell, specific
    for one antigen, to react with a second antigen; a measure of
    relatedness between two antigenic substances, and/or polyspecificity
    of the antibody molecule (e.g., some rheumatoid factors), or of the
    T-cell receptor.

    Cytokines.  Group of substances (biologically active peptides),
    mainly synthesized by lymphocytes (lymphokines) or monocytes/
    macrophages (monokines), that modulate the function of cells in
    immunological reactions; cytokines include interleukins. Some
    cytokines (pleotrophic cytokines) have a broad spectrum of biological
    actions, including: neuromodulation, growth factor activity and
    proinflammatory activity (see also interleukins).

    Cytotoxic T-cell (cytolytic T-cell)(CTL). A subpopulation of T-cells
    with the capacity to lyse target cells displaying a determinant in
    association with MHC gene products, recognized by its antigen receptor
    complex (TCR/CD3).

    Delayed-type hypersensitivity (DTH) (Gell and Coombs Type IV
    reaction).  A form of T-cell-mediated immunity in which the ultimate
    effector cell is the activated mononuclear phagocyte (macrophage); the
    response of DTH appears fully over 24 to 48 h. Previous exposure is
    required. Examples include response to  Mycobacterium tuberculosis 
    (tuberculin test) and contact dermatitis.

    Dendritic cell.  A cell type characterized by extended cytoplasmic
    protrusions and a high expression of adhesion molecules and Class II
    MHC gene products effecting antigen presentation to specific
    lymphocytes (see also Langerhans' cell).

    Dermatitis.  Inflammatory skin disease showing redness, swelling,
    infiltration, scaling and sometimes vesicles and blisters.

    Desensitization.  Generally transient state of specific
    non-reactivity in previously sensitized individual, resulting from
    repeated antigen exposures.

    Eczema.  A dermatitis characterized by non-contagious inflammation
    of skin with typical clinical (itch, erythema, papules, seropapules,
    vesicles, squames, crusts, lichenification) and dermatohistological
    (spongiosis, acanthosis, parakeratosis, lymphocytic infiltration)
    findings. Often due to sensitization.

    Elicitation.  Production of a cell-mediated or antibody-mediated
    allergic response by exposure of a sensitized individual to an
    allergen.

    Endocytosis.  The uptake by a cell of a substance from the
    environment by invagination of its plasma membrane; it includes both
    phagocytosis mediated by receptors and pinocytosis.

    Enzyme-linked immunosorbent assay (ELISA). An assay in which an
    enzyme is linked to an antibody and a labelled substance is used to
    measure the activity of bound enzyme and, hence, the amount of bound
    antibody. With a fixed amount of immobilized antigen, the amount of
    labelled antibody bound decreases as the concentration of unlabelled
    antigen is increased, allowing quantification of unlabelled antigen
    (competitive ELISA). With a fixed amount of one immobilized antibody,
    the binding of a second, labelled antibody increases as the
    concentration of antigen increases, allowing quantification of antigen
    (sandwich ELISA).

    Eosinophil.  A circulating granular leukocyte having prominent
    granules that stain specifically by eosin and containing numerous
    lysosomes. Eosinophils are important effector cells in immune
    reactions to antigens that induce high levels of IgE antibodies (e.g.,
    parasites). Eosinophils are also abundant at sites of immediate
    hypersensitivity reactions.

    Epidemiology.  The study of the distribution and determinants of
    health-related states or events in specified populations, and the
    application of this knowledge to manage health problems.

    Epitope.  A single antigenic determinant.

    Fc receptors.  Receptors expressed on a wide range of cells,
    interacting with the Fc portion of immunoglobulins belonging to
    various isotypes. Membrane-bound Fc receptors mediate different
    effector functions (endocytosis, antibody-dependenT-cellular
    cytotoxicity (ADCC)) and induce mediator release. Both the
    membrane-bound and soluble forms of Fc receptors regulate antibody
    production of B-cells.

    Forced expiratory volume in 1 second (FEV1).  Physiological
    measurement of the volume of air expired in one second with a maximal
    respiratory effort.

    Forced ventilatory capacity (FVC).  The physiological measurement of
    lung volume associated with a complete respiratory effort.

    Glomerulonephropathy.  Disease of the glomeruli, which may show
    either thickening of the basement membrane -- membranous
    glomerulopathy associated with IgG deposits -- due to the accretion of
    proteins, or "minimal change glomerulopathy", in which there is
    functional damage but little structural change by light microscopy.

    Hapten.  A non-immunogenic compound of low relative molecular mass
    which becomes immunogenic after conjugation with a carrier protein or
    cell and in this form induces immune responses. Antibodies, but not
    T-cells, can bind the hapten alone in the absence of carrier.

    Helper T-cell.  A functional subpopulation of T-cells (expressing
    CD4 antigen) that help to generate cytotoxic T-cells and cooperate
    with B-cells in the production of an antibody response. Helper T-cells
    recognize antigen in association with MHC class II gene products.
    Depending on their capacity to produce various cytokines one can
    functionally differentiate Th1 (IL-2 and IFN gamma producing) and Th2
    (IL-3, IL-4 and IL-6 producing) cells.

    Human leukocyte antigen (HLA).  The major human histocompatibility
    complex situated on chromosome 6. Human HLA-A, -B and -C (resembling
    mouse H-2K, D and L) are class I MHC molecules, whereas HLA DP, -DQ
    and -DR (resembling mouse I-A and I-E) are class II MHC molecules.

    Humoral immune response.  An immune response in which specific
    antibodies induce the effector functions (such as phagocytosis and
    activation of the complement system).

    Hyperreactivity.  An abnormally increased response to a stimulus.

    Hypersensitivity.  Abnormally increased immunologically mediated
    response to a stimulus. Sometimes used loosely for any increased
    response (see also hypersusceptibility, hyperreactivity)

    Hypersensitivity pneumonitis (HPS) / extrinsic allergic alveolitis
    (Gell and Coombes Type III reaction).  An immune-mediated
    inflammatory disease of the lung parenchyma caused by exposure to an
    inhaled chemical allergen or organic dust.

    Hypersusceptibility.  Adverse effects in an individual occurring
    under exposure conditions that result in no effects in the great
    majority of the population or an individual exhibiting exaggerated
    effects in comparison with the great majority of those showing some
    adverse effects.

    IgE-binding Fc receptors.  The high-affinity IgE-binding
    Fc-epsilon-R type I is expressed on mast-cells and basophils.
    Interacts with IgE antibodies with high affinity. The cross-linking of
    these receptors results in release of mediators (such as histamine).
    The receptor is composed of alpha, beta and gamma chains; the alpha
    chain contains the IgE binding site, while the gamma chain is
    responsible for signal transfer. The low-affinity IgE binding Fc
    receptor (CD23) is expressed on B-cells, its soluble (truncated) form
    is generated by proteolytic cleavage and regulates IgE production of
    B-cells.

    Immediate-type hypersensitivity (Gell and Coombs Type I reaction). 
    A form of antibody mediated immunity that takes place in minutes to
    hours after the administration of antigen. Previous exposure is
    required. An example is allergic rhinitis to pollen antigen.

    Immunodeficiency.  Defects in one or more components of the immune
    system resulting in inability to eliminate or neutralize non-self
    antigens. Congenital or primary immunodeficiencies are genetic or due
    to developmental disorders (such as congenital thymic aplasia).
    Acquired or secondary immunodeficiencies develop as a consequence of
    malnutrition, malignancies, immunosuppressive compounds, radiation or
    infection of immunocompetent T-cells with human immunodeficiency virus
    (HIV). Defects of the nonspecific defence system may also result in
    immunodeficiency.

    Immunogen.  A substance capable of eliciting a specific immune
    response manifested by the formation of specific antibodies and/or
    specifically committed lymphocytes. To induce an antibody response an
    immunogen must possess structurally and functionally distinct
    determinants for activation of B-cells and T-cells.

    Immunoglobulin (Ig).  Immunity-conferring portion of the plasma- or
    serum-gammaglobulins. Various isotypes (classes and subclasses) of
    immunoglobulins have a common core structure of two identical light
    (L) and two identical heavy (H) polypeptide chains, which contain
    repeating homologous units folded in common globular motifs (Ig
    domains). The amino acid sequences of the N-terminal domains are
    variable (V domains), in contrast to the more conserved constant
    regions (C domains). The V domains contain the
    complementarity-determining regions (CDRS) forming the antigen-binding
    sites, whereas the C domains trigger several effector functions of the
    immune system (see also antibody).

    Immunoglobulin gene superfamily.  Genes encoding proteins containing
    one or more Ig domains (homology units) that are homologous to either
    Ig V or C domains. Cell surface and soluble molecules mediating
    recognition, adhesion or binding functions in and outside the immune
    system, derived from the same precursor, belong to this family of
    molecules (e.g., Ig, TCR, MHC Class I and II, CD4, CD8, Fc-gamma-R,
    NCAM, PDGFR).

    Incidence (epidemiological).  The number of new cases of disease in
    a defined population during a specified period of time.

    Interleukin.  Immunoregulatory proteins, also designated as
    lymphokines, monokines or cytokines. General features are: low
    relative molecular mass (<80 000) and frequently glycosylated;
    regulate immune cell function and inflammation by binding to specific
    cell surface receptors; transient and local production; act in
    paracrine, autocrine or endocrine manner, with stimulatory or blocking
    effect on growth/differentiation; very potent, function at picomolar
    concentrations. Interleukins represent an extensive series of
    mediators with a wide range of overlapping functions. Other mediators
    in this series are  c-kit ligand, interferons, tumour necrosis
    factor, transforming growth factor, and a family of low relative
    molecular mass mediators, called chemokines.

    Intolerance.  Non-immunologically mediated adverse reactions. In
    food intolerances these may be due to pharmacological properties of
    food constituents, metabolic disorders or responses of unknown
    etiology.

    Langerhans' cells.  Bone-marrow-derived epidermal cells with a
    dendritic morphology, expressing CD1 marker in humans and containing
    the cytoplasmic organelle, called the Birbeck granule. They express
    Class II MHC antigen and are capable of antigen presentation (see also
    dendritic cells).

    Lymphocyte.  Bone-marrow-derived cell with little cytoplasm, with
    the ability to migrate and exchange between the circulation and
    tissues, to home to sites of antigen exposure, and to be held back at
    these sites. The only cells that specifically recognize and respond to

    antigens (mainly with the help of accessory cells). Lymphocytes
    consist of various subsets differing in their function and products
    (e.g., B-lymphocytes, helper-T-cells, cytolytic T-cells).

    Macrophage.  Mononuclear cells derived from monocytes residing in
    tissues. Activated by different stimuli they may appear in various
    forms such as epitheloid cells and multinucleate giant cells.
    Macrophages found in different organs and connective tissues have been
    named according the specific locations, e.g., as microglia, alveolar
    macrophages or Kupffer cells. Macrophages may function as
    antigen-presenting cells, effector cells of cell-mediated immunity,
    and phagocytes eliminating opsonized antigens.

    Major basic protein.  A small basic arginine-rich peptide (pI 10.9,
    relative molecular mass of 13 800) in the granules of eosinophils that
    kills helminths and protozoa.

    Major histocompatibility complex (MHC).  A cluster of genes encoding
    cell surface antigens that are polymorphic within a species and have a
    crucial function in signalling between lymphocytes and cells
    expressing antigen and in recognition of self.

    Mast cell. Tissue bound mononuclear granular cells with staining
    affinity for basic dyes at low pH. The specific granules contain
    mediators of allergic inflammation, e.g., histamine. Upon stimulation
    with antigen via membrane-bound IgE antibodies, they release preformed
    and newly generated mediators. Two types of mast cells exist.
    Tryptase-containing T-mast cells are mainly associated with mucosal
    epithelial ceils. Chymase-containing TC mast cells are long-living
    connective tissue cells.

    Mitogen.  A substance that causes cells to synthesize DNA and
    proliferate without acting as an antigen.

    Monocyte. Bone-marrow-derived mononuclear phagocytic leukocyte, with
    bean-shaped nucleus and fine granular cytoplasm containing lysosomes,
    phagocytic vacuoles and cytoskeletal filaments. Once transported to
    tissues they develop into macrophages.

    Natural killer (NK) cell.  A subset of lymphocytes found in blood
    and some lymphoid tissues, derived from the bone marrow and appearing
    as large granular lymphocytes (LGL). NK cells possess the capacity to
    kill certain tumour cells or virus-infected normal cells. The killing
    is not induced by specific antigen and is not restricted by MHC
    molecules.

    Nephropathy.  Disease of the kidney that may involve either or both
    the glomeruli (specialized structures where blood is filtered) and the
    renal tubules (connected structures where the composition of the
    filtrate is greatly modified in accordance with the physiological
    needs of the body).

    Nephrotic syndrome. A clinical disease in which damage to glomeruli
    has caused leaky filtration, resulting in major loss of protein from
    the body.

    Neutrophil (polymorphonuclear leukocyte).  Granular leukocytes
    having a nucleus with three to five lobes and fine cytoplasmic
    granules stainable by neutral dyes. The cells have properties of
    chemotaxis, adherence to immune complexes, and phagocytosis. The cells
    are involved in a variety of inflammatory processes including
    late-phase allergic reactions.

    Occupational asthma.  Asthma caused by a sensitizing agent present
    in the workplace, usually after a period of asymptomatic exposure.

    Opsonization.  Coating of antigens with antibody and/or complement
    components. The interaction of opsonized complexes with Fc- or
    complement-receptors facilitates their uptake by the receptor-bearing
    phagocytic cells.

    Oral tolerance.  Orally induced and immune-mediated
    non-responsiveness.

    Peak expiratory flow rate (PEFR).  A physiological measure of the
    maximum air flow.

    Plasma cell.  A terminally differentiated B-lymphocyte with little
    or no capacity for mitotic division, that can synthesize and secrete
    antibody. Plasma cells have eccentric nuclei, abundant cytoplasm and
    distinct perinuclear haloes. The cytoplasm contains dense rough
    endoplasmic reticulum and a large Golgi complex.

    Platelet activating factor.  Low relative molecular mass
    phospholipid generated from alkyl phospholipids in mast cells,
    basophilic and neutrophilic granulocytes, and monocytes-macrophages,
    which mediates microthrombus formation of platelets in
    hypersensitivity reactions.

    Prevalence (epidemiology). The number of cases of disease occurring
    in a given population at a designated time.

    Prevention of allergy.   Primary prevention is the control of the
    exposures inducing allergy.  Secondary prevention is the control of
    exposure of sensitized individuals.

    Pseudo-allergy.  Non-immunological "hypersensitivity" with clinical
    symptoms and signs mimicking those of allergic diseases.

    Radioallergosorbent test (RAST).  A solid-phase radioimmunoassay for
    detecting IgE antibody specific for a particular antigen.

    Rate (epidemiology).  The frequency with which an event occurs in a
    defined population.

    Reactive airways dysfunction syndrome (RADS).  A syndrome
    characterized by reversible airflow limitation and complicating
    bronchial hyperresponsiveness induced by acute exposure to high
    concentrations of non-sensitizing irritant gases at work.

    Sensitization.  Induction of specialized immunological memory in an
    individual by exposure to an allergen.

    Stem cell.  Pluripotent cells, representing 0.01% of bone marrow
    cells, having the capacity for self renewal, and committed to
    differentiate along particular lineages, e.g., erythroid,
    megakaryocytic, granulocytic, monocytic and lymphocytic. Cytokines
    stimulate the proliferation and maturation of distinct precursors.

    Suppressor T-lymphoctye.  A subpopulation of T-lymphocytes that
    inhibits the activation phase of immune responses. They are CD8+, and
    their growth and differentiation may be regulated by CD4+ cells.

    Tolerance.  Persistent condition of specific immunological
    unresponsiveness, resulting from previous non-sensitizing exposure to
    the antigen.

    Urticaria.  Transient eruption of skin characterized by erythematous
    or oedematous swelling (wheal) of the dermis or subcutaneous tissue.
    

    9.  CONCLUSIONS

         Allergy is an important, world-wide, health problem. It affects a
    substantial proportion of the population and all age groups. For
    reasons that are poorly understood, the overall frequency of allergy
    is increasing.

         The various and complex interactions between chemicals, drugs and
    proteins, the immune system and the target organ(s) that lead to the
    manifestation of allergic hypersensitivity and autoimmunity are
    reviewed in this monograph. Extensive research into these topics
    continues, so new developments are anticipated. The multiplicity of
    endogenous and exogenous factors that have an impact upon allergy are
    considered. Exogenous factors, including allergens themselves, as well
    as infection, air pollution and life style (e.g., tobacco smoking) are
    all of importance. In addition, genetic predisposition to a particular
    allergic disorder is an important determinant of reactivity.

         The epidemiology of allergy demonstrates the widespread nature of
    this group of disorders and, as a consequence, highlights the need for
    proper attention to the identification of allergic hazards and their
    assessment in the implementation of appropriate risk management
    strategies.

         Methods of hazard identification for skin sensitizers are well
    established. They have still to be standardized for respiratory
    sensitizers and are not yet available for other types of allergen.
    Techniques to measure the potency of skin sensitizers are being
    applied for the purpose of risk assessment.

         Once allergic hazards have been well characterized, risk
    assessment, risk management and risk communication are critical
    elements to reduce the incidence of allergic disorders. Risk
    assessment requires that the hazard, of known potency, is evaluated in
    the light of the nature and extent of exposure. Risk management
    measures, such as control of exposure and product labelling, must be
    implemented when the risk assessment indicates the need.

         More is being learned of the physicochemical and immunological
    features of food allergens, which may eventually be of predictive
    value.

         The complexity of the mechanisms of allergic disorders makes it
    difficult at present to suggest  in vitro predictive methods of
    general applicability, although application of structure-activity
    relationships deserves further consideration.
    

    10.  RECOMMENDATIONS FOR PROTECTION OF HUMAN HEALTH

    a)   Effective strategies to prevent allergy should be employed, based
         on good information about allergens in products and the
         environment. Control of exposure should be the basis for
         preventing or minimizing the occurrence of allergic disease.

    b)   There is urgent need to determine the cause of the increased
         frequency of allergy.

    c)   Methods of surveillance should be instituted to define the
         frequency of allergies of different types.

    d)   The measurement of exposure of individuals and of populations may
         be difficult, but adequate assessment is essential to any
         analysis of the association between exposure and effect. The
         specific nature of immune responses represents a unique type of
         biomarker in studying past exposure, for example, by the use of
         skin patch or prick testing, or assay of IgE antibodies to detect
         sensitization by specific allergens.

    e)   Worker surveillance systems, the quality of medical examination
         of workers and education of workers exposed to chemicals should
         be improved in order to reveal occupational allergic diseases at
         an early stage. Relatively simple notification schemes for
         occupational disorders and post-marketing surveillance of
         medicines provide economic screening and alerting systems for
         allergic diseases. Monitoring these disorders in the workplace is
         particularly valuable because exposure there is likely to be
         greater than anywhere else.

    f)   The efficacy and value of primary and secondary prevention and
         intervention strategies should be assessed at intervals using
         validated epidemiological techniques.

    g)   For allergic contact dermatitis, the available  in vivo
         predictive models are of proven value for antigens of low
         relative molecular mass. The need is to discover how best they
         can be used to show the potency of allergens.

    h)   For allergic disorders of the respiratory tract, available  in 
          vivo test methods for substances of low relative molecular mass
         are promising, but their predictive value and specificity need to
         be substantiated. For protein allergens, some animal test methods
         are being developed, but they require further evaluation using
         substances of known allergenic potential in humans.

    i)   It is important that information about the presence of allergens
         in products (e.g., food) be readily available, for instance in
         databases and on product labels, so that regulators and
         individuals can adopt appropriate precautions.

    j)   The main method for avoiding occupational chemically induced
         autoimmune disease is the control of exposure. Pre-exposure
         assessment of those exposed to chemicals of immunomodulating
         potential should be considered in order to document any
         pre-existing features of connective tissue diseases. Strict
         adherence to guidelines to avoid or minimize exposure is advised,
         including the use of good occupational hygiene practices. Other
         risk factors such as smoking should be minimized and regular
         occupational medical examinations should be considered.

    k)   There is a need for investigation of the quantitative
         relationships between immune responses induced by chemicals and
         the severity of allergic reactions.

    l)   Determination of the minimal exposure and the duration of it
         required to cause sensitization or elicit an allergic response is
         important in controlling allergic disease.

    m)   It is important to devise standard strategies for the clinical
         investigation and diagnosis of allergy and to apply them
         internationally to examine the causes and incidence of allergic
         disorders. This will require the production and availability of
         standardized extracts of biological allergens of controlled
         potency, as well as regular consideration of the components of
         standard series of allergens used in testing.

    n)   Public health authorities, health professionals and government
         agencies should consider how to estimate the human and economic
         costs to individuals and society of allergic diseases.

    o)   Public health authorities, health professionals, the public and
         especially the workforce would benefit from better information
         about the occurrence, causes, clinical manifestations and
         consequences of different types of allergy.
    

    11.  FURTHER RESEARCH

    a)   The adjuvant effect of environmental factors, such as pollutants,
         particulates, tobacco smoke and UV radiation, on the induction
         and elicitation of allergic responses to other chemicals should
         be investigated.  The influence of other forms of toxicity,
         including direct immunotoxicity, should also be studied.

    b)   The influence of formulation and mixtures on the induction and
         elicitation of chemical allergy needs to be explored.

    c)   The relevance of route of exposure to chemicals to the
         development of allergic disease, particularly allergic
         sensitization of the respiratory tract, needs to be investigated.

    d)   The relevance of route of exposure to the development of
         tolerance should be explored.

    e)   The role of IgE antibody and other immune effector mechanisms in
         the development of chemical respiratory allergy needs to be
         clarified.

    f)   Understanding of the cellular mechanisms responsible for the
         induction of allergy, such as how allergens are processed and
         presented to T-cells, is necessary to facilitate the development
         of predictive  in vitro tests.

    g)   Improved, reliable, sensitive, specific and robust biomarkers of
         exposure to allergens are needed.  Ideally, they should be
         non-invasive and suitable for field use.  Similarly, better
         techniques for dosimetry of allergens are needed. 

    h)   The basis for the difference in the response of asthmatics and
         non-asthmatics to airborne pollutants needs exploration.

    i)   It is important to understand the relative contributions of
         lifestyle, nutrition, pollution and change in the pattern of
         childhood infection and immunization to the development of
         allergy.

    j)   Mechanisms of sensitization to food allergens need further
         research. In addition, more needs to be known about the
         occurrence and clinical importance of cross-reactivity of complex
         allergens, especially those of natural origin, such as pollen and
         food components.

    k)   It is necessary to identify the properties that control the
         allergenic characteristics and potency of proteins, including the
         reasons why they induce transient or persistent food allergy.

    l)   In the case of autoimmune disease, predictive methods need to be
         developed and validated.
    

    REFERENCES

    Aalberse RC, Koshte V, & Clemens JG (1981) Immunoglobulin E antibodies
    that cross react with vegetable foods, pollen and hymenoptera venom. J
    Allergy Clin Immunol, 68: 356-364.

    Abbey DE, Hwang BL, Burchette RJ, Vancuren T, & Mills PK (1995)
    Estimated long-term ambient concentrations of PM10 and development of
    respiratory symptoms in a nonsmoking population. Arch Environ Health,
    50(2): 139-152.

    Aberg N, Engstrom I, & Lindberg U (1989) Allergic diseases in Swedish
    school children. Acta Paediatr Scand, 78: 246-252.

    Abramson S, Miller RG, & Phillips RA (1977) The identification in
    adult bone marrow of pluripotent and restricted stem cells of the
    myeloid and lymphoid systems. J Exp Med, 145: 1567-1579.

    Adams LE & Hess EV (1991) Drug-related Lupus: Incidence, mechanisms
    and clinical implications. Drug Saf, 6: 431-449.

    Adams RB, Planchon SM, & Roche JK (1993) Interferon-gamma modulation
    of epithelial barrier function: time course, reversibility, and site
    of cytokine binding. J Immunol, 150: 2356-2363.

    Adinoff AD, Telez P, & Clark RAF (1988) Atopic dermatitis and
    aeroallergen contact sensitivity. J Allergy Clin Immunol, 81: 736-742.

    Alanko K, Keskinen H, Björkstén F, & Ojanen S (1978) Immediate-type
    hypersensitivity to reactive dyes. Clin Allergy, 8: 25-31.

    Alarcon-Segovia D (1985) Scleroderma. In: Rose NR & Mackay IR ed. The
    autoimmune diseases. New York, London, San Diego, Academic Press, pp
    119-143.

    Allison AC (1989) Theories of self tolerance and autoimmunity. In:
    Kammüller M, Bloksma N, & Seinen W ed. Autoimmunity and toxicology:
    Immune disregulation induced by drugs and chemicals. Amsterdam,
    Oxford, New York, Elsevier Science Publishers, pp 67-115.

    Amagi M, Ishii K, Hashimoto T, Gamou S, Shimizu N, & Nishikawa T
    (1995) Conformational epitopes of pemphigus antigens (Dsg1 and Dsg3)
    are calcium dependent and glycosylation dependent. J Invest Dermatol,
    105: 243-247.

    American Medical Association (1987a) Council of Scientific Affairs,
    American Medical Association, Report I, Part I, of the Allergy Panel
    (1987):  in vivo diagnostic testing and immunotherapy for allergy. J
    Am Med Assoc, 258: 1363-1367.

    American Medical Association (1987b) Council of Scientific Affairs,
    American Medical Association, Report I, Part II, of the Allergy Panel
    (1987):  in vitro testing for allergy. J Am Med Assoc, 258:
    1639-1643.

    Ames RG, Hall DS, & Reger RB (1984) Chronic respiratory effects of
    exposure to diesel emissions in coal mines. Arch Environ Health, 39:
    389-394.

    Amin S, Lahti A, & Maibach HI (1996) Contact urticaria and the contact
    urticaria syndrome. In: Marzulli FN & Maibach HI ed.
    Dermatotoxicology, 5th ed. London, Taylor and Francis, pp 485-505.

    Andersen KE & Maibach HI (1985) Guinea pig sensitization assays. In:
    Andersen KE & Maibach HI ed. Contact allergy: Predictive tests in
    guinea pigs. Basel, Karger, pp 263-290.

    Andersen KE & Rycroft RJG (1991) Recommended patch test concentrations
    for preservatives, biocides and antimicrobials. Contact Dermatitis,
    25: 1-18.

    Andersen KE, Burrows D, & White IR (1995) Allergens from the standard
    series. In: Rycroft RJG, Menné T, Frosch PJ, & Benezra C ed. Textbook
    of contact dermatitis, 2nd ed. Berlin, Heidelberg, New York, Springer
    Verlag, pp 416-457.

    Anderson HR (1974) The epidemiological and allergic features of asthma
    in the New Guinea Highlands. Clin Allergy, 4: 171-183.

    Anderson HR (1989) Increase in hospital admissions for childhood
    asthma: trends in referral, severity, and readmissions from 1970 to
    1985 in a health region of the United Kingdom. Thorax, 44: 614-619.

    Anderson JA (1991) The clinical spectrum of food allergy in adults.
    Clin Exp Allergy, 21(suppl 1): 304-314.

    Anderson SD & Smith CM (1991) Osmotic challenges in the assessment of
    bronchial hyperresponsiveness. Am Rev Respir Dis, 143(3/2): S43-S46.

    Anderson LB, Dreyfuss EM, Logan U, Johnstone E, & Glaser J (1970)
    Melon and banana sensitivity coincident with ragweed pollinosis. J
    Allergy Clin. Immunol, 45: 310-319.

    Anderson HR, Bailey PA, Cooper JS, Palmer JC, & West S (1983)
    Morbidity and school absence caused by asthma and wheezing illness.
    Arch Dis Child, 58: 777-784.

    Anderson HR, Pottier AC, & Strachan DP (1992) Asthma from birth to age
    23: incidence and relation to prior and concurrent atopic disease.
    Thorax, 47: 537-542.

    Anderson HR, Butland BK, & Strachan DP (1994) Trends in prevalence and
    severity of childhood asthma. Br Med J, 308: 1600-1604.

    Anderson C, Hehr A, Robbins R, Hasan R, Athar M, Mukhtar H, & Elmets
    CA (1995) Metabolic requirements for induction of contact sensitivity
    to immunotoxic polyaromatic hydrocarbons. J Immunol, 
    155(7): 3530-3537.

    Angelini G (1995) Allergens related to specific exposures: Topical
    drugs. In: Rycroft RJG, Menné T, Frosch PJ, & Benezra C ed. Textbook
    of contact dermatitis, 2nd ed. Berlin, Heidelberg, New York, Springer
    Verlag, pp 477-498.

    Angle CR (1988) Indoor air pollutants. Adv Pediatr, 35: 239-280.

    Anonymous (1982) Breast feeding and eczema/asthma. Lancet, 1(8277):
    910-911.

    Anonymous (1985) Smoking, occupation, and allergic lung disease.
    Lancet, 1(8435): 965.

    Anto JM (1995) Asthma outbreaks: an opportunity for research? Thorax,
    50(3): 220-222.

    Anto JM, Sunyer J, Rodriguez Roisin R, Suarez Cervera M, & Vazquez L
    (1989) Community outbreaks of asthma associated with inhalation of
    soybean dust. Toxicoepidemiological Committee. New Engl J Med, 320:
    1097-1102.

    Armentia A, Martin Santos JM, Quintero A, Fernandez A, Barber D,
    Alonso E, & Gil I (1990) Bakers' asthma: Prevalence and evaluation of
    immunotherapy with a wheat flour extract. Ann Allergy, 65(4): 265-272.

    Arnon R & Teitelbaum D (1993) On the existence of suppressor cells.
    Int Arch Allergy Immunol, 100: 2-7.

    Arshad SH, Matthews S, Gant C, & Hide DW (1992) Effect of allergen
    avoidance on development of allergic disorders in infancy. Lancet,
    339: 1493-1497.

    Arshad SH, Stevens M, & Hide DW (1993) The effect of genetic and
    environmental factors on the prevalence of allergic disorders at the
    age of two years. Clin Exp Allergy, 23: 504-511.

    Ashby J, Basketter DA, Paton D, & Kimber I (1995) Structure activity
    relationships in skin sensitizing sensitization using the murine local
    lymph node assay. Toxicology, 103: 177-194.

    Asher MI, Pattemore PK, Harrison AC, Mitchell EA, Rea HH, Stewart AW,
    & Woolcock AJ (1988) International comparison of the prevalence of
    asthma symptoms and bronchial hyperresponsiveness. Am Rev Respir Dis,
    138: 524-529.

    Asher MI, Keil U, Anderson HR, Beasley R, Crane J, Martinez F,
    Mitchell EA, Pearce N, Sibbald B, Stewart AW, Strachan D, Weiland SK,
    & Williams HC (1995) International study of asthma and allergies in
    childhood (ISAAC): Rationale and methods. Eur Respir J, 8(3): 483-491.

