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
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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
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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
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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
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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.
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
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.
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
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)
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