- Exaggerated immunological response to an antigen by a normal immune
system
- Requires previous exposure to the antigen
- Four types based on mechanism involved
- Type I - Anaphylactic
- Type II - Cytotoxic
- Type III - Immune complex
- Type IV - Cell mediated or delayed
- Various clinical conditions are associated with one or more type of
reaction
Type I hypersensitivity
- Immediate or anaphylactic hypersensitivity
- Reaction occurs within 10 - 15 mins of exposure to antigen
- Extent of reaction can range from mild to life threatening
- Clinical features include:
- Urticaria
- Conjunctivitis
- Rhinitis
- Bronchospasm
- Reaction is mediated via IgE
- The main cellular component is the mast cell or basophil
- Reaction is amplified by platelets and eosinophils
- IgE is produced in response to exposure to an antigen or allergen
- IgE binds to cell surface receptors on mast cells
- Cross linking or receptors results in mast cell degranulation via a
calcium influx
- Mediators released include:
- Histamine
- Tryptase
- Kininogenase
- Leukotriene B4
- Prostaglandin D2
- Platelet activating factor
- Mast cell degranulation can also be triggered by:
- Exercise
- Stress
- Chemicals
- Anaphylotoxins
- Reaction to these agents / stimuli is not mediated via IgE
- Treatment is by agents that
- Block histamine receptors
- Inhibit mast cell degranulation
- Leukotriene receptor blockers
- Examples include hayfever and drug allergies
Type II hypersensitivity
- Cytotoxic hypersensitivity
- Affects various organs and tissues
- Antigens are normally endogenous
- Reactions occurs in hours
- Mediated via IgM and IgG antibodies and complement
- Phagocytes and K cells play a role
- Examples include Goodpasture's syndrome and pemphigus
Type III hypersensitivity
- Immune complex hypersensitivity
- The reaction can be general or affect individual tissues
- Reaction occurs several hours after exposure to antigen
- Mediated via soluble immune complexes and complement
- Antibodies are of IgG class
- Antigen can be:
- Exogenous - bacteria, virus or parasites
- Endogenous
- Examples include SLE and rheumatoid arthritis
Type IV hypersensitivity
- Cell mediated or delayed type hypersensitivity
- Reaction peaks after about 48 hours
- Mediated vua T lymphocytes, monocytes and macrophages
- Cytotoxic T cells cause direct damage
- T helper cells secrete cytokines that recruit and activate
macrophages
- The major lymphokines involved include:
- Monocyte chemotactic factor
- Interleukin 2
- TNF - alpha
- Examples include many autoimmune and infectious diseases (e.g.
tuberculosis, toxoplasmosis)
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