    Asherson GL, Zembala M, Perera MACC, Mayhew B, & Thomas WR (1977)
    Production of immunity and unresponsiveness in the mouse by feeding
    contact sensitizing agents and the role of suppressor cells in the
    Peyers's patches, mesenteric lymph nodes and other lymphoid tissue.
    Cell Immunol, 33: 145-155.

    Atkins FM, Steinberg SS, & Metcalfe DD (1985a) Evaluation of immediate
    adverse reactions to foods in adult patients: I. Correlation of
    demographic, laboratory, and prick skin test data with response to
    controlled oral food challenge. J Allergy Clin Immunol, 75: 348-355.

    Atkins FM, Steinberg SS, & Metcalfe DD (1985b) Evaluation of immediate
    adverse reactions to foods in adult patients: II. A detailed analysis
    of reaction patterns during oral food challenge. J Allergy Clin
    Immunol, 75: 356-355.

    Atkinson NF Jr & Platts-Mills TAE (1988) IgE and atopic disease. In:
    Immunological diseases, 4th ed. Boston, Massachusetts, Little Brown,
    vol 2, pp 1009-1026.

    Avnstorp C (1992) Cement eczema: An epidemiological intervention
    study. Acta Dermatol Venereol, 179(suppl): 1-22.

    Axelsson IG, Johansson SG, Larsson PH, & Zetterstrom O (1990)
    Characterization of allergenic components in sap extract from the
    weeping fig (Ficus benjamina). Int Arch Allergy Appl Immunol, 91(2):
    130-135.

    Bachert C, Hauser U, Prem B, Rudack C, & Ganzer U (1995)
    Proinflammatory cytokines in allergic rhinitis. Eur Arch
    Otorhinolaryngol, 252(Suppl 1): 155-214.

    Baird AW, Coombs RRA, McLaughlan P, & Cuthbert AW (1984) Immediate
    hypersensitivity reactions to cow milk proteins in isolated epithelium
    from ileum of milk-drinking guinea-pigs: Comparison with colonic
    epithelia. Int Arch Allergy Appl Immunol, 75: 255-263.

    Baker DB, Gann PH, Brooks SM, Gallagher J, & Bernstein IL (1990)
    Cross-sensitization study of platinum salts sensitization among
    precious metals refinery workers. Am J Ind Med, 18: 653-664.

    Bakke P, Gulsvikk A, & Eide GE (1990) Hay fever, eczema and urticaria
    in southwest Norway. Allergy, 45: 515-522.

    Ball B, Horstman D, Folinsbee L, Gerrity T, DeWitt P, Abdul-Salaam S,
    & Brown J (1993) Pulmonary responses in asthmatics performing light
    exercise in clean air(air) and 0.16 ppm ozone (O3). Am Rev Respir
    Dis, 147: A640 (abstract).

    Barbee RA (1987) The epidemiology of asthma. Monogr Allergy, 21:
    21-41.

    Barbee RA, Kaltenborn W, Lebowitz MD, & Burrows B (1987) Longitudinal
    changes in allergen skin test reactivity in a community population
    sample. J Allergy Clin Immunol, 79(1): 16-24.

    Bardare M, Vaccari A, Allievi E, Brunelli L, Coco F, de Gaspari GC, &
    Flauto U (1993) Influence of dietary manipulation on incidence of
    atopic disease in infants at risk. Ann Allergy, 71(4): 366-371.

    Barnes PJ, Baranjuk J, & Belvisi MG (1991) Neuropeptides in the
    respiratory tract. Am Rev Respir Dis, 144: 1187-1198.

    Barnes PJ, Jonsson B, & Klim JB (1996) The costs of asthma. Eur Respir
    J, 9: 636-642.

    Barratt MD (1995) Quantitative structure activity relationships for
    skin permeability. Toxicol  in vitro, 9: 27-37.

    Barratt MD, Basketter DA, Chamberlain M, Payne MP, Admans GD, &
    Langowski JJ (1994a) Development of an expert system rule base for
    identifying contact allergens. Toxicol  in vitro, 8: 837-839.

    Barratt MD, Basketter DA, Chamberlain M, Admans GD, & Langowski JJ
    (1994b) An expert system rule base for identifying contact allergens.
    Toxicol  in vitro, 8: 1053-1060.

    Barratt MD, Basketter DA, & Roberts DW (1994c) Skin sensitization
    structure-activity relationships for phenyl benzoates. Toxicol  in 
     vitro, 8: 823-826.

    Barrett JT (1988) Textbook of immunology, 5th ed. St. Louis, Missouri,
    C.V. Mosby & Co.

    Barrett SP, Toh BH, Alderuccio F, van Driel I, & Gleeson PA (1995)
    Organ-specific autoimmunity induced by adult thymectomy and
    cyclophosphamide-induced lymphopenia. Eur J Immunol, 25: 238-244.

    Barry DM, Burr ML, & Limb ES (1991) Prevalence of asthma among 12 year
    old children in New Zealand and South Wales: a comparative survey.
    Thorax, 46: 405-409.

    Bascom R (1996) Environmental factors and respiratory
    hypersensitivity: the Americas. Toxicol Lett, 86(2-3): 115-130.

    Bascom R, Naclerio RM, Fitzgerald TK, Kagey Sobotka A, & Proud D
    (1990) Effect of ozone inhalation on the response to nasal challenge
    with antigen of allergic subjects. Am Rev Respir Dis, 142(3): 594-601.

    Basketter DA & Roberts DW (1990) Structure activity relationships in
    contact allergy. Int J Cosmet Sci, 12: 81-90.

    Basketter DA, Roberts DW, Cronin M, & Scholes EW (1992) The value of
    the local lymph node assay in quantitative structure-activity
    investigations. Contact Dermatitis, 27: 137-142.

    Basketter DA, Gerberich GF, Kimber I, & Loveless SE (1996) The local
    lymph node assay -- a valuable alternative to currently accepted skin
    sensitisation tests. Food Chem Toxicol, 34: 651-660.

    Basketter DA, Cookman G, Gerberich GF, Hamaide N, & Potokar M (1997)
    Skin sensitisation thresholds: Determination in predictive models.
    Food Chem Toxicol, 35: 417-425.

    Bates DV & Sizto R (1987) Air pollution and hospital admissions in
    southern Ontario: The acid summer haze effect. Environ Res, 43:
    317-331.

    Baur V & Fruhmann G (1981) Specific IgE antibodies in patients with
    isocyanate asthma. Chest, 80(suppl): 73S-76S.

    Baur X, Dewair M, & Fruhmann G (1984) Detection of immunologically
    sensitised isocyanate workers by RAST and intracutaneous skin tests. J
    Allergy Clin Immunol, 73: 610-618.

    Baur X, Marek W, Ammon J, Czuppon AB, Marczynski B, Raulf-Heimsoth M,
    Roemmelt H, & Fruhman G (1994) Respiratory and other hazards of
    isocyanates. Int Arch Occup Environ Health, 66: 141-152.

    Beasley R, Roche WR, Roberts JA, & Holgate ST (1989) Cellular events
    in the bronchi in mild asthma and after bronchial provocation. Am Rev
    Respir Dis, 139: 806-817.

    Beasley R, Thomson C, & Pearce N (1991) Selenium, glutathione
    peroxidase and asthma. Clin Exp Allergy, 21(2): 157-159.

    Becher R, Hongslo JK, Jantunen J, & Dybing E (1996) Environmental
    chemicals relevant for respiratory hypersensitivity: the indoor
    environment. Toxicol Lett, 86: 155-162.

    Beeson PB (1994) age and sex associations of 40 autoimmune diseases.
    Am J Med, 96: 457-462.

    Beezhold DH, Sussman GL, Liss GM, & Chang N-S (1996) Latex allergy can
    induce clinical reactions to specific foods. Clin Exp Allergy, 26:
    416-422.

    Beggs PJ & Curson PH (1995) An integrated environmental asthma model.
    Arch Environ Health, 50(2): 87-94.

    Behrendt H, Friedrich KH, Kainka-Stänicke E, Darsow U, Becker WM, &
    Tomingas R (1991) New trends in allergy. In: Ring J & Pryzbilla B ed.
    Allergens and pollutants in the air: A complex interaction. Berlin,
    Heidelberg, New York, Springer Verlag, pp 467-478.

    Behrendt H, Becker W, Friedrichs K, Darsow U, & Tomingas R (1992)
    Interaction between aeroallergens and airborne particulate matter. Int
    Arch Allergy Immunol, 99: 425-428.

    Behrendt H, Krämer U, Dolgner R, Hinrichs J, Willer H, Hagenbeck H, &
    Schlipköter W (1993) Elevated levels of total IgE in East German
    children: Atopy, parasites or pollutants? Allergo J, 2: 31-40.

    Behrendt H, Friedrich KH, Krämer U, Hitzfeld B, Becker WM, & Ring J
    (1996) The role of indoor and outdoor air pollution in allergic
    diseases. In: Vos J, Younes M, & Smith E ed. Allergic
    hypersensitivities induced by chemicals. Boca Raton, Florida, CRC
    Press, pp 173-182.

    Bekesi JG, Roboz JP, Fischbein A, & Mason P (1987) Immunotoxicology:
    Environmental contamination by polybrominated biphenyls and immune
    dysfunction among residents of the State of Michigan. Cancer Detect
    Prev, 1(suppl): 29-37.

    Bell RG (1996) IgE, allergies and helminth parasites: a new
    perspective on an old conundrum. Immunol Cell Biol, 74: 337-345.

    Bencko V, Vasilieva EV, & Symon K (1980) Immunological aspects of
    exposure to emissions from burning coal of high beryllium content.
    Environ Res, 22(2): 439-449.

    Benezra C, Sigman CC, Perry LR, Helmes CT, & Maibach HI (1985) A
    systematic search for structure-activity relationships of skin contact
    sensitizers: methodology. J Invest Dermatol, 85: 351-356.

    Benjamini E & Leskowitz S (1991) Immunology: A short course. New York,
    Wiley-Liss.

    Bentley AM, Maestrelli P, Fabbri LM, Menz G, Storz Chr, Bradley B,
    Jeffrey PK, Durham SR, & Kay AB (1991) Immunohistology of the
    bronchial mucosa in occupational, intrinsic and extrinsic asthma. J
    Allergy Clin Immunol, 87: 246 (abstract).

    Berglund M, Braback L, Bylin G, Jonson JO, & Vahter M (1994) Personal
    NO2 exposure monitoring shows high exposure among ice-skating
    schoolchildren. Arch Environ Health, 49: 17-24.

    Bernhisel-Broadbent J & Sampson HA (1989) Cross-allergenicity in the
    legume botanical family in children with food hypersensitivity. J
    Allergy Clin Immunol, 83: 435-440.

    Bernhisel-Broadbent J, Taylor S, & Sampson HA (1989)
    Cross-allergenicity in the legume botanical family in children with
    food hypersensitivity: II. Laboratory correlates. J Allergy Clin
    Immunol, 84: 701-709.

    Bernstein DI, Patterson R, & Zeiss CR (1982a) Clinical and immunologic
    evaluation of trimellitic anhydride- and phthalic anhydride-exposed
    workers using a questionnaire and comparative analysis of enzyme
    linked immunosorbent and radioimmunoassay studies. J Allergy Clin
    Immunol, 69: 311-318.

    Bernstein M, Day JH, & Welsh A (1982b) Double-blind food challenge in
    the diagnosis of food sensitivity in the adult. J Allergy Clin
    Immunol, 70: 205-211.

    Bernstein DI, Gallagher JA, D'Souza L, & Bernstein IL (1984)
    Heterogeneity of specific IgE responses in workers sensitised to acid
    anhydride compounds. J Allergy Clin Immunol, 74: 794-801.

    Biagini RE, Bernstein IL, Gallagher JS, Moorman WJ, Brooks S, & Gann
    PH (1985) The diversity of reaginic immune responses to platinum and
    palladium salts. J. Allergy Clin Immunol, 76(6): 794-802.

    Biagini RE, Moorman WJ, Lewis TR, & Bernstein IL (1986) Ozone
    enhancement of platinum asthma in a primate model. Annu Rev Respir
    Dis, 134: 719-725.

    Bieber TH & Ring J (1992)  In vivo modulation of the high-affinity
    receptor for IgE (Fc-epsilon-RI) on human epidermal Langerhans cells. 
    Int Arch Allergy Immunol, 99: 204-207.

    Bigazzi PE (1988) Autoimmunity induced by chemicals. J Toxicol Clin
    Toxicol, 26: 125-156.

    Bigazzi PE (1997) Autoimmunity caused by xenobiotics. Toxicology, 119:
    1-21.

    Bignon JS, Aron Y, Ju LY, Kopferschmitt MC, Garnier R, Mapp C, Fabbri
    LM, Pauli G, Lockhart A, Charron D, & Swierczewski E (1994) HLA class
    II alleles in isocyanate-induced asthma. Am J Respir Crit Care Med,
    149: 71-75.

    Bilyk N & Holt PG (1995) Cytokine modulation of the immunosuppressive
    phenotype of pulmonary alveolar macrophage populations. Immunology,
    86: 231-237.

    Bindslev-Jensen C, Hansen TK, Norgaard A, Vestergaard H, & Lars K
    (1994a) New controversial techniques in the diagnosis of food
    hypersensitivity. In: Johansson SGO ed. Progress in allergy and
    clinical immunology. Toronto, Bern, Hans Huber, vol 3, pp 468-274.

    Bindslev-Jensen C, Skov P, Madsen F, & Poulsen LK (1994b) Food allergy
    and food intolerance - what is the difference? Ann Allergy, 72:
    317-320.

    Bircher AJ, Wüthrich B, Langauer S, & Schmid P (1993) [Ficus
    benjamina, a perennial inhalation allergen of increasing importance.]
    Schweiz Med Wochenschr, 123(22): 1153-1159 (in German).

    Björksten B & Kjellman NIM (1987) Perinatal factors influencing the
    development of allergy. Clin Rev Allergy, 5: 339-347.

    Black CM, Welsh KI, Walker AE, Bernstein RM, Catoggio LJ, McGregor AR,
    & Lloyd Jones JK (1983) Genetic susceptibility to scleroderma-like
    syndrome induced by vinyl chloride. Lancet, 1: 53-55.

    Blaikie L, Basketter DA, & Morrow T (1995) Experience with a guinea
    pig model for the assessment of respiratory allergens. Hum Exp
    Toxicol, 14: 73.

    Blanco C, Carrillo T, Castillo R, Quiralte J, & Cuevas M (1994)
    Avocado hypersensitivity. Allergy, 49: 454-459.

    Bloch KJ & Walker WA (1981) Effect of locally induced intestinal
    anaphylaxis on the uptake of a bystander antigen. J Allergy Clin
    Immunol, 67: 312-316.

    Bloksma N, Kammüller ME, Punt P, & Seinen W (1988) Strain-dependence
    of primary popliteal lymph node reactions to subcutaneous injection of
    diphenylhydantoin in mice. In: Kammüller ME ed. A toxicological
    approach to chemical-induced autoimmunity. Utrecht, The Netherlands,
    University of Utrecht, pp 79-92 (PhD Thesis).

    Bloksma N, Kubicka-Muranyi M, Schuppe H-C, Gleichmann E, & Gleichmann
    H (1995) Predictive immunotoxicological test systems: Suitability of
    the popliteal lymph node assay in mice and rats. Crit Rev Toxicol, 25:
    369-396.

    Bloom BR, Salgame P, & Diamons B (1992) Revisiting and revising
    suppressor T cells. Immunol Today, 13(4): 131-136.

    Bock SA (1987) Prospective appraisal of complaints of adverse
    reactions to foods in children during the first 3 years of life.
    Pediatrics, 79(5): 683-688.

    Bock SA, Sampson HA, Atkins FM, Zeiger RS, Lehrer S, Sachs M, Bush RK,
    & Metcalfe DD (1988) Double-blind, placebo-controlled food challenge
    (DBPCFC) as an office procedure: a manual. J Allergy Clin Immunol, 82:
    986-997.

    Boerrigter GH & Scheper RJ (1987) Local and systemic desensitization
    induced by repeated epicutaneous hapten application. J Invest
    Dermatol, 88(1): 3-7.

    Bohadana AB, Massin N, Wild P, Kolopp MN, & Toamain JP (1994)
    Respiratory symptoms and airway responsiveness in apparently healthy
    workers exposed to flour dust. Eur Respir J, 7(6): 1070-1076.

    Bonini S, Magrini L, Rotiroti G, Ronchetti MP, & Onotati P (1994)
    Genetic and environmental factors in the changing incidence of
    allergy. Allergy, 49: 6-14.

    Bonnet M, Angibaud G, Cantagrel A, Montastruc JL, & Janet M (1995)
    Myasthénie induite par la tiopronine au cours du traitement de la
    polyarthrite rhumatoïde. Rev Neurol, 151: 67-68.

    Borelli S (1981) [Dermatological indications for climatherapy in the
    high altitude of Davos (1560 m and higher.] In: Borelli S & Düngemann
    H ed. [Allergology and dermatology.] Frankfurt, Germany, IMP-Verlag,
    pp 564-660 (in German).

    Borelli S & Schnyder UW (1962) [Constitutional dermatitis without
    atopy.] In: Miescher G & Storck H ed. [Dermatitis: I. Handbook of
    treatment.] Berlin, Heidelberg, New York, Springer Verlag, vol 1/1, pp
    254-319 (in German).

    Bos JD & Kapsenberg ML (1993) The skin immune system: progress in
    cutaneous biology. Immunol Today, 14: 75-78.

    Braback L, Breborowicz A, Julge K, Knutsson A, Riikjarv MA, Vasar M, &
    Bjorksten B (1995) Risk factors for respiratory symptoms and atopic
    sensitisation in the Baltic area. Arch Dis Child, 72(6): 487-493.

    Braun Fahrländer C, Kunzli N, Domenighetti G, Carell CF, & Ackermann
    Liebrich U (1994) Acute effects of ambient ozone on respiratory
    function of Swiss schoolchildren after a 10-minute heavy exercise.
    Pediatr Pulmonol, 17: 169-177.

    Braun-Fahrländer C, Vuille JC, Sennhauser FH, Neu U, Künzle T, Grize
    L, Gassner M, Minder C, Schindler C, Varonier HS, & Wüthrich B (1997)
    Respiratory health and long-term exposure to air pollutants in Swiss
    schoolchildren -- SCARPOL Team: Swiss study on childhood allergy and
    respiratory symptoms with respect to air pollution, climate and
    pollen. Am J Respir Crit Care Med, 155: 1042-1049.

    Breathnach SM (1988) The Langerhans cell: Centenary review. Br J
    Dermatol, 119(4): 463-469.

    Briatico-Vangosa G, Braun CLJ, Cookman G, Hofmann T, Kimber I,
    Loveless SE, Morrow T, Pauluhn J, Sorensen T, & Niessen HJ (1994)
    Respiratory allergy: Hazard identification and risk assessment. Fundam
    Appl Toxicol, 23: 145-158.

    Britton WJ, Woolcock AJ, Peat JK, Sedgwick CJ, Lloyd DM, & Leeder SR
    (1986) Prevalence of bronchial hyperresponsiveness in children: the
    relationship between asthma and skin reactivity to allergens in two
    communities. Int J Epidemiol, 15(2): 202-209.

    Britton J, Pavord I, Richards K, Knox A, Wisniewski A, Weiss S, &
    Tattersfield A (1994) Dietary sodium intake and the risk of airway
    hyperreactivity in a random adult population. Thorax, 49(9): 875-880.

    Britton JR, Pavord ID, Richards KA, Knox AJ, Wisniewski AF, Lewis SA,
    Tattersfield AE, & Weiss ST (1995) Dietary antioxidant vitamin intake
    and lung function in the general population. Am J Respir Crit Care
    Med, 151(5): 1383-1387.

    Broder I, Higgins MW, Mathews KP, & Keller JB (1974a) Epidemiology of
    asthma and allergic rhinitis in a total community, Tecumseh, Michigan.
    J Allergy Clin Immunol, 53: 127-138.

    Broder I, Higgins MW, Matthews KP, & Keeler JB (1974b) Epidemiology of
    asthma and allergic rhinitis in a total community, Tecumseh, Michigan:
    3. Second survey of the community. J Allergy Clin Immunol, 53:
    127-138.

    Brooks SM (1982) The evaluation of occupational airways disease in the
    laboratory and workplace. J Allergy Clin Immunol, 70: 56-66.

    Broughton A & Thrasher JD (1988) Antibodies and altered cell-mediated
    immunity in formaldehyde-exposed humans. Comments Toxicol, 2: 155-174.

    Bruijnzeel-Koomen C, Van Wichen DF, Toonstra J, Berrens L, &
    Bruijnzeel PL (1986) The presence of IgE molecules on epidermal
    Langerhans cells in patients with atopic dermatitis. Arch Dermatol
    Res, 278(3): 199-205.

    Bruijnzeel-Koomen C, Ortolani C, Aas K, Bindslev-Jensen C, Björkstén
    B, Moneret-Vautrin D, & Wüthrich B (1995) Position paper of the
    European Academy of Allergy and Clinical Immunology on adverse
    reactions to food: Adverse reactions to food. Allergy, 50: 623-635.

    Brunekreef B (1992) Damp housing and adult respiratory symptoms.
    Allergy, 47(5): 498-502.

    Brunekreef B, Dockery DW, Speizer FE, Ware JH, Spengler JD, & Ferris
    BG (1989) Home dampness and respiratory morbidity in children. Am Rev
    Respir Dis, 140: 1363-1367.

    Brunekreef B, Dockery DW, & Krzyzanowski M (1995) Epidemiologic
    studies on short-term effects of low levels of major ambient air
    pollution components. Environ Health Perspect; 103(Suppl 2): 3-13.

    Brunekreef B, Janssen NA, de Hartog J, Harssema H, Knape M, & van
    Vliet P (1997) Air pollution from truck traffic and lung function in
    children living near motorways. Epidemiology, 8(3): 298-303.

    Bruynzeel DP, von Blomberg-van der Flier M, Scheper RJ, van Ketel WG,
    & de Haan P (1985) Penicillin allergy and the relevance of
    epicutaneous tests. Dermatologica, 171: 429-434.

    Bryson JS, Caywood BE, & Kaplan AM (1991) Relationship of cyclosporine
    A-mediated inhibition of clonal deletion and development of syngeneic
    graft-versus-host disease. J Immunol, 147: 391-397.

    Buckley RH (1992) Primary immunodeficiency diseases. In: Wyngaarden J
    & Smith L ed. Cecil's textbook of medicine. Philadelphia,
    Pennsylvania, W.B. Saunders Company, pp 1446-1453.

    Buehler EV (1965) Delayed contact hypersensitivity in the guinea pig.
    Arch Dermatol, 5(91): 171-177.

    Buist AS & Vollmer WM (1990) Reflections on the rise in asthma
    morbidity and mortality. J Am Med Assoc, 264: 1719-1720.

    Burge PS (1989) Diagnosis of occupational asthma. Clin Exp Allergy,
    19(6): 649-652.

    Burlingame RW & Rubin RL (1991) Drug-induced anti-histone
    autoantibodies display two patterns of reactivity with substructures
    of chromatin. J Clin Invest, 88: 680-690.

    Burney P (1987) A diet rich in sodium may potentiate asthma:
    Epidemiologic evidence for a new hypothesis. Chest, 91: 143S-148S.

    Burney PG, Britton JR, Chinn S, Tattersfield AE, Platt HS, Papacosta
    AO, & Kelson MC (1986) Response to inhaled histamine and 24 hour
    sodium excretion. Br Med J Clin Res Ed, 292: 1483-1486.

    Burney PG, Laitinen LA, Perdrizet S, Huckauf H, Tattersfield AE, Chinn
    S, Poisson N, Heeren A, Britton JR, & Jones T (1989a) Validity and
    repeatability of the IUATLD (1984) bronchial symptoms questionnaire:
    An international comparison. Eur Respir J, 2(10): 940-945.

    Burney PG, Neild JE, Twort CH, Chinn S, Jones TD, Mitchell WD, Bateman
    C, & Cameron IR (1989b) Effect of changing dietary sodium on the
    airway response to histamine. Thorax, 44: 36-41.

    Burney PG, Chinn S & Rana RJ (1990) Has the prevalence of asthma
    increased in children? Evidence from the national study of health and
    growth 1973-86. Br Med J, 300: 1306-1310.

    Burney P, Chinn S, Jarvis D, Luczynska C, & Lai E (1996) Variations in
    the prevalence of respiratory symptoms, self-reported asthma attacks,
    and use of asthma medication in the European Community respiratory
    health survey (ECRHS). Eur Respir J, 9: 687-695.

    Burnett RT, Dales R, Krewski D, Vincent R, Dann T, & Brook JR (1995)
    Associations between ambient particulate sulphate and admissions to
    Ontario hospitals for cardiac and respiratory diseases. Am J
    Epidemiol, 142: 15-22.

    Burr ML, Butland BK, King S, & Vaughan Williams E (1989) Changes in
    asthma prevalence: Two surveys 15 years apart. Arch Dis Child 1989,
    64: 1452-1456.

    Burrows B, Halonen M, Barbee RA, & Lebowitz MD (1981) The relationship
    of serum immunoglobulin E to cigarette smoking. Am Rev Respir Dis,
    124(5): 523-525.

    Busse WW (1990) Respiratory infections: their role in airway
    responsiveness and the pathogenesis of asthma. J Allergy Clin Immunol,
    85: 671-683.

    Butcher BT & Salvaggio JE (1986) Occupational asthma. J Allergy Clin
    Immunol, 78(4): 547-556.

    Butler JM, Chan SC, Stevens S, & Hanifin JM (1983) Increased leukocyte
    histamine release with elevated cyclic AMP-phosphodiesterase activity
    in atopic dermatitis. J Allergy Clin Immunol, 71: 490-497.

    Byers VS, Levin AS, Ozonoff DM, & Baldwin RW (1988) Association
    between clinical symptoms and lymphocyte abnormalities in a population
    with chronic domestic exposure to industrial solvent contaminated
    domestic water supply and a high incidence of leukaemia. Cancer
    Immunol Immunother, 27: 77-81.

    Calkhoven P, Aalbers M, Koshte L, Pos O, Oei HD, & Aalberse RC (1987)
    Cross-reactivity among birch pollen, vegetables and fruits as detected
    by IgE antibodies is due to at least three distinct cross-reactive
    structures. Allergy, 42: 382-390.

    Calvin G (1992) Risk management case history -- detergents. In:
    Richardson ML ed. Risk management of chemicals. London, Royal Society
    of Chemistry, pp 120-136.

    Carr RI, Etheridge PD, & Tilley D (1985) Failure of oral tolerance in
    NZB/W mice is antigen dependent and parallels antibody patterns in
    human systemic lupus erythematosus (SLE). Fed Proc, 44: 1542
    (abstract).

    Cartier A, Thompson NC, Frith PA, Roberts R, & Hargreave FE (1982)
    Allergen-induced increase in bronchial responsiveness to histamine:
    relationship to the late asthmatic response and change in airway
    caliber. J Allergy Clin Immunol, 70: 170-177.

    Cartier A, Bernstein IL, Burge PS, Cohn JR, Fabbri LM, Hargreave FE,
    Malo JL, McKay RT, & Salvaggio JE (1989) Guidelines for
    bronchoprovocation on the investigation of occupational asthma: Report
    of the Subcommittee on Bronchoprovocation for Occupational Asthma. J
    Allergy Clin Immunol, 84(5/2): 823-829.

    Catto-Smith AG, Patrick MK, Hardin JA, & Gall DG (1989a) Intestinal
    anaphylaxis in the rat: Mediators responsible for the ion transport
    abnormalities. Agents Actions, 28: 185-191.

    Catto-Smith AG, Patrick MK, Scott RB, Davidson JS, & Gall DG (1989b)
    Gastric response to mucosal IgE-mediated reactions. Am J Physiol, 257:
    G704 (abstract).

    CDC (US Center for Disease Control) (1994) Populations at risk from
    particulate air pollution -- United States, 1992. Morbid Mortal Wkly
    Rep, 43: 290-293.

    CDC (US Center for Disease Control) (1995) Asthma--United States,
    1982-1992. Morbid Mortal Wkly Rep, 43: 952-955.

    Challacombe SJ (1983) Salivary antibodies and systemic tolerance in
    mice after oral immunization with bacterial antigens. Ann NY Acad Sci,
    409: 177-193.

    Challacombe SJ & Tomasi TB (1987) Oral tolerance. In: Brostoff J &
    Challacombe SJ ed. Food allergy and intolerance, Section F: Model
    systems of antigen handling. Philadelphia, Pennsylvania, Ballière
    Tindall, pp 255-268.

    Chambers CA & Allison JP (1997) Co-stimulation in T cell responses.
    Curr Opin Immunol, 9: 396-404.

    Chan PK, Baldwin RC, Parsons RD, Moss JN, Stiratelle R, Smith JM, &
    Hayes AW (1983) Kathon biocide: Manifestation of delayed contact
    dermatitis in guinea pigs is dependent on the concentrations for
    induction and challenge. J Invest Dermatol, 81: 409-411.

    Chan-Yeung M (1990) Occupational asthma. Chest, 98(suppl 5):
    148S-161S.

    Chan-Yeung M (1995) Occupational Asthma. Environ Health Perspect,
    103(suppl 6): 249-252.

    Chan-Yeung M & Malo JL (1995a) Occupational asthma. N Engl J Med,
    333(2): 107-112.

    Chan Yeung M & Malo JL (1995b) Epidemiology of occupational asthma.
    In: Asthma and rhinitis. Oxford, London, Boston, Scientific
    Publications, pp 44-57.

    Chan-Yeung M, Malo JL, Busse W, & Holgate ST ed. (1995) Asthma and
    rhinitis: Epidemiology of occupational asthma. Oxford, London, Boston,
    Blackwell Scientific Publications, pp 44-57.

    Chand N, Harrison JE, Rooney S, Pillar J, Jakubicki R, Nolan K,
    Diamantis W, & Sofia RD (1992) Anti-IL-5 monoclonal antibody inhibits
    allergic late phase bronchial eosinophilia in guinea pigs: a
    therapeutic approach. Eur J Pharmacol, 211: 121-123.

    Chandor SB (1988) Autoimmune phenomena in lymphoid malignancies. Clin
    Lab Med, 8: 373-384.

    Chapman JR & Roberts DW (1984) Humoral immune dysfunction as a result
    of prenatal exposure to diphenylhydantoin: correlation with the
    occurrence of physical defects. Teratology, 30: 107-117.

    Charous BL (1994) The puzzle of latex allergy: some answers, still
    more questions. Ann Allergy, 73(4): 277-281.

    Chase MW (1946) Inhibition of experimental drug allergy by prior
    feeding of the sensitizing agent. Proc Soc Exp Biol Med, 61: 257-259.

    Chen Y, Kuchroo VK, Inobe J, Hafler DA, & Weiner HL (1994) Regulatory
    T cell clones induced by oral tolerance: Suppression of autoimmune
    encephalomyelitis. Science, 265: 1237-1240. 

    Chua KY, Stewart GA, Thomas WR, Simpson RJ, Dilworth RJ, Plozza TM, &
    Turner KJ (1988) Sequence analysis of cDNA coding for a major house
    dust mite allergen, Der p 1. J Exp Med, 167: 175-182.

    Cirla AM (1994) Cobalt-related asthma: clinical and immunological
    aspects. Sci Total Environ, 150(1-3): 85-94.

    Cleland LG & Bell DA (1978) The occurrence of systemic lupus
    erythematosus in two kindreds in association with severe selective IgA
    deficiency. J Rheumatol, 5: 288-293.

    Clinton PM, Kemeny DM, Amlot P, Urbanek R, & Lessof MH (1986)
    Histamine release from peripheral blood leucocytes in egg-allergic
    patients. Clin Allergy, 16: 345-354.

    Coca AF & Cooke RA (1923) On the classification of the phenomena of
    hypersensitiveness. J Immunol, 8: 163-182.

    Cockcroft DW (1988) Allergens. In: Barnes PJ, Rodger IW, & Thomson NC
    ed. Asthma: Basic mechanisms and clinical management. New York,
    London, San Diego, Academic Press, pp 445-464.

    Coenraads PJ & Smit J (1995) Epidemiology. In: Rycroft RJG, Menné T,
    Frosch PJ, & Benezra C ed. Textbook of contact dermatitis, 2nd ed.
    Berlin, Heidelberg, New York, Springer Verlag, pp 133-150.

    Coffman RL, O'Hara J, Bond MW, Carty J, Zlotnik A, & Paul WE (1986)
    B-cell stimulatory factor-1 enhances the IgE response of
    lipopolysaccharide activated B-cells. J Immunol, 136(12): 4538-4541.

    Coffman RL, Lebman D, & Shrader B (1989) Transforming growth
    factor-beta specifically enhances IgA production by
    lipopolysaccharide-stimulated murine B-lymphocytes. J Exp Med, 170:
    1039-1044.

    Cohen IR & Miller A ed. (1994) Autoimmune disease models. New York,
    London, San Diego, Academic Press.

    Colloff MJ, Ayres J, Carswell F, Howarth PH, Merrett TG, Mitchell EB,
    Walshaw MJ, Warner JO, Warner JA, & Woodcock AA (1992) The control of
    allergens of dust mites and domestic pets: A position paper. Clin Exp
    Allergy, 22(suppl 2): 1-28.

    Conell JT (1969) Quantitative intranasal pollen challenges: III. The
    priming effect in allergic rhinitis. J Allergy, 43: 33-44.

    Constantinescu CS, Hilliard B, Fujioka T, Bhopale MK, Calida D, &
    Rostami AM (1998) Pathogenesis of neuroimmunologic diseases. Immunol
    Res, 17: 217-227.

    Cookson WOCM, Sharp PA, Faux JA, & Hopkin JM (1989) Linkage between
    immunoglobulin E responses underlying asthma and rhinitis and
    chromosome 11q. Lancet, 1: 1292-1295.

    Cookson WOCM, Young RP, Sandford AJ, Moffatt MF, Shirakawa T, Sharp
    PA, Faux JA, Julier C, Le Souef PN, Lathrop GM, & Hopkin JM (1992)
    Maternal inheritance of atopic IgE responsiveness on chromosome 11q.
    Lancet, 340: 381-384.

    Cooper KD (1994) Atopic dermatitis: recent trends in pathogenesis and
    therapy. J Invest Dermatol, 102: 128-137.

    Cooper KD, Kang K, Chan SC, & Hanifin JM (1985) Phosphodiesterase
    inhibition by Ro20-1724 reduces hyper-IgE synthesis by atopic
    dermatitis cells  in vitro. J Invest Dermatol, 804: 477-482

    Cormio L, Turjanmaa K, Talja M, Andersson LC, & Ruutu M (1993)
    Toxicity and immediate allergenicity of latex gloves. Clin Exp
    Allergy, 23(7): 618-623.

    Corrigan CJ & Kay AB (1992) T cells and eosinophils in the
    pathogenesis of asthma. Immunol Today, 13: 501-507.

    Cotterill JA (1991) Psychosomatic approaches in the treatment of
    atopic eczema. In: Ruzicka T, Ring J, & Przybilla B ed. Handbook of
    atopic eczema. Berlin, Heidelberg, New York, Springer Verlag, pp
    459-465.

    Crespo JF, Pascual C, Burks AW, Helms RM, & Esteban MM (1995)
    Frequency of food allergy in a pediatric population from Spain.
    Pediatr Allergy Immunol, 6: 39-43.

    Croner S & Kjellman NI (1990) Development of atopic disease in
    relation to family history and cord blood IgE levels. Pediatr Allergy
    Immunol, 1: 14-20.

    Cronin E (1991) Formaldehyde is a significant allergen in women with
    hand eczema. Contact Dermatitis, 25: 276-283.

    Cronin MTD & Basketter DA (1994) Multivariate QSAR analysis of a skin
    sensitization database: SAR and QSAR. Environ Res, 2: 1-21.

    Crowe SE & Perdue MH (1992) Gastrointestinal food hypersensitivity:
    Basic mechanisms of pathophysiology. Gastroenterology, 103: 1075-1095.

    Cullinan P, Lowson D, Nieuwenhuijsen MJ, Sandiford C, Tee RD, Venables
    KM, McDonald JC, & Newman Taylor AJ (1994) Work related symptoms,
    sensitisation, and estimated exposure in workers not previously
    exposed to flour. Occup Environ Med, 51(9): 579-583.

    Curtis GH, Patrick MK, Catto-Smith AG, & Gall DG (1990) Intestinal
    anaphylaxis in the rat: Effect of chronic antigen exposure.
    Gastroenterology, 98: 1558-1566.

    Czernielewski JM & Demarchez M (1987) Further evidence for the
    self-reproducing capacity of Langerhans cells in human skin. J Invest
    Dermatol, 88: 17-20.

    Dakin R (1982) Remarks on cutaneous affection, produced by certain
    poisonous vegetables. Am J Med Sci, 4: 98-100.

    Dally MB, Hunter JV, Hughes EG, Stewart M, & Newman Taylor AJ (1980)
    Hypersensitivity to platinum salts: a population study. Am Rev Respir
    Dis, 12(4): 120 (abstract).

    Dannaeus A & Inganäs M (1981) A follow-up study of children with food
    allergy: Clinical course in relation to serum IgE-antibody levels to
    milk, egg and fish. Clin Allergy, 11: 533-539.

    Darsow U, Vieluf D, & Ring J (1995) Atopy patch test with different
    vehicles and allergen concentrations: an approach to standardisation.
    J. Allergy Clin Immunol, 95: 677-684.

    Davies RJ, Butcher BR, O'Neil CE, & Salvaggio JE (1977) The  in vitro
    effect of toluene di-isocyanate on lymphocyte cyclic adenosine
    monophosphate production by isoproterenol, prostaglandin and
    histamine. J Allergy Clin Immunol, 60: 223-229.

    Dawson KP & Mitchell EA (1990) Asthma in New Zealand children. J
    Asthma, 27: 291-297.

    Daynes RA, Dudley DJ, & Araneo BA (1990) Regulation of murine
    lymphokine production  in vivo. II. Dehydroepiandrosterone is a
    natural enhancer of interleukin synthesis by helper T cells. Eur J
    Immunol, 20(49): 793-802.

    Dearman RJ & Kimber I (1991) Differential stimulation of immune
    function by respiratory and contact chemical allergens. Immunology,
    72: 563-570.

    Dearman RJ & Kimber I (1992) Divergent immune responses to respiratory
    and contact chemical allergens: antibody elicited by phthalic
    anhydride and oxazolone. Clin Exp Allergy, 22: 241-250.

    Dearman RJ, Hegarty JM, & Kimber I (1991) Inhalation exposure of mice
    to trimellitic anhydride induces both IgG and IgE anti-hapten
    antibody. Int Arch Allergy Appl Immunol, 95: 70-76.

    Dearman RJ, Basketter DA, & Kimber I (1995) Differential cytokine
    production following chronic exposure of mice to chemical respiratory
    and contact allergens. Immunology, 86: 545-550.

    Dearman RJ, Basketter DA, & Kimber I (1996) Characterization of
    chemical allergens as a function of divergent cytokine secretion
    profiles induced in mice. Toxicol Appl Pharmacol, 138: 308-316.

    De Bakker JM, Kammüller ME, Muller ESM, Lam AW, Seinen W, & Bloksma N
    (1990) Kinetics and morphology of chemically induced popliteal lymph
    node reactions compared with antigen-, mitogen-, and
    graft-versus-host-reaction-induced responses. Virchows Arch B Cell
    Pathol, 58: 279-287.

    Dekker C, Dales R, Bartlett S, Brunekreef B, & Zwanenburg H (1991)
    Childhood asthma and the indoor environment. Chest, 100: 922-926.

    De Martino M, Novembre E, & Cozza G (1988) Sensitivity to tomato and
    peanut allergens in children monosensitized to grass pollen. Allergy,
    43: 206-213.

    Deo YM, Graziano RF, Repp R, & van de Winkel JGJ (1997) Clinical
    significance of IgG Fc receptors and Fc-gamma-directed
    immunotherapies. Immunol Today, 18: 127-135.

    Derynck R, Lindquist PB, Lee A, Wen D, Tamm T, Graycar JL, Rhee L,
    Mason AJ, Miller DA, Coffey RJ, Moses HL, & Chen EY (1988) A new type
    of transforming growth factor-beta. EMBO J, 7: 3737-3743.

    Devalia JL, Rusznak C, Herdman MJ, Trigg CJ, Tarraf H, & Davies RJ
    (1994) Effect of nitrogen dioxide and sulphur dioxide on airway
    response of mild asthmatic patients to allergen inhalation. Lancet,
    344: 1668-1671.

    Devlin RB, McDonnell WF, Mann R, Becker S, House DE, Schreinemachers
    D, & Koren HS (1991) Exposure of humans to ambient levels of ozone for
    6.6 hours causes cellular and biochemical changes in the lung. Am J
    Respir Cell Mol Biol, 4: 72-81.

    De Weck AL, Stadler BM, Urwyler A, Wehner HU, & Bühlmann RP (1993)
    Cellular allergen stimulation test (CAST) -- A new dimension in
    allergy diagnostic. ACI News, 5: 9.

    De Zotti R, Larese F, Bovenzi M, Negro C, & Molinari S (1994) Allergic
    airway disease in Italian bakers and pastry makers. Occup Environ Med,
    51(8): 548-552.

    Diaz-Sanchez D, Tsien A, Fleming J, & Saxon A (1997) Combined diesel
    exhaust particulate and ragweed allergen challenge markedly enhances
    human  in vivo nasal ragweed-specific IgE and skews cytokine
    production to a T helper cell 2-type pattern. J Immunol, 158:
    2406-2413.

    Diaz-Sanchez D, Tsien A, Fleming J, & Saxon A (1999) Effect of topical
    fluticasone propionate on the mucosal allergic response induced by
    ragweed allergen and diesel exhaust particle challenge. Clin Immunol,
    90(3): 313-322.

    Diem JE, John RN, Hendrick DJ, Glindmeyer HW, & Dharmarajan V (1982)
    Five year longitudinal study of workers employed in a new toluene
    di-isocyanate manufacturing plant. Am Rev Respir Dis, 126(3): 420-428.

    Diller WF (1987) Facts and fallacies involved in the epidemiology of
    isocyanate asthma. Bull Eur Physiopathol Respir, 23(6): 551-553.

    Djukanovic R, Wilson J, Britten K, Wilson SJ, Walls AF, Roche WR,
    Howarth PH, & Holgate ST (1990) Quantitation of mast cells and
    eosinophils in the bronchial mucosa of symptomatic atopic individuals
    and healthy control subjects using immunohistochemistry. Am Rev Respir
    Dis, 142: 863-871.

    Dockery DW & Pope CA (1994) Acute respiratory effects of particulate
    air pollution. Annu Rev Public Health, 15: 107-132.

    Dockery DW, Speizer FE, Stram DO, Ware JH, Spengler JD, & Ferris BGJ
    (1989) Effects of inhalable particles on respiratory health of
    children. Am Rev Respir Dis, 139: 587-594.

    Dockery DW, Pope CA III, Xu X, Spengler JD, Ware JH, Fay ME, Ferris BG
    Jr, & Speizer FE (1993) An association between air pollution and
    mortality in six US cities. N Engl J Med, 329: 1753-1759.

    DOH (1998) Report of the Silicone Gel Breast Implants Independent
    Review Group. London, Department of Health, pp 1-36.

    Domen PL, Muckerheide A, & Michael JG III (1987) Abrogation of oral
    tolerance. J Immunol, 139: 3195-3198.

    Dooms-Goossens A & Morren M (1992) Results of routine patch testing
    with corticosteroids in 2073 patients. Contact Dermatitis, 26:
    182-191.

    Dorsch W & Ring J (1981) Induction of late cutaneous reactions by skin
    blister fluid from allergen-tested and normal skin. J Allergy Clin
    Immunol, 67: 117-123.

    Dotterud LK & Falk ES (1994) Metal allergy in north Norwegian
    schoolchildren and its relationship with ear piercing and atopy.
    Contact Dermatitis, 31(5): 308-313.

    Dotterud LK & Falk ES (1995) Contact allergy in relation to hand
    eczema and atopic diseases in north Norwegian schoolchildren.
    Acta-Paediatr, 84(4): 402-406.

    Dowse GK, Turner KJ, Stewart GA, Alpers MP, & Woolcock AJ (1985) The
    association between dermatophagoides mites and the increasing
    prevalence of asthma in village communities within the Papua New
    Guinea highlands. J Allergy Clin Immunol, 75(1): 75-83.

    Draize JH, Woodard G, & Calvery HD (1944) Methods for the study of
    irritation and toxicity of substances applied topically to the skin
    and mucous membranes. J Pharmacol Exp Ther, 83: 377-390.

    Dreborg S (1989) Skin tests used for standardization of allergenic
    preparation. In: Dreborg S ed. Skin tests used in type I allergy
    testing: Position paper prepared by the Subcommittee on Skin Tests of
    the European Academy of Allergology and Clinical Immunology. Allergy,
    44(suppl 10): 13-51.

    Dreborg S & Foucard T (1983) Allergy to apple, carrot and potato in
    children with birch pollen allergy. Allergy, 38: 167-172.

    Drever F ed. (1995) Occupational health -- Decennial supplement: The
    Registrar General`s decennial supplement for England and Wales.
    London, Office of Population Censuses and Surveys, Health and Safety
    Executive, 374 pp (Series DS No. 10).

    Druce HM (1993) Allergic and nonallergic rhinitis. In: Middleton E,
    Reed Cellis EF, Adkinson NF, Junginger JW, & Busse WW ed. Allergy:
    Principles and practice. St. Louis, Missouri, C.V. Mosby & Co., pp
    1433-1453.

    Druet P (1989) Contributions of immunological reactions to
    nephrotoxicity. Toxicol Lett, 46: 55-64.

    Dubost J-J, Souteyrand P, & Sauvezie B (1991) Drug-induced
    vasculitides. Clin Rheumatol, 5: 119-138.

    Dubroff LM & Reid RJ Jr (1980) Hydralazine-pyrimidine interactions may
    explain hydralazine induced lupus erythematosus. Science, 208:
    404-406.

    Ducombs G & Schmidt R (1995) Plant and plant products. In: Rycroft
    RJG, Menné T, Frosch PJ, & Benezra C ed. Textbook of contact
    dermatitis, 2nd ed. Berlin, Heidelberg, New York, Springer Verlag, pp
    589-634.

    Ducombs G, Benezra C, Talaga P, Andersen KE, Burrows D, Camarasa JG,
    Dooms-Goossens A, Lachapelle J-M, Frosch P, Menné T, Rycroft R, White
    IR, & Wilkinson JD (1990) Patch testing with sesquiterpene lactone
    mix: A marker for contact allergy to Compositae and certain other
    plants -- A multicenter study of the E.E.C.D.R.G., 1989. Contact
    Dermatitis, 22: 249-253.

    Duhme H, Weiland SK, Keil U, Kraemer B, Schmid M, Stender M, &
    Chambless L (1996) The association between self-reported symptoms of
    asthma and allergic rhinitis and self-reported traffic density on
    street of residence in adolescents. Epidemiology, 7: 578-582.

    Duhme H, Weiland SK, Rudolph P, Wienke A, Kramer A, & Keil U (1998a)
    Asthma and allergies among children in West and East Germany: a
    comparison between Münster and Greifswald using the ISAAC phase I
    protocol. Eur Respir J, 11: 840-847.

    Duhme H, Weiland SK, & Keil U (1998b) Epidemiological analyses of the
    relationship between environmental pollution and asthma. Toxicol Lett,
    102-103: 307-316.

    Dupuis G & Benezra C (1982) Contact dermatitis to simple chemicals: A
    molecular approach. New York, Marcel Dekker.

    Durham SR, Graneek BJ, Hawkins R, & Newman Taylor AJ (1987) The
    temporal relationship between increases in airway responsiveness to
    histamine and late asthmatic responses induced by occupational agents.
    J Allergy Clin Immunol, 79: 398-406.

    Durie FH, Fava RA, & Noelle RJ (1994) Collagen-induced arthritis as a
    model of rheumatoid arthritis. Clin Immunol Immunopathol, 
    73(1): 11-18.

    Dvoracek JE, Yunginger JW, Kern EB, Hyatt RE, & Gleich GJ (1984)
    Induction of nasal late-phase reaction by insufflation of ragweed
    pollen extract. J Allergy Clin Immunol, 73: 363-368.

    EAACI (European Academy of Allergology and Clinical Immunology) (1992)
    Subcommittee on Occupational Allergy -- Guidelines for the diagnosis
    of occupational asthma. Clin Exp Allergy, 22(1): 103-108.

    Edman B (1988) The usefulness of detailed information to patients with
    contact allergy. Contact Dermatitis, 19: 43-48.

    Edwards J, Walters S, & Griffiths RK (1994) Hospital admissions for
    asthma in preschool children: Relationship to major roads in
    Birmingham, United Kingdom. Arch Environ Health, 49: 223-227.

    Edworthy SM, Martin L, Barr SG, Birdsell DC, Brant RF, & Fritzler MJ
    (1998) A clinical study of the relationship between silicone breast
    implants and connective tissue disease. J Rheumatol, 25(2): 254-260.

    Ehlers A, Stangier U, & Gieler U (1995) Treatment of atopic
    dermatitis: A comparison of psychological and dermatological
    approaches to relapse prevention. J Consult Clin Psychol, 63(4):
    624-635.

    Eidson M, Philen RM, Sewell CM, Voorhees R, & Kilbourne EM (1990)
    L-tryptophan and eosinophilia-myalgia syndrome in New Mexico. Lancet;
    335(8690): 645-648.

    Elsayed S & Apold J (1977) Allergenic structure of allergen M from
    cod: II. Allergenicity of the limited tryptic hydrolysis peptides of
    fragment TM2. Int Arch Allergy Appl Immunol, 54(2): 171-175.

    Emberlin J (1994) The effects of patterns in climate and pollen
    abundance on allergy. Allergy, 49: 15-20.

    Enders F, Przybilla B, Ring J, Burg G, & Braun-Falco O (1988)
    [Epicutaneous testing with standard batteries of allergens: Results
    from 12,026 patients.] Hautarzt, 39: 779-786 (in German).

    Enk AH & Katz SI (1995) Contact sensitivity as a model for T cell
    activation in skin. J Invest Dermatol, 105: 80S-83S.

    Epstein WL, Kligman AM, & Senecal IP (1963) Role of regional lymph
    nodes in contact sensitization. Arch Dermatol, 88: 789-792.

    Eriksson NE, Formgren H, & Svenonius E (1982) Food hypersensitivity in
    patients with pollen allergy in Sweden. Allergy, 37(3): 437-443.

    Ernst P, Demissie K, Joseph L, Locher U, & Becklake MR (1995)
    Socioeconomic status and indicators of asthma in children. Am J Respir
    Crit Care Med, 152(2): 570-575.

    Estlander T & Jolanki R (1988) How to protect the hands. Dermatol
    Clin, 6: 105-114.

    Estlander T, Keskinen H, Jolanki R, & Kanerva L (1992) Occupational
    dermatitis from exposure to polyurethane chemicals. Contact
    Dermatitis, 27(3): 161-165.

    Estlander T, Kanerva L, Tupasela O, Keskinen H, & Jolanki R (1993)
    Immediate and delayed allergy to nickel with contact urticaria,
    rhinitis, asthma and contact dermatitis. Clin Exp Allergy, 23:
    306-310.

    ETFAD (1993) Severity scoring of atopic dermatitis: the SCORAD index.
    Consensus Report of the European Task Force on Atopic Dermatitis.
    Dermatology, 186: 23-31.

    Council of the European Communities (1976) Council directive of 27
    July 1976 on the approximation of the laws of the Member States
    relating to cosmetic products (76/768/EEC). Off J Eur Communities,
    L-262: 169-183.

    Council of the European Communities (1994) European Parliament and
    Council directive 94/27/EC of 30 June 1994: Nickel. Off J Eur
    Communities, L 18811-2.

    Evans R (1993) Epidemiology and natural history of asthma, allergic
    rhinitis and atopic dermatitis (eczema). In: Middleton E, Reed Cellis
    EF, Adkinson NF, Junginger JW, & Busse WW ed. Allergy: Principles and
    practice. St. Louis, Missouri, C.V. Mosby & Co., pp 1109-1136.

    Evans RG, Webb K, Homan S, & Ayres SM (1988a) Cross-sectional and
    longitudinal changes in pulmonary function associated with automobile
    pollution among bridge and tunnel officers. Am J Ind Med, 14: 25-36.

    Evans RG, Webb KB, Knutsen AP, Roodman ST, Roberts DW, Bagby JR,
    Garrett WA, & Andrews JS (1988b) A medical follow-up of the health
    effects of long-term exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin.
    Arch Environ Health, 43: 273-278.

    Eynon BE & Parker DC (1992) Small B cells as antigen-presenting cells
    in the induction of tolerance to soluble protein antigens. J Exp Med,
    175: 131-138.

    Fabbri LM, Boschetto P, Zocca E, Melani G, Pivirotto F, & Plebani M
    (1987) Bronchoalveolar neutrophilia during late asthmatic reactions
    induced by toluene di-isocyanate. Am Rev Respir Dis, 136: 36-42.

    Fairchild RL, Palmer E, & Moorhead JW (1993) Production of
    DNP-specific/Class I MHC-restricted suppressor molecules is linked to
    the expression of T cell receptor alpha- and beta-chain genes. J
    Immunol, 150(1): 67-77.

    Farine J-C (1997) Animal models in autoimmune disease in
    immunotoxicity assessment. Toxicology, 119: 29-35.

    Fauci A, Schnittman SM, & Poli G (1991) Immunopathologenic mechanisms
    in human immunodeficiency virus (HIV) infection. Ann Intern Med, 114:
    678-693.

    Fawcett IW, Newman Taylor AJ, & Pepys J (1977) Asthma due to inhaled
    chemical agents - epoxy resin systems containing phthalic anhydride,
    trimellitic anhydride and triethylene tetramine. Clin Allergy, 7:
    1-14.

    Fireman E, Ben Efraim S, Grief J, Alguetti A, Ayalon D, & Topilsky M
    (1988) Correlation between prostaglandin E2 production and suppressor
    activity of alveolar macrophages from patients with interstitial lung
    diseases. Immunol Lett, 18(2): 159-165.

    Flatt A, Pearce N, Thomson CD, Sears MR, Robinson MF, & Beasley R
    (1990) Reduced selenium in asthmatic subjects in New Zealand. Thorax,
    45: 95-99.

    Fleming DM & Crombie DL (1987) Prevalence of asthma and hay fever in
    England and Wales. Br Med J Clin Res Ed, 294(6567): 279-283.

    Flens MJ, Zaman GJR, Van der Valk P, Izquierdo MA, Schroeijers AB,
    Scheffer GL, van der Groep P, De Haas M, Meijer CJLM, & Scheper RJ
    (1996) Tissue distribution of the multidrug resistance protein. Am J
    Pathol, 148: 1237-1247.

    Flindt-Hansen H & Isager H (1987) Scleroderma after occupational
    exposure to trichloroethylene and trichloroethane. Acta Dermatol
    Venereol, 67: 263-264.

    Flynn GL (1990) Physicochemical determinants of skin absorption. In:
    Gerrity TR & Henry CJ ed. Principles of route-to-route extrapolation
    for risk assessment. Amsterdam, Oxford, New York, Elsevier Science
    Publishers, pp 93-127.

    Flyvholm M-A (1997) Formaldehyde exposure at the workplace and in the
    environment. Allergologie, 20(5): 225-231.

    Flyvholm M-A & Menné T (1992) Allergic contact dermatitis from
    formaldehyde: A case study focusing on sources of formaldehyde
    exposure. Contact Dermatitis, 27: 27-37.

    Flyvholm M-A, Menné T, & Maibach HI (1996) Skin allergy: Exposures and
    dose-response relationships. In: Vos JG, Younes M, & Smith E ed.
    Allergic hypersensitization induced by chemicals: Recommendations for
    prevention. Boca Raton, Florida, CRC Press, pp 261-285.

    Flyvholm M-A, Hall BM, Agner T, Tiedemann E, Greenhill P, Vanderveken
    W, Freeberg FE, & Menné T (1997) Threshold for occluded formaldehyde
    patch test in formaldehyde-sensitive patients. Contact Dermatitis, 36:
    26-33.

    Foerster J (1993) Autoimmune hemolytic anemias. In: Lee GR, Bithell
    TC, Foerster J, Athens JW, & Lukens JN ed. Wintrobe's clinical
    haematology, 9th ed. London, Lea and Febiger, pp 1170-1196.

    Folinsbee LJ (1992) Human health effects of air pollution. Environ.
    Health Perspect, 100: 45-56.

    Forastiere F, Corbo GM, Michelozzi P, Pistelli R, Agabiti N, Brancato
    G, Ciappi G, & Perucci CA (1992) Effects of environment and passive
    smoking on the respiratory health of children. Int J Epidemiol, 21(1):
    66-73.

    Forster H, Topping M, & Newman Taylor AJ (1988) Specific IgG and IgG4
    antibody to tetrachlorophthalic anhydride. Allergy Proc, 9: 296.

    Forsyth JS, Ogston SA, Clark A, Florey CD, & Howie PW (1993) Relation
    between early introduction of solid food to infants and their weight
    and illnesses during the first two years of life. Br Med J, 306(6892):
    1572-1576.

    Foucard T (1991) Allergy and allergy-like symptoms in 1,050 medical
    students. Allergy, 46: 20-26.

    Fowler JF, Skinner SM, & Belsito DV (1992) Allergic contact dermatitis
    from formaldehyde resins in permanent press clothing: An
    underdiagnosed cause of generalized dermatitis. J Am Acad Dermatol,
    27: 962-968.

    Freier S, Eran M, & Goldstein R (1985) The effect of immediate type
    gastrointestinal allergic reactions on brush border enzymes and gut
    morphology in the rat. Pediatr Res, 19: 456-459.

    Freiman DE & Hardy HL (1970) Beryllium disease: The relation of
    pulmonary pathology to clinical course and prognosis based on a study
    of 130 cases from the US Beryllium Case Registry. Hum Pathol, 
    1: 25-44.

    Friedmann PS (1990) The immunology of allergic contact dermatitis: The
    DNCB story. Adv Dermatol, 5: 175-196.

    Friedmann PS, Moss C, Shuster S, & Simpson JM (1983) Quantitative
    relationships between sensitizing dose of DNCB and reactivity in
    normal subjects. Clin Exp Immunol, 53: 706-710.

    Friedman SM, Posnett DN, Tumang JR, Cole BC, & Crow K (1991) A
    potential role for microbial superantigens in the pathogenesis of
    systemic autoimmune disease. Arthritis Rheum, 34: 468-480.

    Fudenberg HH (1966) Immunological deficiency, autoimmune disease, and
    lymphoma: observations, implications, and speculations. Arthritis
    Rheum, 9: 464-472.

    Fujimaki H, Nohara O, Ichinose T, Watanabe N, & Saito S (1994) IL-4
    production in mediastinal lymph node cells in mice intratracheally
    instilled with diesel exhaust particulates and antigen. Toxicology,
    92: 261-268.

    Fuller KA, Pearl D, & Whitacre CC (1990) Oral tolerance in
    experimental autoimmune encephalomyelitis: Serum and salivary antibody
    responses. J Neuroimmunol, 28: 14-26.

    Fye KH & Sack KE (1991) Rheumatic diseases. In: Stites DP & Terr AI
    ed. Basic and clinical immunology, 7th ed. Norwalk, San Mateo,
    Appleton & Lange, pp 438-463.

    Gabriel SE, O`Fallon WM, Kurland LT, Berad CM, Woods JE, & Melton LJ
    (1994) Risk of connective-tissue diseases and other disorders after
    breast implantation. N Engl J Med, 330(24): 1697-1702.

    Gad SC, Dunn BJ, Dobbs DW, Reilly C, & Walsh RD (1986) Development and
    validation of an alternative dermal sensitization test : The mouse ear
    swelling test (MEST). Toxicol Appl Pharmacol, 84: 93-114.

    Gamble J, Jones W, & Minshall S (1987) Epidemiological-environmental
    study of diesel bus garage workers: chronic effects of diesel exhaust
    on the respiratory system. Environ Res, 44: 6-17.

    Gammelgaard B, Fullerton F, Avnstorp C, & Menné T (1992) Permeation of
    chromium salts through human skin  in vitro. Contact Dermatitis, 27:
    302-310.

    Gardner MLG (1988) Gastrointestinal absorption of intact proteins.
    Annu Rev Nutr, 8: 329-350.

    Garssen J, Nijkamp FP, Van der Vliet H, & Van Loveren H (1991) T-cell
    mediated induction of airway hyperreactivity in mice. Am Rev Resp Dis,
    144: 931-938.

    Garssen J, Nijkamp FP, Van de Vliet H, & Van Loveren H (1994) A role
    for cellular immunity in the induction of airway hyperresposiveness
    induced by small molecular weight compounds. Toxicol Lett, 
    72: 151-154.

    Gauggel DL, Sarlo K, & Asquith TN (1993) A proposed screen for
    evaluating low-molecular-weight chemicals as potential respiratory
    allergens. J Appl Toxicol, 13(5): 307-313. 

    Gautam SC, Chikkala NF, & Battisto JR (1985) Orally induced tolerance
    generates an efferently acting suppressor T cell and an acceptor T
    cell that together downregulate contact sensitivity. J Immunol, 135:
    2975-2983.

    Geier J & Schnuch A (1996) No cross-sensitization between MCI/MI,
    benzisothiazolinone and octylisothiazolinone. Contact Dermatitis,
    34(2): 148-149.

    Gell PGH & Coombs RRA (1963) Clinical aspects of immunology. Oxford,
    London, Boston, Blackwell Scientific Publications.

    Gell PGH, Coombs RRA, & Lachman R (1975) Clinical aspects of
    immunology, 3rd ed. Oxford, London, Boston, Blackwell Scientific
    Publications.

    Gergely J & Sarmay G (1996) Fc-gamma-RII-mediated regulation of human
    B cells. Scand J Immunol, 44: 1-10.

    Gergen PJ, Mullally DI, & Evans R (1988) National survey of prevalence
    of asthma among children in the United States, 1976 to 1980.
    Pediatrics, 81: 1-7.

    Gerling I, Serreze DV, Christianson S, & Leiter E (1992) Intrathymic
    islet cell transplantation reduces beta-cell autoimmunity and prevents
    diabetes in NOD/Lt mice. Diabetes, 41: 1672-1676.

    Gilliam JN & Sontheimer RD (1982) Skin manifestations of SLE. Clin
    Rheum Dis, 8: 207-218.

    Glazier A, Tutschka PJ, Farmer ER, & Santos GW (1983)
    Graft-versus-host disease in cyclosporine A-treated rats after
    syngeneic and autologous bone marrow reconstitution. J Exp Med, 158:
    1-8.

    Gleich GJ (1990) The eosinophil and bronchial asthma: Current
    understanding. J Allergy Clin Immunol, 85(2): 422-436.

    Gleichmann H (1981) Studies on the mechanism of drug sensitization:
    T-cell-dependent popliteal lymph node reaction to diphenylhydantoin.
    Clin Immunol Immunopathol, 18: 203-211.

    Gleichmann HIK, Pals ST, & Radaszkiewicz T (1983) T-cell-dependent
    B-cell proliferation and activation induced by administration of the
    drug diphenylhydantoin to mice. Hematol Oncol, 1: 165-176.

    Gleichmann E, Pals ST, Rolink AG, Radaszkiewicz T, & Gleichmann H
    (1984) Graft-versus-host reactions: clues to the etiopathology of a
    spectrum of immunological diseases. Immunol Today, 5: 324-332.

    Gleichmann E, Vohr H-W, Stringer C, Nuyens J, & Gleichmann H (1989)
    Testing the sensitization of T cells to chemicals: From murine
    graft-versus-host (GVH) reactions to chemical-induced GVH-like
    immunological diseases. In: Kammüller ME, Bloksma N, & Seinen W ed.
    Autoimmunity and toxicology. Amsterdam, Oxford, New York, Elsevier
    Science Publishers, pp 363-390.

    Goettsch W, Garssen J, De Gruijl FR, & Van Loveren H (1993) UV-B and
    the immune system: a review with special emphasis on T cell-mediated
    immunity. Thymus, 21: 93-114.

    Goh C (1995) Non eczematous contact reactions. In: Rycroft RJG, Menné
    T, Frosch PJ, & Benezra C ed. Textbook of contact dermatitis, 2nd ed.
    Berlin, Heidelberg, New York, Springer Verlag, pp 221-233.

    Goldman M, Druet P, & Gleichmann E (1991) TH2 cells in systemic
    autoimmunity: insights from allogeneic diseases and chemically-induced
    autoimmunity. Immunol Today, 12: 223-227.

    Gorski P & Tarkowski M (1992) Non specific environmental factors and
    asthma development. Pol J Occup Med Environ Health, 5: 227-236.

    Goust JM (1993) Immediate hypersensitivity. Immunol Ser, 58: 343-359.

    Graham C, Gealy R, Macina OT, Karol MH, & Rosenkranz HS (1996) QSAR
    for allergic contact dermatitis. Quant Struct-Activ Relat, 15: 1-7.

    Graham C, Rosenkranz HS, & Karol MH (1997) Structure-activity model of
    chemicals that cause human respiratory sensitization. Regul Toxicol
    Pharmacol, 26: 296-306.

    Granato DA & Piguet PF (1986) A mouse monoclonal IgE antibody anti
    bovine milk beta-lactoglobulin allows studies of allergy in the
    gastrointestinal tract. Clin Exp Immunol, 63(3): 703-710.

    Graycar JL, Miller DA, Arrick BA, Lyons RM, Moses HL, & Derynck R
    (1989) Human transforming growth factor-beta-3: recombinant 
    expression, purification, and biologic activities in comparison with 
    transforming growth factors beta 1 and beta 2. Mol Endocrinol,
    3: 1977-1986.

    Greaves IA, Wegman DH, Smith TJ, & Spiegelman DL (1984) Respiratory
    effects of two types of solder flux used in the electronics industry.
    J Occup Med, 26(2): 81-85.

    Greenland S (1993) Basic problems in interaction assessment. Environ
    Health Perspect, 101(suppl): 59-66.

    Gryboski JD (1967) Gastrointestinal milk allergy in infants.
    Pediatrics, 40: 354-362.

    Guillet GG & Guillet MH (1992) Natural history of sensitizations in
    atopic dermatitis. Arch Dermatol, 128: 187-192.

    Gulbenkian AR, Egan RW, Fernandez X, Jones H, Kreutner W, Kung T,
    Payvandi F, Sullivan L, Zurcher JA, & Watnik AS (1992) Interleukin-5
    modulates eosinophil accumulation in allergic guinea pig lung. Am Rev
    Respir Dis, 146: 263-266.

    Gut J, Christen U, Frey N, Koch V, & Stoffler D (1995) Molecular
    mimicry in halothane hepatitis: biochemical and structural
    characterization of lipoylated autoantigens. Toxicology, 97: 199-224.

    Haahtela T, Lindholm H, Bjorksten F, Koskenvuo K, & Laitinen LA (1990)
    Prevalence of asthma in Finnish young men. Br Med J, 301: 266-268.

    Hackney JD, Linn WS, & Avol EL (1989) Acid fog: Effects on respiratory
    function and symptoms in healthy and asthmatic volunteers. Environ
    Health Perspect, 79: 159-162.

    Haddad ZH, Kalra V, & Verma S (1979) IgE antibodies to peptic and
    peptic-tryptic digests of beta-lactoglobulin: significance in food
    hypersensitivity. Ann Allergy, 42: 368-371.

    Halken S, Host A, Hansen LG, & Osterballe O (1992) Effect of an
    allergy prevention programme on incidence of atopic symptoms in
    infancy. Allergy, 47: 545-553.

    Halken S, Host A, Nilsson L, & Taudorf E (1995) Passive smoking as a
    risk factor for development of obstructive respiratory disease and
    allergic sensitization. Allergy, 50(2): 97-105.

    Hallas TE, Yi X, & Schou C (1993) Does guanine concentration in
    house-dust samples reflect house-dust mite exposure? Allergy, 48(5):
    303-305.

    Hananan D (1990) Transgenic mouse models of self-tolerance and
    autoreactivity by the immune system. Annu Rev Cell Biol, 6: 493-537.

    Hang L, Aguado MT, Dixon FJ, & Theofilopoulos AN (1985) Induction of
    severe autoimmune disease in normal mice by simultaneous action of
    multiple immunostimulators. J Exp Med, 161: 423-428.

    Hanifin JM (1983) Atopic dermatitis and other endogenous eczemas.
    Semin Dermatol, 2: 1-44.

    Hanifin JM (1987) Epidemiology of atopic dermatitis. Monogr Allergy,
    21: 116-131.

    Hanifin JM (1993) Atopic dermatitis. In: Middleton E, Reed Cellis EF,
    Adkinson NF, Junginger JW, & Busse WW ed. Allergy: Principles and
    practice. St. Louis, Missouri, C.V. Mosby & Co., pp 1581-1604.

    Hanifin JM & Chan SC (1995) Monocyte phosphodiesterase abnormalities
    and dysregulation of lymphocyte function in atopic dermatitis. J
    Invest Dermatol, 105(suppl 1): 84S-88S.

    Hanifin JM & Lobitz WC Jr (1977) Newer concepts of atopic dermatitis.
    Arch Dermatol, 113(5): 663-670.

    Hanifin JM & Rajka G (1980) Diagnostic features of atopic dermatitis.
    Acta Dermatol Venereol, 92(suppl): 44-47.

    Hanson DG, Roy MJ, Green GM, & Miller SD (1993) Inhibition of
    orally-induced immune tolerance in mice by prefeeding an endopeptidase
    inhibitor. Region Immunol, 5: 76-84.

    Hariya T, Ikezawa Z, Aihara M, Kitamura K, Osawa J, & Nakajima H
    (1994) Allergenicity and tolerogenicity of amlexanox in the guinea
    pig. Contact Dermatitis, 31: 31-36.

    Hasselmark L, Malmgren R, Zetterstrom O, & Unge G (1993) Selenium
    supplementation in intrinsic asthma. Allergy, 48(1): 30-36.

    Hatch M & Thomas D (1993) Measurement issues in environmental
    epidemiology. Environ Health Perspect, 101(Suppl 4): 49-57.

    Hattevig G, Kjellman B, Sigurs N, Bjorksten B, & Kjellman NI (1989)
    Effect of maternal avoidance of eggs, cow's milk and fish during
    lactation upon allergic manifestations in infants. Clin Exp Allergy,
    19(1): 27-32.

    Heine J, Ignotz RA, Hemler ME, Crouse C, & Massague J (1989)
    Regulation of cell adhesion receptors by transforming growth 
    factor-beta. Concomitant regulation of integrins that share a common 
    beta-1 subunit. J Biol Chem, 264: 380-388.

    Heiner DC, Sears JW, & Kniker WT (1962) Multiple precipitins to cow's
    milk chronic respiratory disease: A syndrome including poor growth,
    gastrointestinal symptoms, evidence of allergy, iron deficiency anemia
    and pulmonary hemosiderosis. Am J Dis Child, 103: 634-651.

    Henderson CR & Riley EC (1945) Certain statistical considerations in
    patch testing. J Invest Dermatol, 6: 227-232.

    Hennekens LIM & Buring JE (1987) Epidemiology in medicine. Boston,
    Toronto, Little, Brown & Co., 400 pp.

    Hennekens LIM, Cook NR, Hebert PR, Karlson EW, LaMotte F, Manson JE, &
    Buring JE (1996) Self-reported breast implants and connective tissue
    diseases in female health professionals. J Am Med Assoc, 275: 616-621.

    Henschler D, Assman W, & Meyer K (1962) [The toxicology of toluene
    diisocyanate.] Arch Toxicol, 19: 364-387 (in German).

    Heppel LMJ & Kilshaw PJ (1982) Immune responses of guinea pigs to
    dietary protein: I. Induction of tolerance by feeding with ovalbumin.
    Int Arch Allergy Appl Immunol, 68: 54-59.

    Herbert CA, King CM, Ring PC, Holgate ST, Stewart GA, Thompson PJ, &
    Robinson C (1995) Augmentation of permeability in the bronchial
    epithelium by the house dust mite allergen Der p 1. Am J Respir Cell
    Mol Biol, 12: 369-378.

    Hertzman PA, Blevins WL, Mayer J, Greenfield B, Ting M, & Gleich GJ
    (1990) Association of the eosinophilia-myalgia syndrome with the
    ingestion of tryptophan. N Eng J Med, 322(130): 869-873.

    Hess AD & Fischer AC (1989) Immune mechanisms in cyclosporine-induced
    syngeneic graft-versus- host disease. Transplantation, 48: 895-900.

    Heufler C, Koch F, & Schuler G (1988) Granulocyte/macrophage
    colony-stimulating factor and interleukin-1 mediate the maturation of
    murine epidermal Langerhans cells into potent immunostimulatory
    dendritic cells. J Exp Med, 167: 700-705.

    Hewitt CRA, Brown AP, Hart BJ, & Pritchard DI (1995) A major house
    dust mite allergen disrupts the immunoglobulin E network by
    selectively cleaving CD23: innate protection by antiproteases. J Exp
    Med, 182: 1537-1544.

    Hide DW, Matthews S, Matthews L, Stevens M, Ridout S, Twiselton R,
    Gant C, & Arshad SH (1994) Effect of allergen avoidance in infancy on
    allergic manifestations at age two years. J Allergy Clin Immunol,
    93(5): 842-846.

    Hill DJ & Hosking CS (1997) Emerging disease profiles in infants and
    young children with food allergy. Pediatr Allergy Immunol, 8(suppl
    10): 21-26.

    Hill LW & Sulzberger MB (1935) Evolution of atopic dermatitis. Arch
    Dermatol Syph, 32: 451.

    Hjorth N & Menné T (1990) Prevention of allergic contact
    sensitization: a historical perspective. In: Menné T & Maibach HI ed.
    Exogenous dermatoses. Boca Raton, Florida, CRC Press, chapter 30.

    Hjorth N & Roed-Petersen J (1976) Occupational protein contact
    dermatitis in food handlers. Contact Dermatitis, 2(1): 28-42.

    Hochberg MC, Perlmutter DL, Medsger TA Jr, Nguyen K, Steen V, Weisman
    MH, White B, & Wigley FM (1996) Lack of association between
    augmentation mammoplasty and systemic sclerosis (scleroderma).
    Arthritis Rheum, 39: 1125-1131.

    Hodge L, Salome CM, Peat JK, Haby MM, Xuan W, & Woolcock AJ (1996)
    Consumption of oily fish and childhood asthma risk. Med J Aust,
    164(3): 137-140.

    Holgate ST, Beasley R, & Twentyman OP (1987) The pathogenesis and
    significance of bronchial hyperresponsiveness in airways disease. Clin
    Sci, 73(6): 561-572.

    Hollady SD & Luster MI (1996) Alterations in fetal thymic and liver
    hematopoietic cells as indicators of exposure to developmental
    immunotoxicants. Environ Health Perspect, 104(suppl 4): 809-813.

    Holt PG (1994) A potential vaccine strategy for asthma and allied
    atopic diseases during early childhood. Lancet, 344: 456-458.

    Holt PG (1996) Infections and the development of allergy. Toxicol
    Lett, 86: 205-210.

    Holt PG & Sedgwick JD (1987) Suppression of IgE responses following
    antigen inhalation: A natural homeostatic mechanism which limits
    sensitization to aeroallergens. Immunol Today, 60(1): 97-102.

    Holt PG, Schon-Hegrad MA, & McMenamin PG (1990) Dendritic cells in the
    respiratory tract. Int. Rev Immunol, 6(2-3): 139-149.

    Hopkins JM (1997) Genetics of atopy. In: Kay AB ed. Allergy and
    allergic diseases. Oxford, London, Boston, Blackwell Scientific
    Publications, pp 1187-1195.

    Host A & Halken S (1990) A prospective study of cow milk allergy in
    Danish infants during the first 3 years of life. Allergy, 45: 587.

    Host A, Husby S, & Osterballe O (1988) A prospective study of cow's
    milk allergy in exclusively breast-fed infants: Incidence,
    pathogenetic role of inadvertent exposure to cow's milk formula, and
    characterisation of bovine milk protein in human milk. Acta Paediatr
    Scand, 77(5): 663-670.

    Hostynek JJ, Magee PS, & Maibach HI (1996) QSAR predictive of contact
    allergy: scope and limitations. Curr Probl Dermatol, 14: 59-106.

    Howe W, Venables KM, Topping MD, Dally MB, Hawkins R, Law SJ, & Newman
    Taylor AJ (1983) Tetrachlorophthalic anhydride asthma: evidence for
    specific IgE antibody. J Allergy Clin Immunol, 71: 5-11.

    HSE (1996) Good health is good business: Employers guide
    -- Work-related dermatitis. London, United Kingdom, Health and Safety
    Executive, 3 pp.

    Hsieh KH & Shen JJ (1988) Prevalence of childhood asthma in Taipei,
    Taiwan, and other Asian Pacific countries. J Asthma, 25(2): 73-82.

    Hunter D, Milton R, & Perry KMA (1945) Asthma caused by the complex
    salts of platinum. Br J Ind Med, 2: 92-98.

    Hurtenbach U, Gleichmann H, Nagata N, & Gleichmann E (1987) Immunity
    to D-penicillamine: genetic, cellular, and chemical requirements for
    induction of popliteal lymph node enlargement in the mouse. J Immunol,
    139: 411-416.

    IARC (1993) Beryllium, cadmium, mercury, and exposures in the glass
    industry. Lyon, International Agency for Research on Cancer, pp 88-89
    (IARC Monographs on the Evaluation of Carcinogenic Risks to Humans,
    Volume 58).

    Ignotz RA & Massague J (1986) Transforming growth factor-beta 
    stimulates the expression of fibronectin and collagen and their 
    incorporation into the extracellular matrix. J Biol Chem, 261(9): 
    4337-4345.

    Imbus HR (1985) Clinical evaluation of patients with complaints
    related to formaldehyde exposure. J Allergy Clin Immunol, 76(6):
    831-840.

    International Rhinitis Management Working Group (1994) International
    consensus report on the diagnosis and management of rhinitis. Allergy,
    49(suppl 19): 1-34.

    ISAAC (International Study of Asthma and Allergies in Childhood)
    (1998) ISAAC Steering Committee -- Worldwide variation in the
    prevalence of symptoms of asthma, allergic rhinoconjunctivitis, and
    atopic eczema. Lancet, 351: 1225-1232.

    IPCS (1996) Environmental health criteria 180: Principles and methods
    for assessing direct immunotoxicity associated with exposure to
    chemicals. Geneva, World Health Organization, International Programme
    on Chemical Safety, 390 pp.

    Irani AA, Schechter NM, Craig SS, DeBlois G, & Schwartz LB (1986) Two
    types of human mast cells that have distinct neutral protease
    compositions. Proc Natl Acad Sci (USA), 83: 4464-4468.

    Irvine C, Pugh CE, Hansen EJ, & Rycroft RJG (1994) Cement dermatitis
    in underground workers during construction of the Channel Tunnel.
    Occup Med, 44: 17-23.

    Ishizaka K & Ishizaka T (1966) Physicochemical properties of reginic
    antibody: 1. Association of reaginic activity with an immunoglobulin
    other than gammaA- or gammaG-globulin. J Allergy, 37(3): 169-185.

    Ishizaki T, Koizumi K, Ikemori R, Ishiyama Y, & Kushibiki E (1987)
    Studies of prevalence of Japanese cedar pollinosis among the residents
    in a densely cultivated area. Ann Allergy, 58: 265-270.

    Iverson GM (1970) Ability of CBA mice to produce anti-idiotypic sera
    to 5563 myeloma protein. Nature (Lond), 227: 273-274.

    Iwami T, Nagai H, Tsuruoka N, & Koda A (1993) Effect of murine
    recombinant interleukin-5 on bronchial reactivity in guinea-pigs. Clin
    Exp Allergy, 23(1): 32-38. 

    Izquierdo MA, Scheffer GL, Flens MJ, Giaccone G, Broxterman HJ, Van
    der Valk P, Meijer CJLM, & Scheper RJ (1996) Broad distribution of the
    multidrug resistance -- related Vault Lung Resistance Protein in
    normal human tissues and tumors. Am J Pathol, 148: 877-887.

    Jacobs MC, White IR, Rycroft RJG, & Taub N (1995) Patch testing with
    preservatives at St. John's from 1982 to 1993. Contact Dermatitis, 33:
    247-255.

    Jakob T, Hermann K, & Ring J (1991) Eosinophil cationic protein in
    atopic eczema. Arch Dermatol Res, 283: 5-6. 

    Jakobsson I & Lindberg T (1979) A prospective study of cow's milk
    protein intolerance in Swedish infants. Acta Pædiatr Scand, 68:
    853-859.

    Janeway CA, Travers P, Hunt S, & Walport M (1997) Immunobiology: The
    immune system in health and disease. Edinburgh, Churchill Livingstone,
    400 pp.

    Jarrett EEE (1984) Immunoregulation of IgE responses: the role of the
    gut in perspective. Ann Allergy, 53: 550-556.

    Jarrett EEE & Hall E (1984) The development of IgE suppressive
    immunocompetence in young animals: influence of exposure to antigen in
    the presence or absence of maternal immunity. Immunology, 53: 365-373.

    Jarrett EEE, Haig DM, McDougall W, & McNulty E (1976) Rat IgE
    production: II. Primary and booster reaginic antibody responses
    following intradermal or oral immunization. Immunology, 30: 671-677.

    Jarvis J, Agius R, & Sawyer L (1996) Odds on for asthma. Chem Br, 32:
    51-53.

    Jenkins MK, Schwartz RH, & Pardoll DM (1988) Effects of cyclosporine A
    on T-cell development and clonal deletion. Science, 24: 1655-1658.

    Jiang X, Khursigara G, & Rubin RL (1994) Transformation of
    lupus-inducing drugs to cytotoxic products by activated neutrophils.
    Science, 266: 810-813.

    Johansen J & Menné T (1995) The fragrance mix and its constituents: a
    14-year material. Contact Dermatitis, 32: 18-23.

    Johansen J, Rastogi SC, & Menné T (1996a) Exposure to selected
    fragrance materials. A case study of fragrance-mix-positive eczema
    patients. Contact Dermatitis, 34: 106-110.

    Johansen JD, Andersen KE, & Menné T (1996b) Quantitative aspects of
    isoeugenol contact allergy assessed by use and patch tests. Contact
    Dermatitis, 34: 414-418.

    Johansen JD, Rastogi SC, Andersen KE, & Menné T (1997) Content and
    reactivity to product perfumes in fragrance mix positive and negative
    eczema patients: A study of perfumes used in toiletries and skin-care
    products. Contact Dermatitis, 36(6): 291-296.

    Johansson SGO & Juhlin L (1970) Immunoglobulin-E in "healed" atopic
    dermatitis and after treatment with corticosteroids and azathioprine.
    Br J Dermatol, 82: 10-13.

    Johansson SGO, Dannaeus A, & Lilja G (1984) The relevance of anti-food
    antibodies for the diagnosis of food allergy. Ann Allergy, 53: 665.

    Johnston SL, Pattemore PK, Sanderson G, Smith S, Lampe F, Josephs L,
    Symington P, O`Toole S, Myint SH, & Tyrrell DAJ (1995) Community study
    of role of viral infections in exacerbations of asthma in 9-11 year
    old children. Br Med J, 310: 1225-1229.

    Jones RJ, Vogelsang GB, Hess AD, Farmer ER, Mann RB, Geller RB,
    Piantadosi S, & Santos GW (1989) Induction of graft-versus-host
    disease after autologous bone marrow transplantation. Lancet, 1:
    754-757.

    Jones LA, Chin LT, Longo DL, & Kruisbeek AM (1990) Peripheral clonal
    elimination of functional T cells. Science, 250: 1726-1729.

    Jones RN, Rando RJ, Glindmeyer HW, Foster TA, Hughes JM, O'Neil CE, &
    Weill H (1992) Abnormal lung function in polyurethane foam producers:
    Weak relationship to toluene diisocyanate exposures. Am Rev Respir
    Dis, 146(4): 871-877.

    Jones DB, Coulson AFW, & Duff GW (1993) Sequence homologies between
    hsp60 and autoantigens. Immunol Today, 14: 115-118.

    Jones Williams W (1988) Beryllium disease. Postgrad Med J, 64:
    511-516.

    Jones Williams W & Wallach ER (1989) Laser probe mass spectrometry
    (LAMMS) analysis of beryllium, sarcoidosis and other granulomatous
    diseases. Sarcoidosis, 6: 111-117.

    Jordan JM & Whitlock FA (1972) Emotions and the skin: the conditioning
    of scratch responses in cases of atopic dermatitis. Br J Dermatol,
    86(6): 574-585.

    Jordan JM & Whitlock FA (1974) Atopic dermatitis, anxiety and
    conditioned scratch responses. J Psychosom Res, 18(5): 297-299.

    Jungers P, Dougados M, Pelissier C, Kutten F, Tron F, Lesavre P, &
    Bach JF (1982) Influence of oral contraceptive therapy on the activity
    of systemic lupus erythematosus. Arthritis Rheum, 25: 618-623.

    Juniper CP, How MJ, Goodwin BFJ, & Kinshott AK (1977)  Bacillus 
     subtilis enzymes: a seven year clinical, epidemiological and
    immunological study of an industrial allergen. J Soc Occup Med, 27:
    3-12.

    Kagnoff MF (1982) Oral tolerance. Ann NY Acad Sci, 392: 248-265.

    Kagnoff MF (1992) Celiac disease: A gastrointestinal disease with
    environmental, genetic, and immunologic components. Gastr Clin North
    Am, 21(2): 405-425.

    Kajosaari M & Saarinen UM (1983) Prophylaxis of atopic disease by six
    month's total solid food elimination. Acta Paediatr Scand, 72:
    411-414.

    Kammüller ME (1996) Drug-induced autoimmune disorders. Drug Inf J, 30:
    293-299.

    Kammüller ME & Seinen W (1988) Structural requirements for hydantoins
    and 2-thiohydantoins to induce lymphoproliferative popliteal lymph
    node reactions in the mouse. Int J Immunopharmacol, 10: 997-1010.

    Kammüller ME, Penninks AH, & Seinen W (1984) Spanish toxic oil
    syndrome is a chemically induced GVHD-like epidemic. Lancet, 1:
    1174-1175.

    Kammüller ME, Bloksma N, & Seinen W ed. (1989a) Autoimmunity and
    toxicology: Immune disregulation induced by drugs and chemicals.
    Amsterdam, Oxford, New York, Elsevier Science Publishers, pp 3-34.

    Kammüller ME, Thomas C, De Bakker JM, Bloksma N, & Seinen W (1989b)
    The popliteal lymph node assay in mice to screen for the immune
    disregulating potential of chemicals -- a preliminary study. Int J
    Immunopharmacol, 11: 293-300.

    Kaposi M (1895) Pathology and treatment of diseases of the skin, 4th
    ed. New York, William Wood.

    Kapp A (1995) [Atopic dermatitis: The skin manifestations.] Allergo J,
    4: 229-238 (in German).

    Kapsenberg ML, Wierenga, EA, Bos JD, & Jansen HM (1991) Functional
    subsets of allergen-reactive human CD4+ T cells. Immunol Today, 12:
    392-395.

    Karol MH (1983) Concentration-dependent immunologic response to
    toluene diisocyanate (TDI) following inhalation exposure. Toxicol Appl
    Pharmacol, 68: 229-241.

    Karol MH (1986) Respiratory effects of inhaled isocyanates. CRC Crit
    Rev Toxicol, 16: 349-380.

    Karol MH (1991) Allergic reactions to indoor air pollutants. Environ
    Health Perspect, 95: 45-51.

    Karol MH (1992) Occupational asthma and allergic reactions to inhaled
    compounds. In: Miller K, Turk J, & Nicklin S ed. Principles and
    practice of immunotoxicology. Oxford, London, Boston, Blackwell
    Scientific Publications, pp 228-241.

    Karol MH (1993) Concentration-dependent immunologic response to
    toluene diisocyanate (TDI) following inhalation exposure. Toxicol Appl
    Pharmacol, 68: 229.

    Karol MH (1994a) Occupational asthma pig predictive tests for
    respiratory allergy. In: Dean JH, Luster MI, Munson AE, & Kimber I ed.
    Immunotoxicology and immunopharmacology, 2nd ed. New York, Raven
    Press, pp 703-720.

    Karol MH (1994b) Animal models of occupational asthma. Eur Respir J,
    7(3): 555-568.

    Karol MH & Thorne PS (1988) Pulmonary hypersensitivity and
    hyperreactivity: implications for assessing allergic responses. In:
    Gardner DE, Crapo JD, & Massaro EJ ed. Toxicology of the lung. New
    York, Raven Press, p 427.

    Karol MH, Graham C, Gealy R, Macina OT, Sussman N, & Rosenkranz HS
    (1996) Structure- activity relationships and computer-assisted
    analysis of respiratory sensitization potential. Toxicol Lett, 86:
    187-191.

    Karpatkin S (1988) Immunological platelet disorders. In: Immunological
    diseases, 4th ed. Boston, Little Brown, vol 2, pp 1631-1662.

    Karpus WJ & Swanborg RH (1991) CD4+ suppressor cells inhibit the
    function of effector cells of experimental autoimmune
    encephalomyelitis through a mechanism involving transforming growth
    factor-beta. J Immunol, 146: 1163-1168.

    Katsutani N & Shionoya H (1992) Popliteal lymph node enlargement
    induced by procainamide. Int J Immunopharmacol, 14: 681-686.

    Kaufman LD, Gruber BL, & Gregersen PK (1991) Clinical follow-up and
    immunogenetic studies of 32 patients with eosinophilia-myalgia
    syndrome. Lancet, 337: 1071-1074. 

    Kaufman DL, Clare-Salzler M, Tian J, Forsthuber T, Ting GSP, Robinson
    P, Atkinson MA, Sercarz EE, Tobin AJ, & Lehmann PV (1993) Spontaneous
    loss of T-cell tolerance to glutamic acid decarboxylase in murine
    insulin-dependent diabetes. Nature (Lond), 366: 69-72.

    Kayser D & Schlede E (1995) [Chemicals and contact allergy -- a
    summary evaluation.] Berlin, Federal Institute for Consumer's Health
    Protection and Veterinary Medicine (BGVV) (in German).

    Keeley DJ, Neill P, & Gallivan S (1991) Comparison of the prevalence
    of reversible airways obstruction in rural and urban Zimbabwean
    children. Thorax, 46: 549-553.

    Keil U, Weiland SK, Duhme H, & Chambless L (1996) The international
    study of asthma and allergies in childhood (ISAAC): Objectives and
    methods -- Results from German ISAAC centres concerning traffic
    density and wheezing and allergic rhinitis. Toxicol Lett, 86: 99-103.

    Kemp T, Pearce N, Fitzharris P, Crane J, Fergusson D, St. George I,
    Wickens K, & Beasley R (1997) Is infant immunization a risk factor for
    childhood asthma or allergy? Epidemiology, 8: 678-680.

    Keskinen H, Alanko K, & Saarinen L (1978) Occupational asthma in
    Finland. Clin Allergy, 8(6): 569-579.

    Keskinen H, Tupasela O, Tiikkainen U, & Nordman H (1988) Experience of
    specific in IgE asthma due to diisocyanates. Clin Allergy, 18(6):
    597-604.

    Khoury SJ, Hancock WW, & Weiner HL (1992) Oral tolerance to myelin
    basic protein and natural recovery from experimental autoimmune
    encephalomyelitis are associated with downregulation of inflammatory
    cytokines and differential upregulation of transforming growth factor
    beta, interleukin 4 and prostaglandin E expression in the brain. J Exp
    Med, 176: 1355-1364.

    Kimber I & Basketter DA (1997) Contact sensitisation: a new approach
    to risk assessment. Hum Ecol Risk Assess, 3: 385-395.

    Kimber I & Cumberbatch M (1992a) Dendritic cells and cutaneous immune
    responses to chemical allergens. Toxicol Appl Pharmacol, 117: 137-146.

    Kimber I & Cumberbatch M (1992b) Stimulation of Langerhans cell
    migration by tumour necrosis factor alpha. J Invest Dermatol, 99:
    48S-50S.

    Kimber I & Maurer T ed. (1996) Toxicology of contact hypersensitivity.
    London, Taylor & Francis, 170 pp.

    Kimber I & Wilks MF (1995) Chemical respiratory allergy: Toxicological
    and occupational health issues. Hum Exp Toxicol, 14(9): 735-736.

    Kimber I, Hilton J, & Weisenberger C (1989) The murine local lymph
    node assay for identification of contact allergens: a preliminary
    evaluation of in situ measurement of lymphocyte proliferation. Contact
    Dermatitis, 21: 215.

    Kimber I, Kinnaird A, Peters SW, & Mitchell JA (1990) Correlation
    between lymphocyte proliferative responses and dendritic cell
    migration in regional lymph nodes following skin painting with contact
    sensitizing agents. Int Arch Allerg Appl Immunol, 93: 47-53.

    Kimber I, Dearman RJ, Scholes EW, & Basketter DA (1994) The local
    lymph node assay developments and applications. Toxicology, 93: 13-31.

    King TP, Hoffman D, Lowenstein H, Marsh DG, Platts-Mills TAE, & Thomas
    W (1995) Allergen nomenclature. J Allergy Clin Immunol, 96: 5-14.

    Kirton V (1978) Contact urticaria and cinnamic aldehyde. Contact
    Dermatitis, 4(6): 374-375.

    Kishimoto T & Hirano T (1984) beta-Lymphocyte activation, 
    proliferation, and immunoglobulin secretion. In: Paul WE ed. 
    Fundamental immunology, 2nd ed. New York, Raven Press, pp 385-411.

    Kitagaki H, Fujisawa S, Watanabe K, Hayakawa K, & Shiohara T (1995)
    Immediate type hypersensitivity response followed by a late reaction
    is induced by repeated epicutaneous application of contact sensitizing
    agents in mice. J Invest Dermatol, 105: 749-755.

    Kjellman NI & Croner S (1984) Cord blood IgE determination for allergy
    prediction -- a follow-up to seven years of age in 1,651 children. Ann
    Allergy, 53(2): 167-171.

    Kligman AM (1958a) Poison ivy (Rhus) dermatitis. Arch Dermatol, 77:
    159-180.

    Kligman AM (1958b) Hyposensitization against Rhus dermatitis. Arch
    Dermatol, 78: 47-72.

    Kligman AM (1966a) The identification of contact allergies by human
    assay: II. Factors influencing the induction and measurement of
    allergic contact dermatitis. J Invest Dermatol, 47: 375-393.

    Kligman AM (1966b) The identification of contact allergens by human
    assay: III. The maximization test -- A procedure for screening and
    rating contact sensitizers. J Invest Dermatol, 47: 393-409.

    Kligman AM (1966c) The SLS provocative patch test in allergic
    sensitization. J Invest Dermatol, 46: 573-595.

    Knippels LM, Penninks AH, Spanhaak S, & Houben GF (1998) Oral
    sensitization to food proteins: a Brown Norway rat model. Clin Exp
    Allergy, 28(3): 368-375.

    Koenig JQ (1988) Indoor and outdoor pollutants and the upper
    respiratory tract. J Allergy Clin Immunol, 81: 1055-1059.

    Koenig JQ (1995) Effect of ozone on respiratory responses in subjects
    with asthma. Environ Health Perspect, 103(suppl 2): 103-105.

    Koenig JQ, Pierson WE, & Horike M (1983) The effects of inhaled
    sulfuric acid on pulmonary function in adolescent asthmatics. Am Rev
    Respir Dis, 128(2): 221-225.

    Koenig JQ, Pierson WE, Covert DS, Marshall SG, Morgan MS, & van Belle
    G (1988) The effects of ozone and nitrogen dioxide on lung function in
    healthy and asthmatic adolescents. Res Rep Health Eff Inst, 14: 5-24.

    Koenig JQ, Covert DS, Hanley QS, van Belle G, & Pierson WE (1990)
    Prior exposure to ozone potentiates subsequent response to sulfur
    dioxide in adolescent asthmatic subjects. Am Rev Respir Dis, 141:
    377-380.

    Koëter HBWM (1995) International harmonisation of immunotoxicity
    testing. Hum Exp Toxicol, 14: 151-154.

    Kogevinas M, Anto J, Soriano JB, Tobias A, & Burney P (1996) The risk
    of asthma attributable to occupational exposures. Ann Resp Crit Care
    Med, 154: 137-143.

    Kohler C, Jeanvoine G, Pierrez J, Olive D, & Gerard H (1987)
    Modifications of the thymus and splenic thymic dependent zones after
    in utero exposure to phenytoin: Qualitative and quantitative analysis
    in C3H mice. Dev Pharmacol Therap, 10: 405-412.

    Koren HS (1995) Associations between criteria air pollutants and
    asthma. Environ Health Perspect, 103(suppl 6): 235-242.

    Koren HS & Bromberg PA (1995) Respiratory responses of asthmatics to
    ozone. Int Arch Allergy Immunol, 107: 236-238.

    Koren HS, Devlin RB, Graham DE, Mann R, McGee MP, Horstman DH, Kozumbo
    WJ, Becker S, House DE, McDonnel WF, & Bromberg PA (1989)
    Ozone-induced inflammation in the lower airways of human subjects. Am
    Rev Respir Dis, 139: 407-415.

    Koren HS, Hatch GE, & Graham DE (1990) Nasal lavage as a tool in
    assessing acute inflammation in response to inhaled pollutants.
    Toxicology, 60: 15-25.

    Korman NJ (1995) In situ bound antibodies eluted from the skin of
    patients with bullous pemphigoid are preferentially directed against
    the 230-kD bullous pemphigoid antigen. J Invest Dermatol, 
    105: 824-830.

    Korman NJ, Eyre RW, Zone J, & Stanley JR (1991) Drug-induced
    pemphigus: autoantibodies directed against the pemphigus antigen
    complexes are present in penicillamine and captopril-induced
    pemphigus. J Invest Dermatol, 96: 273-276.

    Kortekangas-Savolainen O, Savolainen J, & Einarsson R (1993)
    Gastrointestinal stability of baker's yeast allergens: an  in vitro 
    study. Clin Exp Allergy, 23(7): 578-590.

    Krämer U, Behrendt H, & Ring J (1996) Air pollution as a risk factor
    for allergy: The East-West German experience. In: Ring J, Behrendt H,
    & Vieluf D ed. New trends in allergy. Berlin, Heidelberg, New York,
    Springer Verlag, vol IV, pp 25-35.

    Kremer AM, Pal TM, Schouten JP, & Rijcken B (1995) Airway
    hyperresponsiveness in workers exposed to low levels of irritants. Eur
    Respir J, 8(1): 53-61.

    Kremser M (1989) [Food allergies -- oropharyngeal reactions.] Wien Med
    Wochenschr, 139(6/7): 135-139.

    Kröger S, Neuber K, Gruseck E, Ring J, & Abeck D (1995)  Pityrosporum
     ovale extracts increase interleukin-4, interleukin-10 and IgE
    synthesis in patients with atopic eczema. Acta Dermatol Venereol, 75:
    357-360.

    Krzystyniak K, Brouland J-P, Panayi G, Patriarca C, Verdier F,
    Descotes J, & Revillard J-P (1992) Activation of CD4+ and CD8+
    lymphocyte subsets by streptozotocin in murine popliteal lymph node
    (PLN) test. J Autoimmun, 5: 183-197.

    Krzyzanowski M, Quackenboss JJ, & Lebowitz MD (1992) Relation of peak
    expiratory flow rates and symptoms to ambient ozone. Arch Environ
    Health, 47(2): 107-115.

    Kubicka-Muranyi M, Goebels R, Goebel C, Uetrecht J, & Gleichmann E
    (1993) T lymphocytes ignore procainamide, but respond to its reactive
    metabolites in peritoneal cells: demonstration by the adoptive
    transfer popliteal lymph node assay. Toxicol Appl Pharmacol, 122:
    88-94.

    Kubicka-Muranyi M, Griem P, Lübben B, Rottmann N, Lührmann R, &
    Gleichmann E (1996) Mercuric chloride-induced autoimmunity in mice
    involves an upregulated presentation of altered and unaltered
    nucleolar self antigen. Int Arch Allergy Immunol, 108: 1-10.

    Kuchroo VK, Byrne MC, Greenfield E, Whitters MJ, Nalefsky EA, Rao A,
    Collins M, & Dorf ME (1995) Transfection of TCR alpha-chains into
    suppressor and T helper cell hybridomas. J Immunol, 154: 5030-5038.

    Kuhn R, Rajewsky K, & Muller W (1991) Generation and analysis of
    interleukin-4 deficient mice. Science, 254(5032): 707-710.

    Kurisaki J, Konishi Y, Kaminogawa S, & Yamauchi K (1981) Studies on
    the allergenic structure of hen ovomucoid by chemical and enzymatic
    fragmentation. Agric Biol Chem, 45: 879.

    Küster W, Petersen M, Christophers E, Goos M, & Sterry W (1990) A
    family study of atopic dermatitis. Arch Dermatol Res, 282: 98-102.

    Lake AM, Kagey-Sobotka A, Jakubowicz T, & Lichtenstein LM (1984)
    Histamine release in acute anaphylactic enteropathy of the rat. J
    Immunol, 133(3): 1529-1534.

    Lammintausta K, Kalimo K, & Havu VK (1982) Contact allergy in atopics
    who perform wet work in hospital. Derm Beruf Umwelt, 30(6): 184-188.

    Lammintausta K, Kalimo K, & Fagerlund VL (1992) Patch test reactions
    in atopic patients. Contact Dermatitis, 26(4): 234-240.

    Lamont AG, Mowat A McI, Browning MJ, & Parrott DMV (1988) Genetic
    control of oral tolerance to ovalbumin in mice. Immunology, 63:
    737-739.

    Landsteiner K & Rostenberg A Jr (1939) Individual differences in
    susceptibility to eczematous sensitization with simple chemical
    substances. J Invest Dermatol, 2: 25-29.

    Lane HC & Fauci AS (1985) Immunologic abnormalities in the acquired
    immunodeficiency syndrome. Annu Rev Immunol, 3: 477-500.

    Lange CE, Jühe S, Stein G, & Veltman G (1974) [The so-called vinyl
    chloride disease -- an occupational scleroderma?] Int Arch Arbeitsmed,
    32: 1-32 (in German).

    Laor A, Cohen L, & Danon YL (1993) Effects of time, sex, ethnic
    origin, and area of residence on prevalence of asthma in Israeli
    adolescents. Br Med J, 307: 841-844.

    Larsen CG, Thomsen MK, Gesser B, Deleuran BW, & Thestrup-Pedersen K
    (1995) The delayed type hypersensitivity reaction is dependent on
    IL-8. Inhibition of a tuberculin skin reaction by an anti-IL-8
    monoclonal antibody. J Immunol, 155(4S): 2151-2157.

    Lau S, Falkenhorst G, Weber A, Werthmann I, Lind P, Buettner Goetz P,
    & Wahn U (1989) High mite-allergen exposure increases the risk of
    sensitization in atopic children and young adults. J Allergy Clin
    Immunol, 84: 718-725.

    Lau S, Ehnert B, Cremer B, Nasert S, Buettner P, Czarnetzki BM, & Wahn
    U (1995) [Reduction of house dust mite exposure in sensitized patients
    with atopic eczema.] Allergo J, 4: 432-437 (in German).

    Lauer K (1990) Environmental nitrophenols and autoimmunity (Letter to
    the Editor). Mol Immunol, 27: 697-698.

    Lauerma AI (1992) Contact hypersensitivity to glucocorticosteroids. Am
    J Contact Dermatitis, 3: 112-132.

    Lauwerys R & Lison D (1994) Health risks associated with cobalt
    exposure -- an overview. Sci Total Environ, 150(1-3): 1-6.

    Law LW, Goldstein AL, & White A (1968) Influence of thymosin on
    immunological competence of lymphoid cells from thymectosized mice.
    Nature (Lond), 219(161): 1391-1392.

    Leach CL, Hatoum NS, Ratajcak HV, Zeiss CR, Roger JC, & Garvin PJ
    (1987) Pathologic and immunologic response to inhaled trimellitic
    anhydride in rats. Toxicol Appl Pharmacol, 87: 67-80.

    Ledford DK (1994) Indoor allergens. J Allergy Clin Immunol, 94:
    327-334.

    Lee HK, Alarie Y, & Karol MH (1984) Induction of formaldehyde
    sensitivity in guinea pigs. Toxicol Appl Pharmacol, 75: 147-155.

    Lehmann PV, Secarz EE, Fortshuber T, Dayan CM, & Gammon G (1993)
    Determinant spreading and the dynamics of the autoimmune T cell
    repertoire. Immunol Today, 14: 203-208.

    Lejman E, Stoudemayer T, Grove G, & Kligman AM (1984) Age differences
    in poison ivy dermatitis. Contact Dermatitis, 11: 163-167.

    Lenshow DJ, Walunas TL, & Bluestone JA (1996) CD28/B7 system of T cell
    co-stimulation. Annu Rev Immunol, 14: 233-258.

    Leung R & Ho P (1994) Asthma, allergy, and atopy in three south-east
    Asian populations. Thorax, 49: 1205-1210.

    Lewis S, Richards D, Bynner J, Butler N, & Britton J (1995)
    Prospective study of risk factors for early and persistent wheezing in
    childhood. Eur Respir J, 8(3): 349-356.

    Leyden JJ & Kligman AM (1977) Allergic contact dermatitis: Sex
    differences. Contact Dermatitis, 3: 333-336.

    Leyden JE, Marples RR, & Kligman AM (1974)  Staphylococcus aureus in
    the lesions of atopic dermatitis. Br J Dermatol, 90(5): 525-530.

    Li LF (1995) A clinical and patch test study of contact dermatitis
    from traditional Chinese medical materials. Contact Dermatitis, 33:
    392-395.

    Lidén C, Menné T, & Burrows D (1996) Nickel-containing alloys and
    platings and their ability to cause dermatitis. Br J Dermatol, 134:
    193-198.

    Lindskov R & Holmer G (1992) Polyunsaturated fatty acid in plasma, red
    blood cells and mononuclear phospholipids in patients with atopic
    dermatitis. Allergy, 47: 517-521.

    Linn WS, Shamoo DA, Spier CE, Valencia LM, Anzar UT, Venet TG, &
    Hackney JD (1983a) Respiratory effects of 0.75 ppm sulfur dioxide in
    exercising asthmatics: influence of upper-respiratory defenses.
    Environ Res, 30: 340-348.

    Linn WS, Venet TG, Shamoo DA, Valencia LM, Anzar UT, Spier CE, &
    Hackney JD (1983b) Respiratory effects of sulfur dioxide in heavily
    exercising asthmatics: A dose-response study. Am Rev Respir Dis, 127:
    278-283.

    Lippmann M (1989) Health effects of ozone: a critical review. J Air
    Pollut Control Assoc, 39: 672-695.

    Liu MC, Bleecker ER, Lichtenstein LM, Kagey-Sobotka A, Niv Y, Mclemore
    TL, Permutt S, Proud D, & Hubbard WC (1990) Evidence for elevated
    levels of histamine, prostaglandin D2, and other bronchoconstricting
    prostaglandins in the airways of subjects with mild asthma. Am Rev
    Respir Dis, 142: 126-132.

    Lo D (1996) Animal models of human disease -- Transgenic and knockout
    models of autoimmunity: building a better disease? Clin Immunol
    Immunopathol, 79(2): 96-104.

    Logan RFA (1992) In: Auricchio S & Visakorpi JK ed. Problems and
    pitfalls in epidemiological studies of coeliac disease, common food
    intolerances: 1. Epidemiology of coeliac disease. Basel, Karger, p 14.

    Lonkar A, Mitchell JC, & Calnan CD (1974) Contact dermatitis from
    parthenium hysterophorus. Trans St. John's Hosp Derm Soc, 60: 43-53.

    Lovik M, Hogseth A-K, Gaarder PI, Hagemann R, & Eide I (1997) Diesel
    exhaust particles and carbon black have adjuvant activity on the local
    lymph node response and systemic IgE production to ovalbumin.
    Toxicology, 121: 165-178.

    Lucas A, Brooke OG, Morley R, Cole TJ, & Bamford MF (1990) Early diet
    of preterm infants and development of allergic or atopic disease:
    randomised prospective study. Br Med J, 300(6728): 837-840.

    Lucente FE (1989) Rhinitis and nasal obstruction. Otolaryngol Clin
    North Am, 22: 307-318.

    Luczynska CM & Topping MD (1986) Specific IgE antibodies to reactive
    dye-albumin conjugates. J Immunol Methods, 95: 177-186.

    Luo J-C, Nelsen KG, & Fischbein A (1990) Persistent reactive airways
    dysfunction syndrome after exposure to toluene di-isocyanate. Br J Ind
    Med, 47: 239-241.

    Lutsky I, Baum GL, Teichtahl H, Mazar A, Aizer F, & Bar Sela S (1985)
    Occupational respiratory disease in veterinarians. Ann Allergy, 55(2):
    153-156.

    Lynch NR, Hagel IA, Palenque ME, Di Prisco MC, Escudero JE, Corao LA,
    Sandia JA, Ferreira LJ, Botto C, Perez M, & Le Souef PN (1998)
    Relationship between helminthic infection and IgE response in atopic
    and nonatopic children in a tropical environment. J Allergy Clin
    Immunol, 101(2): 217-221.

    McAlindon T, Gianotta L, Taub N, D'Cruz D, & Hughes GRV (1993)
    Environmental factors predicting nephritis in systemic lupus
    erythematosus. Ann Rheum Dis, 52: 720-724.

    Maccia CA, Berstein IL, Emmett EA, & Brookes SSM (1976)  In vitro 
    demonstration of specific IgE in phthalic anhydride sensitivity. Am
    Rev Respir Dis, 113: 701-704.

    MacDonald TT (1983) Immunosuppression caused by antigen feeding: II.
    Suppressor T cells mask Peyer's patch B cell priming to orally
    administered antigen. Eur J Immunol, 13: 138-142.

    McGrath H Jr, Bell JM, & Haycock JW (1994) Fluorescent light activates
    the immunomodulator cis-urocanic acid  in vitro: Implications for
    patients with systemic lupus erythematosus. Ann Rheum Dis, 53(6):
    396-399.

    Machado DC, Horton D, Harrop R, Peachell PT, & Helm BA (1996)
    Potential allergens stimulate the release of mediators of the allergic
    response from cells of mast cell lineage in the absence of
    sensitization with antigen-specific IgE. Eur J Immunol, 26: 2972-2980.

    McMillan C & Burrows D (1995)  In vitro testing in contact
    hypersensitivity. In: Rycroft RJG, Menné T, Frosch PJ, & Benezra C ed.
    Textbook of contact dermatitis, 2nd ed. Berlin, Heidelberg, New York,
    Springer Verlag, pp 306-318.

    Madara JL & Stafford J (1989) Interferon-gamma directly affects
    barrier function of cultured intestinal epithelial monolayers. J Clin
    Invest, 83(2): 724-727.

    Maestrelli P, Di Stefano A, Occari P, Turato G, Milani G, Pivirotto F,
    Mapp CE, Fabbri LM, & Saetta M (1995) Cytokines in the airway mucosa
    of subjects with asthma induced by toluene diisocyanate. Am J Resp
    Crit Care Med, 151: 607-612.

    Magnus P & Jaakkola JJ (1997) Secular trend in the occurrence of
    asthma among children and young adults: critical appraisal of repeated
    cross sectional surveys. Br Med J, 314: 1795-1799

    Magnusson CG (1986) Maternal smoking influences cord serum IgE and IgD
    levels and increases the risk for subsequent infant allergy. J Allergy
    Clin Immunol, 78: 898-904.

    Magnusson B & Gilje O (1973) Allergic contact dermatitis from a
    dish-washing liquid containing laurylethersulphate. Acta Dermatol
    Venereol, 53: 136-140.

    Magnusson B & Hersle K (1965) Patch test methods. Acta Dermatol
    Venereol, 45: 123-128.

    Magnusson B & Kligman AM (1970a) Allergic contact dermatitis in the
    guinea pig. Springfield, Charles C Thomas, pp 3-120.

    Magnusson B & Kligman AM (1970b) The identification of contact
    allergens by animal assay: The guinea pig maximisation test. J Invest
    Dermatol, 52: 268-276.

    Magnussen H, Holz O, & Jörres RA (1998) Asthma and the environment: Is
    ozone really important? Eur Respir Rev, 8: 141-144.

    Mahzoon S, Yamamoto S, & Greaves MW (1977) Response of skin to
    ammonium persulphate. Acta Dermatol Venereol, 57(2): 125-126.

    Maibach H (1976) Immediate hypersensitivity in hand dermatitis. Arch
    Dermatol, 112(9): 1289-1291.

    Malo J-L, Ouimet G, Cartier A, Lebitz D, & Ziess CR (1983) Combined
    alveolitis and asthma due to hexamethylene di-isocyanate (HDI) with
    evidence of crossed respiratory and immunologic reactivities to
    diphenylmethane di-isocyanate MDI. J Allergy Clin Immunol, 
    72: 413-419.

    Manetti R, Parronchi P, Giudizi MG, Piccinni MP, Maggi E, Trinchieri
    G, & Romagnani S (1993) Natural killer cell stimulatory factor
    (interleukin 12 [IL-12]) induces T helper type 1 (Th1)-specific immune
    responses and inhibits the development of IL-4 producing Th cells. J
    Exp Med, 177(4): 1199-1204.

    Mantle J & Pepys J (1974) Asthma amongst Tristan da Cunha islanders.
    Clin Allergy, 4: 161-170.

    Marchal G, Seman M, Milon G, Truffa-Bacchi P, & Zilberfarb V (1982)
    Local adoptive transfer of skin delayed type hypersensitivity
    initiated by a single T lymphocyte. J Immunol, 129(3): 954-958.

    Marchionini A (1960) [New studies of constitutional neurodermatitis.]
    In: Marchionini A & Röckl M ed. [Dermatology and venereology.] Berlin,
    Heidelberg, New York, Springer Verlag, vol 3, pp 42-46 (in German).

    Marcon-Genty D, Tomé D, Dumontier AM, Kheroua O, & Desjeux JF (1989)
    Permeability of milk protein antigens across the intestinal
    epithelium  in vitro. Reprod Nutr Dev, 29(6): 717-723.

    Marfaing-Koka A, Devergne O, Gorgone G, Portier A, Schall TJ, Galanaud
    P, & Emilie D (1995) Regulation of the production of RANTES chemokine
    by endothelial cells -- Synergistic induction by IFN-gamma plus
    TNF-alpha and inhibition by IL-4 and IL-13. J Immunol, 154(suppl 4):
    1870-1878.

    Marsh DG (1990) Immunogenetic and immunochemical factors determining
    immune responsiveness to allergens: studies in unrelated subjects. In:
    Marsh DG & Blumenthal M ed. Genetic and environmental factors in
    clinical allergy. Minneapolis, University of Minneapolis Press, pp
    97-120.

    Marsh DG & Norman PS (1988) Antigens that cause atopic disease. In:
    Immunological diseases, 4th ed. Boston, Little Brown, vol 2, pp
    981-1008.

    Martin S & Weltzien HU (1994) T cell recognition of haptens, a
    molecular view. Int Arch Allergy Immunol, 104: 10-16.

    Martinez FD (1994) Role of viral infections in the inception of asthma
    and allergies during childhood: could they be protective? Thorax, 49:
    1189-1191.

    Martinez FD (1995) Viral infections and the development of asthma. Am
    J Respir Crit Care Med, 151: 1644-1648.

    Martinez FD, Antognoni G, Macri F, Bonci E, Midulla F, De Castro G, &
    Ronchetti R (1988) Parental smoking enhances bronchial responsiveness
    in nine-year-old children. Am Rev Respir Dis, 138(3): 518-523.

    Martinez FD, Cline M, & Burrows B (1992) Increased incidence of asthma
    in children of smoking mothers. Pediatrics, 89: 21-26.

    Martinez FD, Wright AL, Taussig LM, Holberg CJ, Halonen M, & Morgan WJ
    (1995) Asthma and wheezing in the first six years of life. The Group
    Health Medical Associates. N Engl J Med, 332: 133-138.

    Marzulli FN & Maibach HI (1973) Antimicrobials: Experimental contact
    sensitization in man. J Soc Cosmet Chem, 24: 399-421.

    Marzulli FN & Maibach HI (1974) The use of graded concentrations in
    studying skin sensitizers: experimental contact sensitization in man.
    Food Cosmet Toxicol, 12: 219-227.

    Marzulli FN & Maibach HI (1996) Test methods for allergic contact
    dermatitis in humans.
    In: Marzulli FN & Maibach HI ed. Dermatotoxicology, 5th ed.
    Washington, New York, London, Hemisphere Publishing Corporation, pp
    477-482.

    Matricardi PM (1997) Infections preventing atopy: Facts and new
    questions. Allergy, 52: 879-882.

    Matricardi PM, Rosmini F, Ferrigno L, Nasion R, Rapicetta M, Chionne
    P, Stroffolini T, Pasquini P, & D`Amelio R (1997) Cross sectional
    retrospective study of the prevalence of atopy among Italian military
    students with antibodies against hepatitis A virus. Br Med J, 314:
    999-1003.

    Mattingly JA (1984) Immunologic suppression after oral administration
    of antigen: III. Activation of suppressor-inducer cells in the Peyer's
    patches. Cell Immunol, 86: 46-52.

    Matzner Y, Erlich HA, Brautbar C, Sanilevitch A, Landau M, Brenner S,
    & Friedmann A (1995) Identical HLA class II alleles predispose to
    drug-triggered and idiopathic pemphigus vulgaris. Acta Dermatol
    Venereol, 75: 12-14.

    Maulitz RM, Pratt DS, & Schocket AL (1979) Exercise-induced
    anaphylactic reaction to shellfish. J Allergy Clin Immunol, 63(6):
    433-434.

    Maurer T, Thomann P, Weirich EG, & Hess R (1975) The optimization test
    in the guinea pig: A method for the predictive evaluation of the
    contact allergenicity of chemicals. Agents Actions, 5: 174-179.

    Maynard AD, Northage C, Hemingway M, & Bradley SD (1997) Measurement
    of short-term exposure to airborne soluble platinum in the platinum
    industry. Ann Occup Hyg, 41(1): 77-94.

    Meade R, Askenase PW, Geba GP, Neddermann K, Jacoby RO, & Pasternak RD
    (1992) Transforming growth factor-beta 1 inhibits murine immediate and
    delayed type hypersensitivity. J Immunol, 149(2): 521-528.

    Medici TC & Vetter W (1991) [Bronchial asthma and kitchen salt.]
    Schweiz Med Wochenschr, 121(14): 501-508 (in German).

    Meding B (1990) Epidemiology of hand eczema in an industrial city.
    Acta Dermatol Venereol, 153(suppl): 2-43.

    Melamed D & Friedman A (1993) Direct evidence for anergy in T
    lymphocytes tolerized by oral administration of ovalbumin. Eur J
    Immunol, 23: 935-942.

    Mellström G, Lindahl G, & Wahlberg J (1989) DAISY: reference database
    on protective gloves. Semin Dermatol, 8: 75-79.

    Melnik B & Plewig G (1991) Atopic dermatitis and disturbances of
    essential fatty acid in prostaglandin E metabolism. J Am Acad
    Dermatol, 25: 859-860.

    Menné T & Bachmann E (1979) Permanent disability from skin diseases.
    Dermatosen, 27: 37-42.

    Menné T & Holm NV (1983) Nickel allergy in a female twin population.
    Int J Dermatol, 22: 22-28.

    Menné T & Holm NV (1986) Gene susceptibility in human allergic contact
    sensitization. Semin Dermatol, 5: 301-306.

    Menné T & Maibach HI (1991) Systemic contact-type dermatitis. In:
    Marzulli FN & Maibach HI ed. Dermatotoxicology. Washington, New York,
    London, Hemisphere Publishing Corporation, p 453.

    Menné T & Maibach HI ed. (1994) Hand eczema. Boca Raton, Florida, CRC
    Press, 334 pp.

    Menné T & Wilkinson JD (1995) Individual predisposition to contact
    dermatitis. In: Rycroft RJG, Menné T, Frosch PJ, & Benezra C ed.
    Textbook of contact dermatitis. Berlin, Heidelberg, New York, Springer
    Verlag, pp 123-130.

    Menné T, Christophersen J, & Maibach HI (1987) Epidemiology of
    allergic contact sensitization. Monogr Allergy, 21: 132-161.

    Menné T, Dooms-Goossens A, Wahlberg JE, White I, & Wilkinson J (1992)
    How large a proportion of contact sensitivities are diagnosed with the
    European standard series. Contact Dermatitis, 26: 201-202.

    Menné T, Veien N, Sjolin KE, & Maibach HI (1994) Systemic contact
    dermatitis. Am J Contact Dermatitis, 5: 1-12.

    Meredith S (1993) Reported incidence of occupational asthma in the
    United Kingdom, 1989-90. J Epidemiol Community Health, 47(6): 459-463.

    Meredith SK & McDonald JC (1994) Work-related respiratory disease in
    the United Kingdom, 1989-1992: Report on the SWORD project. Occup Med,
    44(4): 183-189.

    Meredith S & Nordman H (1996) Occupational asthma: measures of
    frequency from four countries. Thorax, 51(4): 435-440.

    Merget R, Reineke M, Rueckmann A, Bergmann EM, & Schultze Werninghaus
    G (1994) Nonspecific and specific bronchial responsiveness in
    occupational asthma caused by platinum salts after allergen avoidance.
    Am J Respir Crit Care Med, 150(4): 1146-1149.

    Merget R, Dierkes A, Rueckmann A, Bergmann EM, & Schultze Werninghaus
    G (1996) Absence of relationship between degree of nonspecific and
    specific bronchial responsiveness in occupational asthma due to
    platinum salts. Eur Respir J, 9(2): 211-216.

    Metcalfe DD (1985) Food allergens. Clin Rev Allergy, 3(3): 331-349.

    Metcalfe DD & Sampson HA (1990) Workshop on experimental methodology
    for clinical studies of adverse reactions to foods and food additives.
    J Allergy Clin Immunol, 86: 421-442.

    Metzger WJ, Zavala D, Richerson HB, Mosely P, Iwamota P, Monick M,
    Sjoerdsma K, & Hunninghake GW (1987) Local allergen challenge and
    bronchoalveolar lavage of allergic asthmatic lungs. Am Rev Respir Dis,
    135: 433-440.

    Miedema I, Feskens EJ, Heederik D, & Kromhout D (1993) Dietary
    determinants of long-term incidence of chronic nonspecific lung
    diseases: The zutphen study. Am J Epidemiol, 138(1): 37-45.

    Miller K & Nicklin S (1988) Mechanisms of food intolerances:
    development and use of experimental animal models. In: Walker R &
    Quattrucci E ed. Nutritional and toxicological aspects of food
    processing. London, New York, Philadelphia, Taylor & Francis, pp
    351-364.

    Minowada G & Welch WJ (1995) Clinical implications of the stress
    response. J Clin Invest, 95: 3-12.

    Mitchell EA, Stewart AW, Pattemore PK, Asher MI, Harrison AC, & Rea HH
    (1989) Socioeconomic status in childhood asthma. Int J Epidemiol,
    18(4): 888-890.

    Molfino NA, Wright SC, Katz I, Tarlo S, Silverman F, McClean PA,
    Szalai JP, Raizenne M, Slutsky AS, & Zamel N (1991) Effect of low
    concentrations of ozone on inhaled allergen responses in asthmatic
    subjects. Lancet, 338: 199-203.

    Moller DR, Gallagher JS, Benstein DI, Wilcox TG, Burroughs HE, &
    Bernstein IL (1985) Detection of IgE mediated respiratory
    sensitisation in workers exposed to hexahydrophthalic anhydride. J
    Allergy Clin Immunol, 75: 663-672.

    Monick M, Glazier J, & Hunninghake GW (1987) Human alveolar
    macrophages suppress interleukin 1 activity via the secretion of
    prostaglandin E2. Am Rev Respir Dis, 135: 72-77.

    Montgomery-Smith J (1983) Epidemiology and natural history of asthma,
    allergic rhinitis and atopic dermatitis (eczema). In: Allergy:
    Principles and practice. St Louis, Missouri, C.V. Mosby & Co., pp
    771-803.

    Montgomery-Smith J, Middleton E, Reed CE, Ellis EF, Adkinson NF, &
    Yunginger JW ed. (1983) Allergy: Principles and practice
    -- Epidemiology and natural history of asthma, allergic rhinitis and
    atopic dermatitis (eczema). St Louis, Missouri, C.V. Mosby & Co., pp
    771-803.

    Moore SA, Strieter RM, Rolfe MW, Standiford TJ, Burdick MD, & Kunkel
    SL (1992) Expression and regulation of human alveolar
    macrophage-derived interleukin-1 receptor antagonist. Am J Respir Cell
    Mol Biol, 6(6): 569-575.

    Morel E, Feuillet-Fieux MN, Vernet-der Garabedian B, Raimond F,
    D'Anglejan J, Bataille R, Sany J, & Bach JF (1991) Autoantibodies in
    D-penicillamine-induced myasthenia gravis: a comparison with
    idiopathic myasthenia and rheumatoid arthritis. Clin Immunol
    Immunopathol, 58: 318-330.

    Morgenstern H & Thomas D (1993) Principles of study design in
    environmental epidemiology. Environ Health Perspect, 101(suppl 4):
    23-28.

    Morren MA, Przybilla B, Bamelis M, Heykants B, Reynaers A, & Degreef H
    (1994) Atopic dermatitis: triggering factors. J Am Acad Dermatol, 31:
    467-473.

    Mosimann B, Peitrequin R, Blanc C, & Pecoud A (1992) Allergie aux
    blattes (cafards) dans une population suisse souffrant d'asthme et de
    rhinite chroniques. Schweiz Med Wochenschr, 122(34): 1245-1248.

    Mosmann TR & Sad S (1996) The expanding universe of T-cell subsets:
    Th1, Th2 and more. Immunol Today, 17: 138-146.

    Moulon C, Vollmer J, & Weltzien HU (1995) Characterization of
    processing requirements and metal cross-reactivities in T cell clones
    from patients with allergic contact dermatitis to nickel. Eur J
    Immunol, 25: 3308-3315.

    Mowatt AMcI (1984) The immunopathogenesis of food-sensitive
    enteropathies. In: Newly TJ & Stokes CR ed. Local immune responses of
    the gut. Boca Raton, Florida, CRC Press, pp 199-225.

    Mowatt AMcI (1987) The regulation of immune responses to dietary
    protein antigens. Immunol Today, 8: 93-98.

    Mowatt AMcI, Lamont AG, Strobel S, & Mackenzie S (1986) The role of
    antigen processing and suppressor T cells in immune responses to
    dietary proteins in mice. Adv Exp Med Biol, 216A: 709-720.

    M'Raihi L, Charpin D, Pons A, Bongrande P, & Vervloet D (1991)
    Cross-reactivity between latex and banana. J Allergy Clin Immunol, 87:
    129-130.

    Mullan RJ & Murthy LI (1991) Occupational sentinel health events: An
    up-dated list for physician recognition and public health
    surveillance. Am J Ind Med, 19(6): 775-799.

    Mullick FG, McAllister HA, Wagner BM, & Fenoglio JJ (1979) Drug
    related vasculitis: Clinicopathologic correlations in 30 patients. Hum
    Pathol, 10: 313-325.

    Munir AK, Einarsson R, Schou C, & Dreborg SK (1993) Allergens in
    school dust: I. The amount of the major cat (Fel d I) and dog (Can f
    I) allergens in dust from Swedish schools is high enough to probably
    cause perennial symptoms in most children with asthma who are
    sensitized to cat and dog. J Allergy Clin Immunol, 91(5): 1067-1074.

    Munir AK, Einarsson R, & Dreborg SK (1994a) Indirect contact with pets
    can confound the effect of cleaning procedures for reduction of animal
    allergen levels in house dust. Pediatr Allergy Immunol, 5(1): 32-39.

    Munir AK, Bjorksten B, Einarsson R, Schou C, Ekstrand Tobin A, Warner
    A, & Kjellman NI (1994b) Cat (Fel d I), dog (Can f I), and cockroach
    allergens in homes of asthmatic children from three climatic zones in
    Sweden. Allergy, 49(7): 508-516.

    Muraille E & Leo O (1998) Revisiting the Th1/Th2 paradigm. Scand J
    Immunol, 47: 1-9.

    Muranaka M, Suzuki S, Koizumi K, Takafuji S, Miyamoto T, Ikemori R, &
    Tokiwa H (1986) Adjuvant activity of diesel-exhaust particulates for
    the production of IgE antibody in mice. J Allergy Clin Immunol, 77:
    616-623.

    Mygind N (1986) Essential allergy, 1st ed. Oxford, London, Boston,
    Blackwell Scientific Publications, 480 pp.

    Mygind N (1989) Nasal allergy, 2nd ed. Oxford, London, Boston,
    Blackwell Scientific Publications, pp 219-223.

    Nagata S, Yamashiro Y, Ohtuska Y, Shioya T, Oguchi S, Shimizu T, &
    Maeta M (1995) Quantitative analysis and immunohistochemical studies
    on small intestinal mucosa of food-sensitive enteropathy. J Pediatr
    Gastroenterol Nutr, 20: 44-48.

    Naparstek Y & Plotz PH (1993) The role of autoantibodies in autoimmune
    disease. Annu Rev Immunol, 11: 79-104.

    NAS (National Academy of Science) (1983) Committee on the
    Institutional Means for Assessment of Risks to Public Health -- Risk
    assessment in the Federal Government: Managing the process.
    Washington, DC, National Academy Press, pp 1-50.

    NAS (National Academy of Science) (1993) Committee on Risk Assessment
    Methodology, Board on Environmental Studies and Toxicology -- Issues
    in risk assessment. Washington, DC, National Academy Press, 356 pp.

    Neas LM, Dockery DW, Ware JH, Spengler JD, Speizer FE, & Ferris BG Jr
    (1991) Association of indoor nitrogen dioxide with respiratory
    symptoms and pulmonary function in children. Am J Epidemiol, 134(2):
    204-219.

    Nelson HS (1985) The atopic diseases. Ann Allergy, 55: 441-447.

    Nelson HS, Rosloniec DM, McCall LI, & Ikle D (1993) Comparative
    performance of five commercial prick skin test devices. J Allergy Clin
    Immunol, 92(5): 750-756.

    Nemery B, Casier P, Roosels D, Lahaye D, & Demedts M (1992) Survey of
    cobalt exposure and respiratory health in diamond polishers. Am Rev
    Respir Dis, 145(3): 610-616.

    Newly TJ, Stokes CR, & Bourne FJ (1980) Effects of feeding bacterial
    lipopolysaccharide and dectran sulphate on the development of oral
    tolerance to contact sensitizing agents. Immunology, 41: 617-621.

    Newman LS & Kreiss K (1992) Nonoccupational beryllium disease
    masquerading as sarcoidosis: identification by blood lymphocyte
    proliferative response to beryllium. Am Rev Resp Dis, 145: 1212-1214.

    Newman LS, Bobka C, Schumacher B, Daniloff E, Zhen B, Mroz MM, & King
    TE Jr (1994) Compartmentalized immune response reflects clinical
    severity of beryllium disease. Am J Resp Crit Care Med, 150: 135-142.

    Newman Taylor AJ (1988) Occupational asthma. Postgrad Med J, 
    64: 505-510.

    Newman Taylor AJ (1995) Environmental determinants of asthma. Lancet,
    345(8945): 296-299.

    Newman Taylor AJ, Venables KM, Durham S, Graneek BJ, & Topping MD
    (1987) Acid anhydrides and asthma. Int Arch Allergy Appl Immunol, 82:
    435-439.

    Ng TP & Tan WC (1994a) Epidemiology of chronic (perennial) rhinitis in
    Singapore: prevalence estimates, demographic variation and clinical
    allergic presentation. Ann Acad Med Singap, 23: 83-88.

    Ng TP & Tan WC (1994b) Epidemiology of allergic rhinitis and its
    associated risk factors in Singapore. Int J Epidemiol, 23(3): 553-558.

    Niebel G (1995) [Behavioural medicine of chronic skin disease.] Bern,
    Göttingen, Toronto, Seattle, Hogrefe & Huber Publishers.

    Nielsen NH & Menné T (1992) Allergic contact sensitization in an
    unselected Danish population -- the Glostrup allergy study, Denmark.
    Acta Dermatol Venereol, 72: 456-460.

    Nielsen NH & Menné T (1993) Nickel sensitization and ear piercing in
    an unselected Danish population -- The Glostrup allergy study,
    Denmark. Contact Dermatitis, 29: 16-21.

    Niermann H (1964) [Dermatology in twins.] Berlin, Heidelberg, New
    York, Springer Verlag, 108 pp.

    Niestijl Jansen JJ, Kardinaal AFM, Huijbers G, Vlieg-Boerstra BJ,
    Martens BPM, & Ockhuizen T (1994) Prevalence of food allergy and
    intolerance in the adult Dutch population. J Allergy Clin Immunol, 93:
    446-456.

    NIH (National Institutes of Health) (1991) National Asthma Education
    Program Expert Panel -- Guidelines for the diagnosis and management of
    asthma. Bethesda, Maryland, National Heart, Lung and Blood Institute,
    72 pp (NIH Publication No. 91-3042A).

    Nilsson B & Kristofferson A (1989) Zimelidine: Febrile reaction and
    peripheral neuropathy. In: Kammüller ME, Bloksma N, & Seinen W ed.
    Autoimmunity and toxicology. Amsterdam, Oxford, New York, Elsevier
    Science Publishers, pp 183-214.

    Ninan TK & Russell G (1992) Respiratory symptoms and atopy in Aberdeen
    schoolchildren: Evidence from two surveys 25 years apart. Br Med J,
    304: 873-875.

    NIOSH (1997) NIOSH alert: Preventing allergic reactions to natural
    rubber latex in the workplace. Cincinnati, Ohio, National Institute
    for Occupational Safety and Health, 11 pp.

    Nitta H, Sato T, Nakai S, Maeda K, Aoki S, & Ono M (1993) Respiratory
    health associated with exposure to automobile exhaust. I. Results of
    cross-sectional studies in 1979, 1982, and 1983. Arch Environ Health,
    48: 53-58.

    Noelle RJ, Daum J, Bartlett WC, McCann J, & Shepherd DM (1990) Cognate
    interactions between helper T-cells and B-cells. V. Reconstitution of
    helper T-cell function using purified plasma membranes from activated
    Th1 and Th2 helper T-cells and lymphokines. J Immunol, 146(4):
    1118-1124.

    Norback D, Bjornsson E, Janson C, Widstrom J, & Boman G (1995)
    Asthmatic symptoms and volatile organic compounds, formaldehyde, and
    carbon dioxide in dwellings. Occup Environ Med, 52(6): 388-395.

    Nordman H, Keskinen H, & Tuppurainen M (1985) Formaldehyde asthma
    -- rare or overlooked? J Allergy Clin Immunol, 75: 91-99.

    Nussler AK & Billiar TR (1993) Inflammation, immunoregulation, and
    inducible nitric oxide synthase. J Leuk Biol, 54(2): 171-178.

    Nyren O, Yin L, Josefsson S, McLaughlin JK, Blot WJ, Engqvist M,
    Hakelius L, Boice JD Jr, & Adami HO (1998) Risk of connective tissue
    disease and related disorders among women with breast implants: a
    nation-wide retrospective cohort study in Sweden. Br Med J, 316(7129):
    417-422.

    O'Brien IM, Harries MG, Burge PS, & Pepys J (1979) Toluene
    di-isocyanate induced asthma. Reactions to TDI, MDI, HDI and
    histamine. Clin Allergy, 19: 1-6.

    O'Byrne PM, Dolovich J, & Hargreave FE (1987) Late asthmatic
    responses. Am Rev Respir Dis, 134: 740-751.

    Odom RB & Maibach HI (1976) Contact urticaria: a different contact
    dermatitis. Cutis, 18(5): 672-676.

    O'Donnell TV (1995) Asthma and respiratory problems -- a review. Sci
    Total Environ, 163(1-3): 137-145.

    OECD (1992) Guideline 406 for testing chemicals. Paris, Organisation
    for Economic Cooperation and Development.

    Oettigen HC, Martin TR, Wynshaw-Boris A, Deng C, Drazen JM, & Leder P
    (1994) Active anaphylaxis in IgE-deficient mice. Nature (Lond), 370:
    367-370.

    Ogawa M, Berger PA, McIntyre OR, & Clendenning WE (1971) IgE in atopic
    dermatitis. Arch Dermatol, 103: 575.

    Ohashi PS, Oehnen S, Buerki K, Pircher H, Ohashi CT, Odermatt B,
    Malissen B, Zinkernagel RM, & Hengartner H (1991) Ablation of
    "tolerance" and induction of diabetes by virus infection in viral
    antigen transgenic mice. Cell, 65: 305-317.

    Ollerenshaw S & Woolcock AJ (1992) Characteristic of inflammation in
    biopsies from large airways of subjects with asthma and subjects with
    chronic airflow limitation. Am Rev Respir Dis, 145: 922-927.

    Ollier S & Davies RJ (1994) A report of the relationship between
    allergen load, primary immunologic sensitization and the expression of
    allergic disease. Respir Med, 88: 407-415.

    Oostendorp RAJ, Meijer CJLM, & Scheper RJ (1993) Immunosuppression by
    retroviral-envelope-related peptides, and their role in non-retroviral
    human disease. Crit Rev Oncol Hematol, 14: 189-206.

    Oosterlee A, Drijver M, Lebret E, & Brunekreef B (1996) Chronic
    respiratory symptoms in children and adults living along streets with
    high traffic density. Occup Environ Med, 53: 241-247.

    Ortolani C, Ispano M, Pastorello EA, Ansaloni R, & Magri GC (1989)
    Comparison of results of skin prick tests (with fresh foods and
    commercial food extracts) and RAST in 100 patients with oral allergy
    syndrome. J Allergy Clin Immunol, 83: 683-690.

    Osebold JW, Gershwin LJ, & Zee YC (1980) Studies on the enhancement of
    allergic lung sensitization by inhalation of ozone and sulfuric acid
    aerosol. J Environ Pathol Toxicol, 3: 221-234.

    Osterland CK (1994) Laboratory diagnosis and monitoring in chronic
    systemic autoimmune diseases. Clin Chem, 40(11B): 2146-2153.

    Ostro BD, Lipsett MJ, Wiener MB, & Selner JC (1991) Asthmatic
    responses to airborne acid aerosols. Am J Public Health, 81(6):
    694-702.

    Oswald IP, Gazzinelli RT, Sher A, & James SL (1992) IL-10 synergizes
    with IL-4 and transforming growth factor-beta to inhibit macrophage
    cytotoxic activity. J Immunol, 148(11): 3578-3582.

    Paggiaro P, Bacci E, Paoletti P, Bernard P, Dente FL, Marchetti G,
    Talini D, Menconi GF, & Giuntini C (1990) Bronchoalveolar lavage and
    morphology of the airways after cessation of exposure in asthmatic
    subjects sensitised to toluene di-isocyanate. Chest, 98(3): 536-542.

    Park HS, Yu HJ, & Jung KS (1994) Occupational asthma caused by
    chromium. Clin Exp Allergy, 24: 676-681.

    Parker D, Sommer G, & Turk JL (1975) Variation in guinea pig
    responsiveness. Cell Immunol, 18: 233-238.

    Parronchi P, Macchia D, Piccini M-P, Biswas P, Simonelli C, Maggi E,
    Ricci M, Ansari A, & Romagnani S (1991) Allergen-and bacterial
    antigen-specific T-cell clones established from atopic donors show a
    different profile of cytokine production. Proc Natl Acad Sci (USA),
    88: 4538-4542.

    Pastorello E, Stocchi L, Bigi A, Pravettoni V, Schilke ML, Valente D,
    & Zanussi C (1989) Value and limits of diagnostic tests in food
    hypersensitivity. Allergy, 44(suppl): 151-158.

    Pastorello EA, Incorvaia C, & Ortolani C (1995) The mouth and pharynx.
    In: Ortolani C ed. Atlas of mechanisms in adverse reactions to food.
    Allergy, 50(20): 41-44.

    Pauli G, de Blay F, Bessort JC, & Dietermann A (1992) The association
    between respiratory allergies and food hypersensitivities. ACI News,
    4: 43-47.

    Pauluhn J (1996) Predictive testing for respiratory sensitisation.
    Toxicol Lett, 86(2-3): 177-185.

    Pauluhn J & Eben A (1991) Validation of a non-invasive technique to
    assess immediate or delayed onset of airway hypersensitivity in
    guinea-pigs. J Appl Toxicol, 11: 423-431.

    Pauluhn J & Mohr U (1994) Assessment of respiratory hypersensitivity
    in guinea pigs sensitized to diphenylmethane-4,4-diisocyanate (MDI)
    and challenged with MDI acetylcholine or MDI albumin conjugate.
    Toxicology, 92: 53-74.

    Payne MP & Walsh PT (1994) Structure-activity relationships for skin
    sensitization potential: development of structural alerts for use in
    knowledge-based toxicity prediction systems. J Chem Inf Comput Sci,
    34(1): 154-161.

    PEACE(1998) Study of pollution effects on asthmatic children in Europe
    (PEACE). Eur Respir Rev, 8: 1-130.

    Pearce N, Weiland S, Keil U, Langridge P, Anderson HR, Strachan D,
    Bauman A, Young L, Gluyas P, Ruffin D, Crane J, & Beasley R (1993)
    Self-reported prevalence of asthma symptoms in children in Australia,
    England, Germany and New Zealand: an international comparison using
    the ISAAC protocol. Eur Respir J, 6: 1455-1461.

    Peat JK (1996) Prevention of asthma. Eur Respir J, 9: 1545-1555.

    Peat J., Woolcock A, Leeder SR, & Blackburn CR (1980) Asthma and
    bronchitis in Sydney schoolchildren: II. The effect of social factors
    and smoking on prevalence. Am J Epidemiol, 111(6): 728-735.

    Peat J., van den Berg RH, Green WF, Mellis CM, Leeder SR & Woolcock A
    (1994) Changing prevalence of asthma in Australian children. Br Med J,
    308: 1591-1596.

    Peden DB, Carter J, Dailey LA, & Devlin R (1994) The effect of ozone
    on house dust mite allergen-induced nasal inflammation in asthmatics.
    Am J Respir Crit Care Med, 149: A154 (abstract).

    Pelletier L, Castedo M, Bellon B, & Druet P (1994) Mercury and
    autoimmunity. In: Dean JH, Luster MI, Munson AE, & Kimber I ed.
    Immunotoxicology and immunopharmacology, 2nd ed. New York, Raven
    Press, pp 539-552.

    Pepys J (1994) "Atopy": a study in definition. Allergy, 49(6):
    397-399.

    Pepys J, Pickering CAC, & Hughes EG (1972) Asthma due to inhaled
    chemical agents -- complex salts of platinum. Clin Allergy, 2:
    391-396.

    Peters A, Dockery DW, Heinrich J, & Watchman HE (1997) Short-term
    effects of particulate air pollution on respiratory morbidity in
    asthmatic children. Eur Resp J, 10(4): 872-879.

    Pfeiffer C, Murray J, Madri J, & Bottomly K (1991) Selective
    activation of Th1- and Th2-like cells  in vivo: response to human
    collagen IV. Immunol Rev, 123: 65-84.

    Philen RM & Posada M (1993) Toxic oil syndrome and
    eosinophilia-myalgia syndrome: May 8-10, 1991, World Health
    Organization Meeting report. Semin Arthritis Rheum, 23: 104-124.

    Picker LJ, Treer JR, Ferguson-Darnell B, Collins PA, Bergstresser PR,
    & Terstappen LWMM (1993) Control of lymphocyte recirculation in man:
    II. Differential regulation of the cutaneous lymphocyte associated
    antigen, a tissue-selective homing receptor for skin-homing T cells. J
    Immunol, 150: 1122-1136.

    Pinkston P, Bitterman PB, & Crystal RG (1984) Interleukin-2 in the
    alveolitis of beryllium-induced lung disease. Am Rev Respir Dis,
    129(411): A161 (abstract).

    Pistoor FAM., Kapsenberg ML, Bos JD, Meinardi MMHM, von Blomberg BME,
    & Scheper RJ (1995) Cross-reactivity of human nickel-reactive T
    lymphocyte clones with copper and palladium. J Invest Dermatol, 105:
    92-95.

    Planchon SM, Martins CAP, Guerrant RL, & Roche J. (1994) Regulation of
    intestinal epithelial barrier function by TGF-beta-1. J Immunol, 153:
    5730-5739.

    Platts-Mills TA (1994) How environment affects patients with allergic
    disease: indoor allergens and asthma. Ann Allergy, 72: 381-384.

    Platts-Mills TA & Chapman MD (1987) Dust mites: immunology, allergic
    disease, and environmental control. J Allergy Clin Immunol, 80:
    755-775.

    Platts-Mills TAE, Chapman MD, Mitchell B, Heymann PW, & Deuell B
    (1991) Role of inhalant allergens in atopic eczema. In: Ruzicka T,
    Ring J, & Przybilla B ed. Handbook of atopic eczema. Berlin,
    Heidelberg, New York, Springer Verlag, pp 192-203.

    Platt-Mills TA, Sporik RB, Chapman MD, & Heymann PW (1995) The role of
    indoor allergens in asthma. Allergy, 50(suppl 22): 5-12.

    Podmore P (1995) Shoes. In: Rycroft RJG, Menné T, Frosch PJ, & Benezra
    C ed. Textbook of contact dermatitis, 2nd ed. Berlin, Heidelberg, New
    York, Springer Verlag, pp 516-527.

    Polak L (1980) Immunological aspects of contact sensitivity: An
    experimental study. Monogr Allergy, 15: 1-170.

    Pollart SM, Chapman MD, Fiocco GP, Rose G, & Platts-Mills TA (1989)
    Epidemiology of acute asthma: IgE antibodies to common inhalant
    allergens as a risk factor for emergency room visits. J Allergy Clin
    Immunol, 83: 875-882.

    Pope CA III, Schwarz J, & Ransom MR (1992) Daily mortality and PM10
    pollution in Utah Valley. Arch Environ Health, 47: 211-217.

    Pope CA III, Bates DV, & Raizenne ME (1995) Health effects of
    particulate air pollution: Time for reassessment. Environ Health
    Perspect, 103(5): 472-480.

    Posselt AM, Barker CF, Friedman AL, & Naji A (1992) Prevention of
    autoimmune diabetes in the BB rat by intrathymic islet transplantation
    at birth. Science, 256: 1321-1324.

    Potter DW & Wederbrand KS (1995) Total IgE antibody in BALB/c mice
    after dermal exposure to chemicals. Fundam Appl Toxicol, 26: 127-135.

    Prausnitz C & Kustner H (1921) [Studies concerning sensitivity.] Zent.
    bl Bakteriol I Orig, 86: 160-169

    Prentice RL & Thomas D (1993) Methodologic research needs in
    environmental epidemiology: Data analysis. Environ Health Perspect,
    101(suppl 4): 39-48.

    Prud'Homme GJ, Parfrey NA, & Vanier LE (1991) Cyclosporine-induced
    autoimmunity and immune hyperreactivity. Autoimmunity, 9: 345-356.

    Przybilla B & Ring J (1990) Food allergy and atopic eczema. Semin
    Dermatol, 9: 220-225.

    Przybilla B, Eberlein-König B, & Rueff F (1994) Practical management
    of atopic eczema. Lancet, 343: 1342-1346.

    Punnonen J & de Vries JE (1994) IL-13 induces proliferation, Ig
    isotype switching, and Ig synthesis by immature human fetal B cells. J
    Immunol, 152: 1094-1102.

    Punnonen J, Aversa G, Cocks BG, McKenzie AN, Menon S, Zurawski G, de
    Waal Malefyt R, & de Vries JE (1993) Interleukin-13 induces
    interleukin 4-independent IgG4 and IgE synthesis and CD23 expression
    by human B cells. Proc Natl Acad Sci (USA), 90(8): 3730-3734.

    Raaschou Nielsen O, Nielsen ML, & Gehl J (1995) Traffic-related air
    pollution: exposure and health effects in Copenhagen street cleaners
    and cemetery workers. Arch Environ Health, 50(3): 207-213.

    Radl J, Valentijn RM, Haaijman JJ, & Paul LC (1985) Monoclonal
    gamma-pathies in patients undergoing immunosuppressive treatment after
    renal transplantation. Clin Immunol Immunopathol, 37: 98-102.

    Rajka G (1990) Essential aspects of atopic eczema. Berlin, Heidelberg,
    New York, Springer Verlag, 261 pp.

    Rastogi SK, Gupta BN, Husain T, Mathur N, Pangtey BS, & Garg N (1992)
    Respiratory symptoms and ventilatory capacity in metal polishers. Hum
    Exp Toxicol, 11(6): 466-472.

    Rattray NJ, Botham PA, Hext PM, Woodcock DR, Fielding I, Dearman RJ, &
    Kimber I (1994) Induction of respiratory hypersensitivity to
    diphenylmethane-4,4-diisocyanate in guinea pigs. Influence of route of
    exposure. Toxicology, 88: 15-30.

    Raulf M & Konig W (1991) Modulation of leukotriene generation from
    human polymorphonuclear granulocytes by polychlorinated biphenyls
    (PCB). Immunology, 73(4): 485-490.

    Ravetch JV (1994) Fc receptors: rubor redux. Cell, 78: 553-560

    Ravetch JV (1997) Fc receptors. Curr Opin Immunol, 9: 121-125.

    Rea TH (1979) Quantitative enhancement of dinitrochlorobenzene
    responsivity in women receiving oral contraceptives. Arch Dermatol,
    115(3): 361-362.

    Reijula K & Patterson R (1994) Occupational allergies in Finland in
    1981-91. Allergy Proc, 15(3): 163-168.

    Reitamo S, Visa K, Kahonen K, Kayhko K, Stubb S, & Salo OP (1986)
    Eczematous reactions in atopic patients caused by epicutaneous testing
    with inhalant allergens. Br J Dermatol, 114: 303-309.

    Renstrom A, Malmberg P, Larsson K, Sundblad BM, & Larsson PH (1994)
    Prospective study of laboratory-animal allergy: factors predisposing
    to sensitization and development of allergic symptoms. Allergy, 49(7):
    548-552.

    Richardson BC, Liebling MR, & Hudson JL (1990) CD4+ cells treated with
    DNA methylation inhibitors induce autologous B cell differentiation.
    Clin Immunol Immunopathol, 55: 368-381.

    Richardson B, Powers D, Hooper F, Yung RL, & O'Rourke K (1994)
    Lymphocyte function-associated antigen 1 overexpression and T cell
    autoreactivity. Arthritis Rheum, 37: 1363-1372.

    Ridings JE, Barrat MD, Cary R, Earnshaw CG, Eggington CE, Ellis MK,
    Judson PN, Langowski JJ, Marchant CA, Payne MP, Watson WP, & Yih TD
    (1996) Computer prediction of possible toxic action from chemical
    structure: an update on the DEREK system. Toxicology, 106: 267-279.

    Riedel F, Kramer M, Scheibenbogen C, & Rieger CH (1988) Effects of
    SO2 exposure on allergic sensitization in the guinea pig. J Allergy
    Clin Immunol, 82: 527-534.

    Riedler J, Reade T, Dalton M, Holst D, & Robertson C (1994) Hypertonic
    saline challenge in an epidemiologic survey of asthma in children. Am
    J Respir Crit Care Med, 150: 1632-1639.

    Ring J (1988) [Applied allergology], 2nd ed. Munich, MMW Medizin
    Verlag GmbH, pp 85-86 (in German). 

    Ring J (1991) Atopy: Conditions, disease or syndrome? In: Ruzicka T,
    Ring J, & Przybilla B ed. Handbook of atopic eczema. Berlin,
    Heidelberg, New York, Springer Verlag, pp 3-8.

    Ring J & Thomas P (1989) Histamine and atopic eczema. Acta Dermatol
    Venereol, 144: 70-77.

    Ring J, Sedlmeier F, von der Helm D, Mayr T, Walz U, Ibel H, Przybilla
    B, Reimann HJ, & Dorsch W (1988) Histamine and allergic disease. In:
    Ring J & Burg G ed. New trends in allergy. Berlin, Heidelberg, New
    York, Springer Verlag, pp 44-77.

    Ring J, Bieber T, Vieluf D, Kunz B, & Przybilla B (1991a) Atopic
    eczema, Langerhans cells and allergy. Int Arch Allergy Immunol, 94:
    194-201.

    Ring J, Ruzicka T, & Przybilla B (1991b) The pathophysiology of atopic
    eczema. In: Ruzicka T, Ring J, & Przybilla B ed. Handbook of atopic
    eczema. Berlin, Heidelberg, New York, Springer Verlag, pp 330-335.

    Ring J, Abeck D, & Neuber K (1992) Atopic eczema: role of
    microorganisms on the skin surface. Allergy, 47: 265-269.

    Ring J, Behrendt H, Schäfer T, Vieluf D, & Krämer U (1995) Impact of
    air pollution on allergic diseases: Clinical and epidemiologic
    studies. In: Johansson SGO ed. Progress in allergy and clinical
    immunology. Bern, Göttingen, Toronto, Seattle, Hogrefe & Huber
    Publishers, vol 3, pp 174-179.

    Ring J, Brockow K, & Abeck D (1996) The therapeutic concept of patient
    management in atopic eczema. Allergy, 51: 206-215.

    Ritz HL, Conner DS, & Sauter ED (1975) Contact sensitization of
    guinea-pigs with unsaturated and halogenated sultones. Contact
    Dermatitis, 1: 349-359.

    Ritz HL, Evans BL, Bruce RD, Fletcher ER, Fisher GL, & Sarlo K (1993)
    Respiratory and immunological responses of guinea pigs to
    enzyme-containing detergents: A comparison of intratracheal and
    inhalation modes of exposure. Fundam Appl Toxicol, 21: 31-37.

    Roberts DW & Basketter DA (1990a) A quantitative structure
    activity/dose relationship for contact allergenic potential of alkyl
    group transfer agents. Toxicology  in vitro, 4(4/5): 686-687.

    Roberts DW & Basketter DA (1990b) A quantitative structure
    activity/dose response relationship for contact allergic potential of
    alkyl group transfer agents. Contact Dermatitis, 23: 331-335.

    Roberts SA, Reinhardt MC, Paganelli R, & Levinsky RJ (1981) Specific
    antigen exclusion and non-specific facilitation of antigen entry
    across the gut in rats allergic to food proteins. Clin Exp Immunol,
    45: 131-136.

    Robertson CF, Heycock E, Bishop J, Nolan T, Olinsky A, & Phelan PD
    (1991) Prevalence of asthma in Melbourne schoolchildren: Changes over
    26 years. Br Med J, 302: 1116-1118.

    Robinson DS (1998) T cell co-stimulation: a potential therapeutic
    target in asthma? Clin Exp Allergy, 28: 788-790.

    Roche WR, Beasley R, Williams JH, & Holgate ST (1989) Subepithelial
    fibrosis in the bronchi of asthmatics. Lancet, 1: 520-523.

    Röcken M, Racke M, & Shevach EM (1996) IL-4-induced immune deviation
    as antigen-specific therapy for inflammatory autoimmune disease.
    Immunol Today, 17: 225-231.

    Roed-Petersen J (1989) A new glove material protective against epoxy
    and acrylate monomer. In: Frosch PJ, Dooms-Goossens A, Lachapelle J-M,
    Rycroft RJG, & Scheper RJ ed. Current topics in contact dermatitis.
    Berlin, Heidelberg, New York, Springer Verlag, pp 603-606.

    Roitt IM, Brostoff J, & Male DK ed. (1998) Immunology, 5 th ed. St.
    Louis, Missouri, C.V. Mosby & Co., 424 pp.

    Romagnani S (1991) Human Th1 and Th2 subsets: doubt no more.
    Immmunol Today, 12(8): 256-257.

    Romagnani S (1992a) Induction of Th1 and Th2 responses: A key role for
    the "natural" immune response? Immunol Today, 13(10): 379-381.

    Romagnani S (1992b) Human TH1 and TH2 subsets: Regulation of
    differentiation and role in protection and immunopathology. Int Arch
    Allergy Immunol, 98: 279-285.

    Romanski B (1987) The pathology of food allergy studied by gastric
    allergen challenge. In: Brostoff J & Challacombe SJ ed. Food allergy
    and intolerance. London, Baillière Tindall, pp 917-931.

    Romanski B (1989) The pathology of food allergy studied by gastric
    allergen challenge. In: Paul WE ed. Fundamental immunology. New York,
    Raven Press, p 41.

    Rose G (1985) Sick individuals and sick populations. Int J Epidemiol,
    14: 32-38.

    Rose NR & Potter M (1995) The silicone controversy: towards a
    resolution. Immunol Today, 16: 459-460.

    Rosen FS (1987) Autoimmunity and immunodeficiency disease. In: Evered
    D & Whelan J ed. Autoimmunity and autoimmune disease. New York,
    Chichester, Brisbane, Toronto, John Wiley & Sons, pp 135-148 (Ciba
    Foundation Symposium No. 129).

    Rosenstreich DL, Eggleston P, Kattan M, Baker D, Slavin RG, Gergen P,
    Mitchell H, McNiff-Mortimer K, Lynn H, Ownby D, & Malveaux F (1997)
    The role of cockroach allergy and exposure to cockroach allergen in
    causing morbidity among inner-city children with asthma. N Engl J Med,
    336(19): 1356-1363.

    Ross B, McCullough J, & Ownby DR (1991) Evidence of allergenic
    cross-reactivity between banana and watermelon. J Allergy Clin
    Immunol, 87: 274 (abstract).

    Rossman MD, Kern JA, Elias JA, Cullen MR, Epsein PE, Preuss OP,
    Markham TN, & Daniele RP (1988) Proliferative response of
    bronchoalveolar lymphocytes to beryllium. Ann Intern Med, 108:
    687-693.

    Roth MD & Golub SH (1993) Human pulmonary macrophages utilize
    prostaglandins and transforming growth factor beta 1 to suppress
    lymphocyte activation. J Leuk Biol, 53(4): 366-371.

    Rothman KJ (1993) Methodologic frontiers in environmental
    epidemiology. Environ Health Perspect, 101(suppl 4): 19-21.

    Rubin RL (1989) Autoimmune reactions induced by procainamide and
    hydralazine. In: Kammüller M, Bloksma N, & Seinen W ed. Autoimmunity
    and toxicology: Immune disregulation induced by drugs and chemicals.
    Amsterdam, Oxford, New York, Elsevier Science Publishers, pp 119-150.

    Rubin RL, Burlingame RW, Arnott JE, Totoritis MC, McNally EM, &
    Johnson AD (1995) IgG but not other classes of anti-[(H2A-H2B)-DNA] is
    an early sign of procainamide-induced lupus. J Immunol, 154:
    2483-2493.

    Ruffin J, Liu MYG, Sessions R, Banerjee S, & Banerjee UC (1986)
    Effects of certain atmospheric pollutants (SO2, NO2, CO) on the
    soluble amino acids, molecular weight and antigenicity of some
    airborne pollen grains. Cytobios, 46: 119-129.

    Ruzicka T, Ring J, & Przybilla B ed. (1991) Handbook of atopic eczema.
    Berlin, Heidelberg, New York, Springer Verlag, 481 pp.

    Rycroft RJG (1995) Occupational contact dermatitis. In: Rycroft RJG,
    Menné T, Frosch PJ, & Benezra C ed. Textbook of contact dermatitis,
    2nd ed. Berlin, Heidelberg, New York, Springer Verlag, pp 343-400.

    Rycroft RJG, Menné T, Frosch PJ, & Benezra C ed. (1995) Textbook of
    contact dermatitis, 2nd ed. Berlin, Heidelberg, New York, Springer
    Verlag, 400 pp.

    Rystedt I (1985) Atopic background in patients with occupational hand
    eczema. Contact Dermatitis, 12(5): 247-254.

    Saarinen UM & Kajosaari M (1995) Breastfeeding as prophylaxis against
    atopic disease: Prospective follow-up study until 17 years old.
    Lancet, 346(8982): 1065-1069.

    Saihan EM, Burton JL, & Heaton KW (1978) A new syndrome with
    pigmentation, scleroderma, gynaecomastia, Raynaud's phenomenon and
    peripheral neuropathy. Br J Dermatol, 99: 437-440.

    Saint-Remy J-MR (1997) Epitope mapping, a new method for biological
    evaluation and immunotoxicology. Toxicology, 119: 77-81.

    Sakaguchi S & Sakaguchi N (1989) Organ-specific autoimmune disease
    induced in mice by elimination of T cell subsets: V. Neonatal
    administration of cyclosporine A causes autoimmune disease. J Immunol,
    142: 471-480.

    Sakaguchi S & Sakaguchi N (1990) Thymus and autoimmunity: capacity of
    the normal thymus to produce pathogenic self-reactive T cells and
    conditions required for their induction of autoimmune disease. J Exp
    Med, 172: 537-545.

    Salome CM, Peat J., Britton WJ, & Woolcock A (1987) Bronchial
    hyperresponsiveness in two populations of Australian schoolchildren:
    I. Relation to respiratory symptoms and diagnosed asthma. Clin
    Allergy, 17: 271-281.

    Saltini C, Winestock K, Kirby M, Pinkston P, & Crysal RG (1989)
    Maintenance of the alveolitis in patients with chronic beryllium
    disease by beryllium-specific helper T-cells. N Engl J Med, 320:
    1103-1109.

    Samet JM, Lambert WE, Skipper BJ, Cushing AH, Hunt WC, Young SA,
    McLaren LC, Schwab M, & Spengler JD (1993) Nitrogen dioxide and
    respiratory illnesses in infants. Am Rev Respir Dis, 148: 1258-1265.

    Sampson HA (1982) The immunopathogenic role of food hypersensitivity
    in atopic dermatitis. Acta Dermatol Venereol, 176(suppl): 34-37.

    Sampson HA (1983) Role of immediate food hypersensitivity in the
    pathogenesis of atopic dermatitis. J Allergy Clin Immunol, 71:
    473-480.

    Sampson HA & Albergo R (1984) Comparison of results of skin tests,
    RAST, and double-blind, placebo-controlled food challenges in children
    with atopic dermatitis. J Allergy Clin Immunol, 74: 26-33.

    Sampson HA & Metcalfe DD (1992) Food allergies. J Am Med Assoc,
    268(20): 2840-2844.

    Sanchez-Guerrero J, Schur PH, Sergent JS, & Liang MH (1994) Silicone
    breast implants and rheumatic diseases. Arthritis Rheum, 37: 158-168.

    Sanchez-Guerrero J, Colditz GA, Karlson EW, Hunter DJ, Speizer FE, &
    Liang MH (1995) Silicone breast implants and the risk of
    connective-tissue diseases and symptoms. N Engl J Med, 332(25):
    1666-1670.

    Sanderson CJ (1990) Eosinophil differentiation factor (interleukin-5).
    Immunol Ser, 49: 231-256.

    Sanderson DM & Earnshaw CG (1991) Computer prediction of possible
    toxic action from chemical structure; The DEREK system. Hum Exp
    Toxicol, 10: 261-273.

    Sandford A, Shirakawa T, Moffatt MF, Daniels SE, Ra C, Faux JA, Young
    RP, Nakamura Y, Lathrop GM, Cookson WOCM, & Hopkin JM (1993)
    Localization of atopy and beta subunit of high-affinity IgE receptor
    (Fc-epsilon-R1) on chromosome 11q. Lancet, 341: 332-334.

    Sarlo K & Clark ED (1992) A tier approach for evaluating the
    respiratory allergenicity of low molecular weight chemicals. Fundam
    Appl Toxicol, 18: 107-114.

    Sarlo K & Karol MH (1994) Guinea pig predictive tests for respiratory
    allergy. In: Dean JH, Luster MI, Munson AE, & Kimber I ed.
    Immunotoxicology and immunopharmacology, 2nd ed. New York, Raven
    Press, pp 703-720.

    Sarlo K, Ritz HL, Fletcher ER, Schrotel KR, & Clark ED (1997)
    Proteolytic detergent enzymes enhance the allergic antibody responses
    of guinea pigs to nonproteolytic detergent enzymes in a mixture:
    implications for occupational exposure. J Allergy Clin Immunol, 100:
    480-487.

    Satoh T, Kramarik JA, Tollerud DJ, & Karol MH (1995) A murine model
    for assessing the respiratory hypersensitivity potential of chemical
    allergens. Toxicol Lett, 78: 57-66.

    Saval P, Fuglsang G, Madsen C, & Osterballe O (1993) Prevalence of
    atopic disease among Danish school children. J Pediatr Allergy
    Immunol, 4: 117-122.

    Savilahti E & Kuitunen M (1992) Allergenicity of cow milk proteins. J
    Pediatr, 121: S12-S20.

    Savonius B, Keskinen H, Tuppurainen M, & Kanerva L (1993) Occupational
    respiratory disease caused by acrylates. Clin Exp Allergy, 23(5):
    416-424.

    Schachter J & Higgins MW (1976) Median age at onset of asthma and
    allergic rhinitis in Tecumseh, Michigan. J Allergy Clin Immunol,
    57(4): 342-351.

    Schäfer T & Ring J (1997) Epidemiology of allergic diseases. Allergy,
    52(suppl 38): 14-22.

    Schäfer T, Przybilla B, Überla K, Pöschl C, & Ring J (1994) Frequency
    of atopic diseases in children and parental predisposition -- results
    of an epidemiological survey. ACI News, 2(suppl): 170.

    Scheinbart LS, Johnson MA, Gross LA, Edelstein SR, & Richardson BC
    (1991) Procainamide inhibits DNA methyltransferase in a human T cell
    line. J Rheumatol, 18: 530-534.

    Scheinman PL (1996) Allergic contact dermatitis to fragrances: A
    review. Am J Contact Dermatitis, 7: 65-76.

    Scheper RJ & von Blomberg BME (1992) Cellular mechanisms in allergic
    contact dermatitis. In: Rycroft RJG, Menné T, Frosch PJ, & Benezra C
    ed. Textbook of contact dermatitis. Berlin, Heidelberg, New York,
    Springer Verlag, pp 11-27.

    Scheper RJ, von Blomberg M, Boerrigter GH, Bruynzeel DP,
    von Dinther-Janssen ACHM, & Vos A (1983) Induction of immunological
    memory in the skin. Role of local T cell retention. Clin Exp Immunol,
    51: 141-148.

    Schleimer RP & Bochner BS (1998) The role of adhesion molecules in
    allergic inflammation and their suitability as targets of antiallergic
    therapy. Clin Exp Allergy, 28(suppl 39): 15-23.

    Schlipköter HW, Krämer U, Behrendt H, Dolgner R, Stiller-Winkler R,
    Ring J, & Willer HJ (1992) Impact of air pollution on children's
    health: Results from Saxony-Anhalt and Saxony as compared to
    Northrhine-Westphalia. In: Health and ecological effects -- Critical
    issues in the global environment. Pittsburgh, Pennsylvania, Air and
    Waste Management Association, vol 5 (Publication IU-A 2103).

    Schnyder UW (1960) [Neurodermitis, asthma, rhinitis: A genetic
    allergological study.] Basel, Karger, 106 pp.

    Schon-Hegrad MA, Oliver J, McMenamin PG, & Holt PG (1991) Studies on
    the density, distribution and surface phenotype of intraepithelial
    class II MHC antigen (Ia)-bearing dendritic cells (DC) in the
    conducting airways. J Exp Med, 173(6): 1345-1356.

    Schönrich G, Kalinke U, Momburg F, Malissen M, Schmitt-Verhulst AM,
    Malissen B, Hämmerling GJ, & Arnold B (1991) Down-regulation of T cell
    receptors on self-reactive T cells as a novel mechanism for
    extrathymic tolerance induction. Cell, 65: 293-304.

    Schöpf E, Mueller JM, & Ostermann T (1995) [Significance of adjuvant
    basis therapy in chronic relapsing skin disease.] Hautarzt, 44:
    451-454 (in German).

    Schuhmann D, Kubicka-Muranyi M, Mirtschewa J, Gunther J, Kind P, &
    Gleichmann E (1990) Adverse reactions to gold: I. Chronic treatment
    with a Au(I) drug sensitizes mouse T cells not to Au(I) but to Au(III)
    and induces autoantibody formation. J Immunol, 145: 2132-2139.

    Schulz O, Sutton BJ, Beavil RL, Shi J, Sewell HF, Gould HJ, Laing P, &
    Shakib F (1997) Cleavage of the low-affinity receptor for human IgE
    (CD23) by a mite cysteine protease: nature of the cleaved fragment in
    relation to the structure and function of CD23. Eur J Immunol, 27:
    584-588.

    Schulz O, Sewell HF, & Shakib F (1998) Proteolytic cleavage of CD25,
    the a subunit of the human T cell interleukin 2 receptor, by Der p 1,
    a major mite allergen with cysteine protease activity. J Exp Med, 18:
    271-275.

    Schultz Larsen F (1991) Genetic aspects of atopic eczema. In: Ruzicka
    T, Ring J, & Przybilla B ed. Handbook of atopic eczema. Berlin,
    Heidelberg, New York, Springer Verlag, pp 15-26.

    Schultz-Larsen F (1993) The epidemiology of atopic dermatitis. Monogr
    Allergy, 31: 9-28.

    Schultz Larsen F & Hanifin JM (1992) Secular change in the occurrence
    of atopic dermatitis. Acta Dermatol Venereol, 176(suppl): 7-12.

    Schultz-Larsen F, Holm NV, & Hennigsen K (1986) Atopic dermatitis: A
    genetic-epidemiology study in a population-based twin sample. J Am
    Assoc Dermatol, 15: 487-494.

    Schultze-Werninghaus G, Roesch A, Wilhelms OH, Gonsior E, & Meir Sydow
    J (1978) [Bronchial asthma due to occupational allergy of immediate
    type (I) to platinum salts.] Dtsch Med Wochenschr, 103: 972-975 (in
    German).

    Schutz-Kiss D, Popp W, Wagner C, Reiser K, Havelec L, & Zwick H (1995)
    [Sensitization to inhaled allergens in the Vienna population.] Wien
    Klin Wochenschr, 107(11): 331-335 (in German).

    Schuurman H-J, Van Loveren H, Rozing J, & Vos JG (1992) Chemicals
    trophic for the thymus: risk for immunodeficiency and autoimmunity.
    Int J Immunopharmac, 14: 369-375.

    Schwartz J & Morris R (1995) Air pollution and hospital admissions for
    cardiovascular disease in Detroit, Michigan. Am J Epidemiol, 142:
    23-35.

    Schwartz J & Weiss ST (1990) Dietary factors and their relation to
    respiratory symptoms -- The second national health and nutrition
    examination survey. Am J Epidemiol, 132(1): 67-76.

    Schwartz HR, Nerurkar LS, Spies JR, Scanlon RT, & Bellanti JA (1980)
    Milk hypersensitivity: RAST studies using new antigens generated by
    pepsin hydrolysis of beta-lactoglobulin. Ann Allergy, 45: 242-245.

    Scott P (1993) IL-12: Initiation cytokine for cell-mediated immunity.
    Science, 260(5107): 496-497.

    Seager J, Jamison DL, Wilson J, Hayward AR, & Soothill JF (1975) IgA
    deficiency, epilepsy, and phenytoin treatment. Lancet, II: 632-635.

    Sears MR, Herbison GP, Holdaway MD, Hewitt CJ, Flannery EM, & Silva PA
    (1989) The relative risks of sensitivity to grass pollen, house dust
    mite and cat dander in the development of childhood asthma. Clin Exp
    Allergy, 19: 419-424.

    Seaton A, Godden DJ, & Brown K (1994) Increase in asthma: a more toxic
    environment or a more susceptible population? Thorax, 49: 171-174.

    Seifert J, Ring J, & Brendel W (1974) Prolongation of skin allografts
    after oral application of ALS in rats. Nature (Lond), 249(459): 776.

    Seifert J, Ring J, Steininger J, & Brendel W (1977) Influence of the
    immune response on the absorption of protein from the gut. Nutr Metab,
    21(Suppl 1): 256-258.

    Sell S (1987) Immune deficiency states. In: Immunology,
    immunopathology, and immunity. Amsterdam, Oxford, New York, Elsevier
    Science Publishers, 617 pp.

    Senthilselvan A & Habbick BF (1995) Increased asthma hospitalizations
    among registered Indian children and adults in Saskatchewan,
    1970-1989. J Clin Epidemiol, 48(10): 1277-1283.

    Sequeira J, Cesic D, Keser G, Bukelica M, Karanagnostis S, Khamashta
    MA, & Hughes GRV (1993) Allergic disorders in systemic lupus
    erythematosus. Lupus, 2: 187-192.

    Serafini U (1997) Do infections protect against asthma and atopy?
    Allergy, 52(9): 955-957.

    Shaheen SO, Aaby P, Hall A, Barker DJP, Heyes CB, Shiell AW, &
    Goudiaby A (1996) Measles and atopy in Guinea-Bissau. Lancet, 347:
    1792-1796.

    Shaw RA, Crane J, O'Donnell TV, Porteous LE, & Coleman ED (1990)
    Increasing asthma prevalence in a rural New Zealand adolescent
    population: 1975-89. Arch Dis Child, 65: 1319-1323.

    Shaw RA, Crane J, & O'Donnell TV (1991) Asthma symptoms, bronchial
    hyperresponsiveness and atopy in a Maori and European adolescent
    population. NZ Med J, 104: 175-179.

    Shaw RA, Crane J, O'Donnell TV, Lewis ME, Stewart B, & Beasley R
    (1992a) The use of a videotaped questionnaire for studying asthma
    prevalence: A pilot study among New Zealand adolescents. Med J Aust,
    157(5): 311-314.

    Shaw RA, Crane J, Pearce N, Burgess CD, Bremner P, Woodman K, &
    Beasley R (1992b) Comparison of a video questionnaire with the IUATLD
    written questionnaire for measuring asthma prevalence. Clin Exp
    Allergy, 22: 561-568.

    Shaw R, Woodman K, Crane J, Moyes C, Kennedy J, & Pearce N (1994) Risk
    factors for asthma symptoms in Kawerau children. NZ Med J, 107:
    387-391.

    Shaw R, Woodman K, Ayson M, Dibdin S, Winkelmann R, Crane J, Beasley
    R, & Pearce N (1995) Measuring the prevalence of bronchial
    hyper-responsiveness in children. Int J Epidemiol, 24(3): 597-602.

    Sherrill DL, Halonen M, & Burrows B (1994) Relationships between total
    serum IgE, atopy, and smoking: a twenty-year follow-up analysis. J
    Allergy Clin Immunol, 94: 954-962.

    Shimizu H, Masunaga T, Ishiko A, Kikuchi A, Hashimoto T, & Nishikawa T
    (1995)  Pemphigus vulgaris and  Pemphigus foilaceus sera show an
    inversely graded pattern to extracellular regions of desmosomes in
    different layers of human epidermis. J Invest Dermatol, 105: 153-159.

    Shirakawa T, Enomoto T, Shimazu S, & Hopkin JM (1997) The inverse
    association between tuberculin responses and atopic disorder. Science,
    275: 77-79.

    Sibbald B (1993) Epidemiology of allergic rhinitis. In: Burr ML ed.
    Epidemiology of clinical allergy. Basel, Karger, pp 61-79 (Monograph
    on Allergy Series).

    Sibbald B (1997) Familial inheritance of asthma and allergy. In: Kay
    AB ed. Allergy and allergic diseases. Oxford, London, Boston,
    Blackwell Scientific Publications, pp 1177-1186.

    Sibbald B & Rink E (1991a) Labelling of rhinitis and hayfever by
    doctors. Thorax, 46: 378-381.

    Sibbald B & Rink E (1991b) Epidemiology of seasonal and perennial
    rhinitis: Clinical presentation and medical history. Thorax, 46:
    895-901.

    Sigurs N, Bjarnson R, Sigurbergsson F, Kjellman B, & Björkstén (1995)
    Asthma and immunoglobulin E antibodies after respiratory syncitial
    virus bronchiolitis: a prospective cohort study with matched controls.
    Pediatrics, 95: 500-505.

    Silman A & Hochberg MC (1996) Occupational and environmental
    influences on scleroderma. Rheum Dis Clin North Am, 22(4): 737-749.

    Silman A, Black CM, & Welsh KL (1996) Epidemiology, demographics,
    genetics. In: Clements PJ & Furst DE ed. Systemic sclerosis.
    Baltimore, Maryland, Williams and Wilkins, pp 23-49.

    Silvennoinen-Kassinen S (1981) The specificity of nickel sulphate
    reaction  in vitro: A family study and a study of chromium-allergic
    subjects. Scand J Immunol, 13: 231-235.

    Silverman GA, Peri BA, & Rothberg RM (1982) Systemic antibody
    responses of different species following ingestion of soluble protein
    antigens. Dev Comp Immunol, 6: 747-752.

    Singer PA & Theofilopoulos AN (1990) T-cell receptor Vb repertoire
    expression in murine models of SLE. Immunol Rev, 118: 103-127.

    Sinigaglia F (1994) The molecular basis of metal recognition by T
    cells. J Invest Dermatol, 102: 398-401.

    Smalley DL, Shanklin DR, Hall MF, Stevens MV, & Hanissian A (1995)
    Immunologic stimulation of T lymphocytes by silica after use of
    silicone mammary implants. FASEB J, 9: 424-427.

    Smalley DL, Levine JJ, Shanklin DR, Hall MF, & Stevens MV (1997)
    Lymphocyte response to silica among offspring of silicone breast
    implant recipients. Immunobiology, 196: 567-574.

    Smith IM (1983) Epidemiology and natural history of asthma, allergic
    rhinitis and atopic dermatitis (eczema). In: Middleton E, Reed Cellis
    EF, Adkinson NF, Junginger JW, & Busse WW ed. Allergy: Principles and
    practice. St. Louis, Missouri, C.V. Mosby & Co., pp 771-804.

    Song YH, Li Y, & Maclaren NK (1996) The nature of autoantigens
    targeted in autoimmune endocrine diseases. Immunol Today, 17: 232-237.

    Song Z, Casolaro V, Chen R, Georas SN, Monos D, & Ono SJ (1996)
    Polymorphic nucleotides within the human IL-4 promoter that mediate
    over expression of the gene. J Immunol, 156: 424-429.

    Sonnhag C, Karlsson E, & Hed J (1979) Procainamide induced lupus
    erythematosus like syndrome in relation to acetylator phenotype and
    plasma levels of procainamide. Acta Med Scand, 206: 245-251.

    Sparrow GP (1977) A connective tissue disorder similar to vinyl
    chloride disease in a patient exposed to perchlorethylene. Clin Exp
    Dermatol, 2: 17-22.

    Speizer FE & Ferris BGJ (1973) Exposure to automobile exhaust: I.
    Prevalence of respiratory symptoms and disease. Arch Environ Health,
    26: 313-318.

    Spier HW & Sixt I (1955) [Investigation of the dependence of eczema on
    the thickness of the stratum corneum.] Hautarzt, 6: 152-159 (in
    German).

    Sporik R, Holgate ST, Platts Mills TA, & Cogswell JJ (1990) Exposure
    to house-dust mite allergen (Der p I) and the development of asthma in
    childhood: A prospective study. N Engl J Med, 323: 502-507.

    Stadler JC & Karol MH (1985) Use of dose-response data to compare the
    skin sensitizing abilities of dicyclohexylmethane-4,4'-diisocyanante
    and picryl chloride in two animal species. Toxicol Appl Pharmacol, 78:
    445-450.

    Stadler JC & Loveless SE (1992) Guinea pigs exhibit extended latency
    period for the development of sensitivity to an amine. Toxicologist,
    12: 44 (abstract).

    Stahl RE, Friedman-Kien A, Dubin R, Marmor M, & Zolla-Pazner S (1982)
    Immunologic abnormalities in homosexual men. Am J Med, 73(2): 171-178.

    Starzl TE, Nalesnik MA, Porter KA, Ho M, Iwatsuki S, Griffith BP,
    Rosenthal JT, Hakala TR, Shaw BW Jr, Hardesty RL, Atchinson RW, Jaffe
    R, & Bahnson HT (1984) Reversibility of lymphomas and
    lymphoproliferative lesions developing under cyclosporin-steroid
    therapy. Lancet, 8377: 583-587.

    Steinman R, Hoffman L, & Pope M (1995) Maturation and migration of
    cutaneous dendritic cells. J Invest Dermatol, 105: 2S-7S.

    Sterk PJ, Fabbri LM, Quanjer PH, Cockcroft DW, O'Byrne PM, Anderson
    SD, Juniper EF, & Malo JL (1993) Airway responsiveness: Standardized
    challenge testing with pharmacological, physical and sensitizing
    stimuli in adults. Report of Working Party on Standardization of Lung
    Function Tests, European Community for Steel and Coal. Official
    Statement of the European Respiratory Society. Eur Respir J,
    16(suppl): 53-83.

    Stewart GA (1994) Molecular biology of allergens. In: Busse WW &
    Holgate ST ed. Asthma and rhinitis. Oxford, London, Boston, Blackwell
    Scientific Publications, pp 898-932.

    Stiller-Winkler R, Radaszkiewicz T, & Gleichmann E (1988)
    Immunopathological signs in mice treated with mercury compounds-I.
    Identification by the popliteal lymph node assay of responder and
    non-responder strains. Int J Immunopharmacol, 10: 475-484.

    Stites DP & Terr AI (1991) Basic and clinical immunology. London,
    Appleton & Lange, p 797.

    Stoddard JJ & Miller T (1995) Impact of parental smoking on the
    prevalence of wheezing respiratory illness in children. Am J
    Epidemiol, 141: 96-102.

    Stokes CR, Newly JR, Huntley JH, Patel D, & Bourne FJ (1979) The
    immune response of mice to bacterial antigens given by mouth.
    Immunology, 38: 497-502.

    Stokes CR, Newly TJ, & Bourne FJ (1983a) The influence of oral
    immunization on local and systemic immune response to heterologous
    antigens. Clin Exp Immunol, 52: 399-406.

    Stokes CR, Swarbrick ET, & Soothill JF (1983b) Genetic differences in
    immune exclusion and partial tolerance to ingested antigens. Clin Exp
    Immunol, 52(3): 678-684.

    Strachan DP (1988) Damp housing and childhood asthma: validation of
    reporting of symptoms. Br Med J, 297: 1223-1226.

    Strachan DP (1989) Hay fever, hygiene, and household size. Br Med J,
    299: 1259-1260.

    Strachan DP (1995) Epidemiology of hay fever: towards a community
    diagnosis. Clin Exp Allergy, 25(4): 296-303.

    Strachan DP (1996) Socioeconomic factors and the development of
    allergy. Toxicol Lett, 86: 199-203.

    Strachan DP & Anderson HR (1992) Trends in hospital admission rates
    for asthma in children. Br Med J, 304: 819-820.

    Strachan DP & Carey IM (1995) Home environment and severe asthma in
    adolescents: a population based case-control study. Br Med J, 311:
    1053-1056.

    Strachan DP, Golding J, & Anderson HR (1990) Regional variations in
    wheezing illness in British children: effect of migration during early
    childhood. J Epidemiol Community Health, 44: 231-236.

    Strachan DP, Cox BD, Erzinclioglu SW, Walters DE, & Whichelow MJ
    (1991)Ventilatory function and winter fresh fruit consumption in a
    random sample of British adults. Thorax, 46: 624-629.

    Strachan DP, Anderson HR, Limb ES, O'Neill A, & Wells N (1994) A
    national survey of asthma prevalence, severity, and treatment in Great
    Britain. Arch Dis Child, 70: 174-178.

    Strachan DP, Sibbald B, Weiland SK, Ait-Khaled N, Anabwani G, Anderson
    HR, Asher MI, Beasley R, Bjorksten B, Burr M, Clayton T, Crane J,
    Ellwood P, Keil U, Lai C, Mallol J, Martinez F, Mitchell E, Montefort
    S, Pearce N, Robertson C, Shah J, Stewart A, von Mutius E, & Williams
    H (1997) Worldwide variations in prevalence of symptoms of allergic
    rhinoconjunctivitis in children: the international study of asthma and
    allergies in childhood (ISAAC). Pediatr Allergy Immunol, 8: 161-176.

    Strandberg I, Boman G, Hassler L, & Sjoqvist F (1976) Acetylator
    phenotype in patients with hydralazine induced lupoid syndrome. Acta
    Med Scand, 200: 367-371.

    Strobel S & Ferguson A (1984) Immune responses to fed protein antigens
    in mice: III. Systemic tolerance or priming is related to age at which
    antigen is first encountered. Pediatr Res, 18: 588-594.

    Suzuki T, Kanoh T, Kanbayashi M, Todome Y, & Ohkuni H (1993) The
    adjuvant activity of pyrene in diesel exhaust on IgE antibody
    production in mice. Jpn J Allergol, 42: 963-968.

    Swennen B, Buchet JP, Stanescu D, Lison D, & Lauwerys R (1993)
    Epidemiological survey of workers exposed to cobalt oxides, cobalt
    salts, and cobalt metal. Br J Ind Med, 50(9): 835-842.

    Szentivanyi A (1968) The beta adrenergic theory of the atopic
    abnormality in asthma. J Allergy, 42: 203.

    Takafuji S, Suzuki S, Koizumi K, Tadokoro K, Miyamoto T, Ikemori R, &
    Muranaka M (1987) Diesel-exhaust particulates inoculated by the
    intranasal route have an adjuvant activity for IgE production in mice.
    J Allergy Clin Immunol, 79: 639-645.

    Takafuji S, Suzuki S, Koizumi K, Tadokoro K, Ohashi H, Muranaka M, &
    Miyamoto T (1989) Enhancing effect of suspended particulate matter on
    the IgE antibody production in mice. Int Arch Allergy Appl Immunol,
    90: 1-7.

    Tanser AR, Bourke MP, & Blandford AG (1973) Isocyanate asthma:
    respiratory symptoms caused by diphenylmethane di-isocyanate. Thorax,
    28(5): 596-600.

    Tariq SM, Stevens M, Matthews S, Ridout S, Twiselton R, & Hide DW
    (1996) Cohort study of peanut and tree nut sensitisation by the age of
    4 years. Br Med J, 313: 514-517.

    Tarvainen K, Jolanki R, Estlander T, Tupasela O, Pfaffli P, & Kanerva
    L (1995) Immunologic contact urticaria due to airborne
    methylhexahydrophthalic and methyltetrahydrophthalic anhydrides.
    Contact Dermatitis, 32(4): 204-209.

    Taylor SL (1986) Immunologic and allergic properties of cow's milk
    proteins in humans. J Food Protect, 49: 239-250.

    Taylor SL (1992) Chemistry and detection of food allergens. Food
    Technol, 46: 146-152.

    Taylor JS & Praditsuwan P (1996) Latex allergy. Arch Dermatol, 132:
    265-271.

    Taylor B, Wadsworth J, Wadsworth M, & Peckham C (1984) Changes in the
    reported prevalence of childhood eczema since the 1939-45 war. Lancet,
    2: 1255-1257

    Tee RD, Gordon DJ, Gordon S, Crook B, Nunn A, Musk AW, Venables KM, &
    Taylor A (1992a) Immune response to flour and dust mites in a United
    Kingdom bakery. Br J Ind Med, 49(8): 581-587.

    Tee RD, Gordon DJ, van Hage Hamsten M, Gordon S, Nunn A, Johansson SG,
    & Taylor A (1992b) Comparison of allergic responses to dust mites in
    UK bakery workers and Swedish farmers. Clin Exp Allergy, 22(2):
    233-239.

    Teichberg S, Isolauri E, Wapnir RA, Roberts B, & Lifshitz F (1990)
    Development of the neonatal rat small intestinal barrier to
    nonspecific macromolecular absorption: Effect of early weaning to
    artificial diets. Pediatr Res, 28(1): 31-37.

    Tenenbaum SA, Rice JC, Espinoza LR, Cuéllar ML, Plymale DR, Sander DM,
    Williamson LL, Haislip AM, Gluck OS, Tesser JRP, Nogy L, Stribrny Km,
    Bevan JA, & Garry RF (1997) Use of antipolymer assay in recipients of
    silicone breast implants. Lancet, 349: 449-454.

    Terr AI (1994a) The atopic diseases. In: Stites DP, Terr AI, & Parslow
    TG ed. Basic and clinical immunology, 8th ed. East Norwalk,
    Connecticut, Appleton & Lange, pp 27-346.

    Terr AI (1994b) Immune-complex allergic disease. In: Stites DP, Terr
    AI, & Parslow TG ed. Basic and clinical immunology, 8th ed. East
    Norwalk, Connecticut, Appleton & Lange, pp 357-362.

    Tharp MD (1990) Immunodermatology. IgE and immediate hypersensitivity.
    Dermatol Clin North Am, 8(4): 619-631.

    Theofilopoulos AN (1995a) The basis of autoimmunity: Part I.
    Mechanisms of aberrant self-recognition. Immunol Today, 16: 90-98.

    Theofilopoulos AN (1995b) The basis of autoimmunity: Part II. Genetic
    predisposition. Immunol Today, 16: 150-158.

    Theofilopoulos AN & Dixon FJ (1985) Murine models of systemic lupus
    erythematosus. Adv Immunol, 37: 269-390.

    Thomas C, Groten J, Kammüller ME, De Bakker JM, Seinen W, & Bloksma N
    (1989) Popliteal lymph node reactions in mice induced by the drug
    zimeldine. Int J Immunopharmacol, 11: 693-702.

    Thomas C, Lippe W, Seinen W, & Bloksma N (1991) Popliteal lymph node
    enlargement and antibody production in the mouse induced by drugs
    affecting monoamine levels in the brain. Int J Immunopharmacol, 13:
    621-629.

    Thompson HSG & Staines NA (1990) Could specific oral tolerance be a
    therapy for autoimmune disease? Immunol Today, 11: 398-399.

    Thorne PS & Karol MH (1989) Association of fever with late-onset
    pulmonary hypersensitivity responses in the guinea pig. Toxicol Appl
    Pharmacol, 100: 247-258.

    Thorne PS, Hillebrand J, Magreni C, Riley EJ, & Karol MH (1986)
    Experimental sensitization to subtilisin: I. Production of
    immediate- and late-onset pulmonary reactions. Toxicol Appl Pharmacol,
    86: 112-123.

    Thorne PS, Hawk C, Kaliszewski SD, & Gurney PD (1991) The noninvasive
    mouse ear swelling assay. I. Refinements for detecting weak contact
    sensitizers. Fundam Appl Toxicol, 17: 790-806.

    Thurston GD (1996) A critical review of PM10-mortality time-series
    studies. J Expo Anal Environ Epidemiol, 6(1): 3-21.

    Thurston GD, Ito K, Kinney PL, & Lippmann M (1992a) A multi-year study
    of air pollution and respiratory hospital admissions in three New York
    State metropolitan areas: Results for 1988 and 1989 summers. J Expo
    Anal Environ Epidemiol, 2: 429-450.

    Thurston G, D'Souza N, Lippmann M, Bartoszek M, & Fine I (1992b)
    Associations between summer haze air pollution and asthma
    exacerbations: A pilot camp study. Am Rev Respir Dis, 145: A429
    (abstract).

    Tisch R, Yang XD, Singer SM, Liblau LS, Fugger L, & McDevitt HO (1993)
    Immune response to glutamic acid decarboxylase correlates with
    insulitis in non-obese diabetic mice. Nature (Lond), 366: 72-75.

    Tollerud DJ, Weiss ST, Elting E, Speizer FE, & Ferris B (1983) The
    health effects of automobile exhaust. VI. Relationship of respiratory
    symptoms and pulmonary function in tunnel and turnpike workers. Arch
    Environ Health, 38: 334-340.

    Tomasi TB, Barr WG, Challacombe SJ, & Curran G (1983) Oral tolerance
    and accessory-cell function of Peyer's patches. Ann NY Acad Sci, 409:
    145-163.

    Tomura T & van Lancker JL (1988) Procainamide -- DNA interaction. J
    Rheumatol, 15: 59-64.

    Topping MD, Venables KM, Luczynska CM, Howe W, & Newman Taylor A
    (1986) Specificity of the human IgE response to inhaled acid
    anhydride. J Allergy Clin Immunol, 77: 834-842.

    Toren K, Brisman J, & Jarvholm B (1993) Asthma and asthma-like
    symptoms in adults assessed by questionnaires: A literature review.
    Chest, 104(2): 600-608.

    Tournade H, Pelletier L, Pasquier R, Vial MC, Mandet C, & Druet P
    (1990) D-penicillamine- induced autoimmunity in Brown Norway rats:
    Similarities with HgCl2-induced autoimmunity. J Immunol, 144:
    2985-2991.

    Tribe RM, Barton JR, Poston L, & Burney PG (1994) Dietary sodium
    intake, airway responsiveness, and cellular sodium transport. Am J
    Respir Crit Care Med, 149(6): 1426-1433.

    Troisi RJ, Willett WC, Weiss ST, Trichopoulos D, Rosner B, & Speizer
    FE (1995) A prospective study of diet and adult-onset asthma. Am J
    Respir Crit Care Med, 151(5): 1401-1408.

    Tsicopoulos A, Hamid Q, Varney V, Ying S, Moqbel R, Durham SR, & Kay
    AB (1992) Preferential messenger RNA expression of Th1-type cells
    (IFN-gamma+, IL2+) in classical delayed-type (tuberculin)
    hypersensitivity reactions in human skin. J Immunol, 148(7):
    2058-2061.

    Tsunoda K, Ohta Y, Shinogami M, & Soda Y (1995) Does passive smoking
    affect the incidence of nasal allergies? Am J Public Health, 85(7):
    1019-1020.

    Turjanmaa K (1987) Incidence of immediate allergy to latex gloves in
    hospital personnel. Contact Dermatitis, 17(5): 270-275.

    Turjanmaa K (1994) Allergy to natural rubber latex: a growing problem.
    Ann Med, 26: 297-300.

    Turjanmaa K, Alenius H, Mäkinen-Kiljunen S, Reunala T, & Palosuo T
    (1996) Natural rubber latex allergy. Allergy, 51: 593-602.

    Turner MW, Boulton P, Shields JG, Strobel S, Gibson S, Miller HRP, &
    Levinsky RJ (1988) Intestinal hypersensitivity reactions in the rat:
    I. Uptake of intact protein, permeability to sugars and their
    correlation with mucosal mast-cell activation. Immunology, 63(1):
    119-124.

    Turner MW, Barnett GE, & Strobel S (1990) Mucosal mast cell activation
    patterns in the rat following repeated feeding of antigen. Clin Exp
    Allergy, 20(4): 421-427.

    Uehara M (1991) Dry skin and inflammation. In: Ruzicka T, Ring J, &
    Przybilla B ed. Handbook of atopic eczema. Berlin, Heidelberg, New
    York, Springer Verlag, pp 84-89.

    Uetrecht JP (1992) The role of leukocyte-generated reactive
    metabolites in the pathogenesis of idiosyncratic drug reactions. Drug
    Metab Rev, 24: 299-366.

    Ulfvarson U & Alexandersson R (1990) Reduction in adverse effect on
    pulmonary function after exposure to filtered diesel exhaust. Am J Ind
    Med, 17: 341-347.

    Utell MJ, Warren J, & Sawyer RF (1994) Public health risks from motor
    vehicle emissions. Annu Rev Public Health, 15: 157-178.

    Vandenplas O (1995) Occupational asthma caused by natural rubber
    latex. Eur Respir J, 8: 1957-1965.

    Vandenplas O, Delwiche JP, Evrard G, Aimont P, van der Brempt X,
    Jamart J, & Delaunois L (1995) Prevalence of occupational asthma due
    to latex among hospital personnel. Am J Respir Crit Care Med, 151(1):
    54-60.

    Van der Heijden FL, Van Neerven RJJ, Van Katwijk M, Bos JD, &
    Kapsenberg ML (1993) Serum IgE-facilitated allergen presentation in
    atopic disease. J Immunol, 150: 3643-3650.

    Van der Valk P, van Kalken CK, Ketelaars H, Broxterman HJ, Kuiper CM,
    Tsuruo T, Lankelma J, Meijer CJLM, Pinedo HM, & Scheper RJ (1990)
    Distribution of multi-drug resistance-associated P-glycoprotein in
    normal and neoplastic human tissues -- Analysis with 3 monoclonal
    antibodies recognizing different epitopes of the P-Glycoprotein
    molecule. Ann Oncol, 1: 56-64. 

    Van Hoogstraten IMW, Boden D, von Blomberg BME, Kraal G, & Scheper RJ
    (1992) Persistent immune tolerance to nickel and chromium by oral
    administration prior to cutaneous sensitization. J Invest Dermatol,
    99: 608-616.

    Van Hoogstraten IMW, Boos C, Boden D, von Blomberg BME, Scheper RJ, &
    Kraal G (1993) Oral induction of tolerance to nickel sensitization in
    mice. J Invest Dermatol, 101: 26-31.

    Van Hoogstraten IMW, von Blomberg BME, Boden D, Kraal G, & Scheper RJ
    (1994) Non-sensitizing percutaneous skin contacts prevent subsequent
    induction of immune tolerance. J Invest Dermatol, 102: 80-83.

    Van Lee Uwen BH, Martinsom ME, Webb GC, & Young IG (1989) Molecular
    organization of the cytokine gene cluster, involving the human IL-3,
    IL-4, IL-5, and GM-CSF genes on human chromosome 5. Blood, 73:
    1142-1148.

    Van Loveren H, Redegeld F, Matsuda H, Buckley T, Teppema JS, & Garssen
    J (1997) Mast cells. In: Bos JD ed. Skin immune system (SIS), 2nd ed.
    Boca Raton, Florida, CRC Press, pp 160-184.

    Van Niekerk CH, Weinberg EG, Shore SC, Heese HV, & Van Schalkwyk J
    (1979) Prevalence of asthma: A comparative study of urban and rural
    Xhosa children. Clin Allergy, 9: 319-324.

    Varjonen E, Kalimo K, Lammintausta K, & Terho P (1992) Prevalence of
    atopic disorders among adolescents in Turku, Finland. Allergy, 47:
    243-248.

    Veien NK, Hattel T, Justese O, & Norholm A (1987) Dietary restrictions
    in the treatment of adult patients with eczema. Contact Dermatitis,
    17: 223-228.

    Veien NK, Hattel T, & Laurberg G (1993) Low nickel diet: An open
    prospective trial. J Am Acad Dermatol, 29: 1002-1007.

    Veltman C, Lange CE, Juhe S, Stein G, & Bachner V (1975) Clinical
    manifestations and course of vinyl chloride disease. Proc NY Acad Sci,
    246: 6-17.

    Venables KM & Chan Yeung M (1997) Occupational asthma. Lancet,
    349(9063): 1465-1469.

    Venables KM & Newman Taylor A (1990) Exposure-response relationships
    in tetrachlorophthalic anhydride asthma. J Allergy Clin Immunol, 85:
    55-58.

    Venables KM, Dally MB, Burge PS, Pickering CAC, & Newman Taylor A
    (1985a) Occupational asthma in a steel coating plant. Br J Ind Med,
    42: 517-524.

    Venables KM, Topping MD, Howe W, Luczynska CM, Hawkins R, & Newman
    Taylor A (1985b) Interaction of smoking and atopy in producing
    specific IgE antibody against a hapten protein conjugate. Br Med J,
    290: 201-204.

    Venables KM, Topping MD, Nunn A, Howe W, & Newman Taylor A (1987)
    Immunologic and functional consequences of chemical
    (tetrachlorophthalic anhydride) induced asthma after 4 years of
    avoidance of exposure. J Allergy Clin Immunol, 80: 212-218.

    Venables KM, Dally MB, Nunn A, Stevens JF, Stephens R, Farrer N,
    Hunter JV, Stewart M, Hughes EG, & Newman Taylor A (1989) Smoking and
    occupational allergy in a platinum refinery. Br Med J, 299: 939-942.

    Verdier F, Virat M, & Descotes J (1990) Applicability of the popliteal
    lymph node assay in the Brown-Norway rat. Immunopharmacol
    Immunotoxicol, 12: 669-677.

    Verdier F, Patriarca C, & Descotes J (1997) Autoantibodies in
    conventional toxicity testing. Toxicology, 119: 51-58.

    Verhoeff AP, van Strien RT, van Wijnen JH, & Brunekreef B (1995) Damp
    housing and childhood respiratory symptoms: the role of sensitization
    to dust mites and molds. Am J Epidemiol, 141: 103-110.

    Vial T & Descotes J (1994) Contact sensitization assays in guinea
    pigs: are they predictive of the potential for systemic allergic
    reactions? Toxicology, 93: 63-75.

    Vickers CFH (1991) Natural history of atopic eczema. In: Ruzicka T,
    Ring J, & Przybilla B ed. Handbook of atopic eczema. Berlin,
    Heidelberg, New York, Springer Verlag, pp 80-83.

    Vieluf D, Przybilla B, Traenkner I, & Ring J (1990) Oral provocation
    with food additives in atopic eczema. J Allergy Clin Immunol, 85: 206
    (abstract).

    Vieluf D, Kunz B, Bieber T, Przybilla B, & Ring J (1993) "Atopy patch
    test" with aeroallergens in patients with atopic eczema. Allergo J, 2:
    9-12

    Vincent A, Roberts M, Willison H, Lang B, & Newsom-Davis J (1995)
    Autoantibodies, neurotoxins and the nervous system. J Physiol, 89(3):
    129-136.

    Viner AS & Jackman N (1976) Retrospective survey of 1271 patients
    diagnosed as perennial rhinitis. Clin Allergy, 6(3): 251-259.

    Vivier E & Daeron M (1997) Immunoreceptor tyrosine-based inhibition
    motifs. Immunol Today, 18: 286-291.

    Von Blomberg BME, Bruynzeel DP, & Scheper RJ (1990) Advances in
    mechanisms of allergic contact dermatitis:  in vitro  and  in vivo 
    research. In: Marzulli FN & Maibach HI ed. Dermatotoxicology, 4th ed.
    Washington, New York, London, Hemisphere Publishing Corporation, pp
    255-362.

    Von Mutius E, Fritzsch C, Weiland SK, Roll G, & Magnussen H (1992)
    Prevalence of asthma and allergic disorders among children in united
    Germany: A descriptive comparison. Br Med J, 305: 1395-1399.

    Von Mutius E, Martinez FD, Fritzsch C, Nicolai T, Roell G, & Thiemann
    HH (1994a) Prevalence of asthma and atopy in two areas of West and
    East Germany. Am J Respir Crit Care Med, 149: 358-364.

    Von Mutius E, Martinez FD, Fritzsch C, Nicolai T, Reitmeir P, &
    Thiemann HH (1994b) Skin test reactivity and number of siblings. Br
    Med J, 308: 692-695.

    Von Mutius E, Weiland SK, Fritzsch C, Duhme H, & Keil U (1998)
    Increasing prevalence of hay fever and atopy among children in
    Leipzig, East Germany. Lancet, 351: 862-866.

    Vos JG, Younes M, & Smith E ed. (1996) Allergic hypersensitivities
    induced by chemicals: Recommendations for prevention. Boca Raton,
    Florida, CRC Press, 348 pp.

    Vreeburg KJJ, Wilkinson JD, & Scheper RJ (1991) Reduced frequency of
    nickel allergy upon oral nickel contact at an early age. Clin Exp
    Immunol, 85: 441-445.

    Wade JF 3rd & Newman LS (1993) Diesel asthma: Reactive airways disease
    following overexposure to locomotive exhaust. J Occup Med, 35(2):
    149-154.

    Wahlberg JE (1995) Patch testing. In: Rycroft RJG, Menné T, Frosch PJ,
    & Benezra C ed. Textbook of contact dermatitis, 2nd ed. Berlin,
    Heidelberg, New York, Springer Verlag, pp 241-265.

    Wahlberg JE & Boman A (1985) Guinea pig maximisation test. Curr Probl
    Dermatol, 14: 59-106.

    Waite DA, Eyles EF, Tonkin SL, & O'Donnell TV (1980) Asthma prevalence
    in Tokelauan children in two environments. Clin Allergy, 10: 71-75.

    Walker RB & Warin RP (1956) The incidence of eczema in early
    childhood. Br J Dermatol, 68: 182-183.

    Walker FB, Smith PD, & Maibach HI (1967) Genetic factors in human
    allergic contact dermatitis. Int Arch Allergy, 32: 453-462.

    Walker C, Kaegi MK, Braun MD, & Blaser K (1991) Activated T cells and
    eosinophilia in bronchoalveolar lavages from subjects with asthma
    correlated with disease severity. J Allergy Clin Immunol, 88: 935-942.

    Walker-Smith JA (1992) Immunology of gastrointestinal food allergy in
    infancy and early childhood. In: MacDonald TT ed. Immunology of
    gastrointestinal disease. Dodrecht, Kluwer Academic Publishers, pp
    61-73.

    Ward AM, Udnoon S, Watkins J, Walker AE, & Darke CS (1976)
    Immunological mechanisms in the pathogenesis of vinyl chloride
    disease. Br Med J, 1: 936-938.

    Wardlaw A (1993) The role of air pollution in asthma. Clin Exp
    Allergy, 23: 81-96.

    Ware JH, Dockery DW, Spiro A 3rd, Speizer FE, & Ferris BG Jr (1984)
    Passive smoking, gas cooking, and respiratory health of children
    living in six cities. Am Rev Respir Dis, 129(3): 366-374.

    Ware JH, Spengler JD, Neas LM, Samet JM, Wagner GR, Coultas D,
    Ozkaynak H, & Schwab M (1993) Respiratory and irritant health effects
    of ambient volatile organic compounds: The Kanawha County health
    study. Am J Epidemiol, 137(12): 1287-1301.

    Warren DL, Shiotsuka RN, Sangha GK, & Lyon JP (1993) Comparison of
    respiratory sensitization responses to 2,4 and 2,6 toluene
    diisocyanate. Toxicologist, 13: 41 (abstract).

    Wass U & Belin L (1990) An  in vitro method for predicting
    sensitising properties of inhaled chemicals. Scand J Work Environ
    Health, 16(3): 208-214.

    Weeke ER (1987) Epidemiology of hay fever and perennial allergic
    rhinitis. Monogr Allergy, 21: 1-20.

    Weigle WO (1965) The production of thyroiditis and antibody following
    injection of unaltered thyroglobulin into rabbits previously
    stimulated with altered thyroglobulin. J Exp Med, 122: 1049-1062.

    Weiland SK, Mundt KA, Rückmann A, & Keil U (1994) Self-reported
    wheezing and allergic rhinitis in children and traffic density on
    street of residence. Ann Epidemiol, 4: 243-247.

    Weill H, Butcher B, & Charmarajan V (1981) Respiratory and immunologic
    evaluation of isocyanate exposure in a new manufacturing plant.
    Cincinnati, Ohio, National Institute of Occupational Safety and Health
    (NIOSH Technical Report; DHHS (NIOSH) Publication No. 81-125).

    Weinberg DA, Lesser RL, & Vollmer TL (1994) Ocular myasthenia: A
    protean disorder. Surv Ophthalmol, 39: 169-210.

    Weiner HL, Friedman A, Miller A, Khoury SJ, Al-Sabbagh A, Santos L,
    Sayegh M, Nussenblatt RB, Trentham DE, & Hafler DA (1994) Oral
    tolerance: Immunologic mechanisms and treatment of animal and human
    organ-specific autoimmune diseases by oral administration of
    autoantigens. Annu Rev Immunol, 12: 809-837.

    Weinstein RS, Grogan TM, Kuszak JR, Jakate SM, Kluskens LF, & Coon JS
    (1991) Multidrug resistance gene product (P-glycoprotein) in normal
    tissue and tumors. St. Louis, Missouri, Mosby Year Book, Inc., pp
    207-234 (Advances in Pathology and Laboratory Medicine Series).

    Weiss KB, Wagener DK, Aberg N, Engstrom I, & Lindberg U (1989)
    Geographic variations in US asthma mortality: Small-area analyses of
    excess mortality, 1981-1985 -- Allergic diseases in Swedish school
    children. Am J Epidemiol, 78(2): 246-252.

    Weiss KB, Gergen PJ, & Wagener DK (1993) Breathing better or wheezing
    worse? The changing epidemiology of asthma morbidity and mortality.
    Annu Rev Public Health, 14: 491-513.

    Weitzman M, Gortmaker SL, Sobol AM, & Perrin JM (1992) Recent trends
    in the prevalence and severity of childhood asthma. J Am Med Assoc,
    268: 2673-2677.

    Weltzien HU, Moulon C, Martin S, Padovan E, Hartmann U, & Kohler J
    (1996) T cell immune responses to haptens: Structural models for
    allergic and autoimmune reactions. Toxicology, 107: 141-151.

    White IR (1995) Phototoxic and photoallergic reactions. In: Rycroft
    RJG, Menné T, Frosch PJ, & Benezra C ed. Textbook of contact
    dermatitis. Berlin, Heidelberg, New York, Springer Verlag, pp 75-91.

    WHO (in press) Air quality guidelines. Copenhagen, World Health
    Organization, Regional Office for Europe.

    Wick G, Sundick RS, & Albini B (1974) The obese strain (OS) of
    chickens: An animal model with spontaneous autoimmune thyroiditis.
    Clin Immunol Immunopathol, 3: 272-300.

    Widmann FK (1989) An introduction to clinical immunology.
    Philadelphia, Pennsylvania, F.A. Davis, 424 pp.

    Wierenga EA, Smoek M, de Groot C, Chrétien I, Bos JD, Jansen HM, &
    Kapsemberg ML (1990) Evidence for compartmentalization of functional
    subsets of CD4+ T lymphocytes in atopic patients. J Immunol, 144:
    4651-4656.

    Wilkinson JD (1995) The management of contact dermatitis. In: Rycroft
    RJG, Menné T, Frosch PJ, & Benezra C ed. Textbook of contact
    dermatitis, 2nd ed. Berlin, Heidelberg, New York, Springer Verlag, pp
    660-683.

    Williams WJ (1989) Beryllium workers -- sarcoidosis or chronic
    beryllium disease. Sarcoidosis, 6(suppl): 34-35.

    Williams HC (1992) Is the prevalence of atopic dermatitis increasing?
    Clin Exp Dermatol, 17: 385-391.

    Williams IR & Unanue ER (1990) Costimulatory requirements of murine
    Th1 clones: The role of accessory cell-derived signals in response to
    anti-CD3 antibody. J Immunol, 145(1): 85-93.

    Williams WR & Williams WJ (1982) Development of beryllium lymphocyte
    transformation tests in chronic beryllium disease. Int Arch Allergy
    Appl Immunol, 67: 175-180.

    Williams ME, Lichtman AH, & Abbas AK (1990) Anti-CD3 antibody induces
    unresponsiveness to IL-2 in Th1 clones but not in Th2 clones. J
    Immunol, 144(4): 1208-1214.

    Williams HC, Burney PG, Hay RJ, Archer CB, Shipley MJ, Hunter JJ,
    Bingham EA, Finlay AY, Pembroke AC, Graham Brown RA, Atherton DA,
    Lewis-Jones MS, Holden CA, Harper JI, Champion RH, Poyner TF, Launer
    J, & David TJ (1994a) The UK Working Party's diagnostic criteria for
    atopic dermatitis: I. Derivation of a minimum set of discriminators
    for atopic dermatitis. Br J Dermatol, 131(3): 383-396.

    Williams HC, Strachan DP, & Hay RJ (1994b) Childhood eczema: disease
    of the advantaged? Br Med J, 308: 1132-1135.

    Williams HC, Pembroke AC, Forsdyke H, Boodoo G, Hay RJ, & Burney PG
    (1995a) London-born black Caribbean children are at increased risk of
    atopic dermatitis. J Am Acad Dermatol, 32(2/1): 212-217.

    Williams PB, Buhr MP, Weber RW, Volz MA, Koepke JW, & Selner JC
    (1995b) Latex allergen in respirable particulate air pollution. J
    Allergy Clin Immunol, 95(1/1): 88-95.

    Witt C, Stuckey MS, Woolcock A, & Dawkins RL (1986) Positive allergy
    prick tests associated with bronchial histamine responsiveness in an
    unselected population. J Allergy Clin Immunol, 77: 698-702.

    Wjst M, Reitmeir P, Dold S, Wulff A, Nicolai T, von Loeffelholz
    Colberg EF, & von Mutius E (1993) Road traffic and adverse effects on
    respiratory health in children. Br Med J, 307: 596-600.

    Wjst M, Heinrich J, Liu P, Dold S, Wassmer G, Merkel G, Huelsse C, &
    Watchman HE (1994) Indoor factors and IgE levels in children. Allergy,
    49: 766-771.

    Wolf R & Brenner S (1994) An active amide group in the molecule of
    drugs that induce pemphigus: a casual or causal relationship?
    Dermatology, 189: 1-4.

    Wood RA, Mudd KE, & Eggleston PA (1992) The distribution of cat and
    dust mite allergens on wall surfaces. J Allergy Clin Immunol, 89(1/1):
    126-130.

    Wucherpfennig KW, Yu B, Monos DS, Argyris E, Karr RW, Ahmed AR, &
    Strominger JL (1995) Structural basis for major histocompatibility
    complex (MHC)-linked susceptibility to autoimmunity: charged residues
    of a single MHC binding pocket confer selective presentation of
    self-peptides in pemphigus vulgaris. Proc Natl Acad Sci (USA), 92(25):
    11935-11939.

    Wüthrich B (1975) [Immunopathology of constitutional neurodermatitis.]
    Bern, Göttingen, Toronto, Seattle, Hogrefe & Huber Publishers, 178 pp
    (in German).

    Wüthrich B (1990) [Do food allergies of Type III exist?] Allergologie,
    13: 371 (in German).

    Wüthrich B (1991) Minimal forms of atopic eczema. In: Ruzicka T, Ring
    J, & Przybilla B ed. Handbook of atopic eczema. Berlin, Heidelberg,
    New York, Springer Verlag, pp 46-53.

    Wüthrich B (1993) [Food allergies: Frequency of symptoms and the
    allergy-eliciting foods in 402 patients.] Allergologie, 16: 280-287
    (in German).

    Wüthrich B & Hofer T (1984) [Food allergy: The "celery-mugwort-spices
    syndrome" association with mango allergy?] Dtsch Med Wochenschr,
    109(250): 981-986 (in German).

    Wüthrich B, Stäger J, & Johansson SGO (1990) Celery allergy associated
    with birch and mugwort pollinosis. Allergy, 45(80): 566-571.

    Wüthrich B, Schindler C, Leuenberger P, Ackermann-Liebrich U, &
    SAPALDIA Team (1995) Prevalence of atopy and pollinosis in the adult
    population of Switzerland (SAPALDIA-study). Int Arch Allergy Immunol,
    106: 149.

    Xu X & Wang L (1993) Association of indoor and outdoor particulate
    level with chronic respiratory illness. Am Rev Respir Dis, 148(6/1):
    1516-1522.

    Yamakage A, Ishikawa H, Saito Y, & Hattori A (1980) Occupational
    scleroderma-like disorder occurring in men engaged in the
    polymerization of epoxy resins. Dermatologica, 161: 33-44.

    Yamashita U, Takami T, Hamasaka T, & Kitigawa M (1976) The role of
    hapten-reactive T-lymphocytes in the induction of autoimmunity in
    mice: II. Termination of self tolerance to erythrocytes by
    immunization with hapten-isologous erythrocytes. Cell Immunol, 25:
    32-40.

    Yamashita N, Natsuaki M, & Sagami S (1989) Flare-up reaction on murine
    contact hypersensitivity: I. Description of an experimental model:
    rechallenge system. Immunology, 67(3): 365-369.

    Yokoyama Y, Nitta H, Maeda K, & Aoki S (1985) What interactio