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Hypersensitivity Reactions
Hypersensitivity is defined as an exaggerated or inappropriate state of normal immune response with onset of adverse effects on the body.
The lesions of hypersensitivity are a form of antigen antibody reaction.
These lesions are termed as hypersensitivity reactions or immunologic tissue injury, of which 4 types are described: type I, II, III and IV.
Depending upon the rapidity, duration and type of the immune response, these 4 types of hypersensitivity reactions are grouped into immediate and delayed type:
- Immediate type
- Delayed type
| Immediate type | Delayed type |
|---|---|
|
|
Type I (Anaphylactic, atopic)
- Rapidly developing immune response in a previously sensitised person.
- Peak of action: 15-30 minutes.
- Mediated by Ig E antibodies
- It can be due to pollutant, genetic, viral infections
- Eg ;systemic anaphylaxis due to any drug or sting
- Local anaphylaxis due hay fever, bronchial asthma, food allergy etc
Type II: Cytotoxic (Cytolytic) Reaction
- Reaction of humoral antibodies attack on the cell surface antigen an causes cell lysis.
- Peak of action: 15-30 minutes
- IgG or IgM mediated.
- HLA-linked, exposure to foreign tissues/cells
- Examples of type II
Cytotoxic antibodies to blood cells.
- Autoimmune haemolytic anaemia
- Transfusion reactions due to incompatible or mismatched blood transfusion.
- Haemolytic disease of the newborn (erythroblastosis foetalis)
- Idiopathic thrombocytopenic purpura (ITP)
- Leucopenia with agranulocytosis
- Drug-induced cytotoxic antibodies
Cytotoxic antibodies to tissue components.
- In Graves’ disease
- In myasthenia gravis
- In male sterility, antisperm antibody is formed which reacts with spermatozoa and causes impaired motility as well as cellular injury.
- In type 1 diabetes mellitus,
- In hyperacute rejection reaction, antibodies are formed against donor antigen
Type III: Immune Complex Mediated (Arthus) Reaction
- Results from deposition of antigens antigen-antibody complexes on tissues
- Peak action time Within 6 hours
- Mediated by IgG, IgM antibodies
- May be due Persistence of low grade infections environmental antigens, autoimmune process
Common examples of cell injury by type III injury are as under:
- Immune complex glomerulonephritis in which the antigen may be GBM or exogenous agents (e.g. Streptococcal antigen).
- Good pasture syndrome having GBM as antigen
- SLE
- Rheumatoid arthritis
- Farmer’s lung in which actinomycetes-contaminated hay acts as antigen.
- Polyarteritis nodosa and Wegener’s granulomatosis with antineutrophil cytoplasmic antigen.
- Henoch-Schönlein purpura
- Drug-induced vasculitis
Type IV (Delayed hypersensitivity)
- Cell-mediated slow and prolonged response
- Peak action time After 24 hours
- Cell-mediated
- Etiology -CD8+ T cells,cutaneous antigens
- Common examples:
- Reaction against mycobacterial infection e.g. tuberculin reaction, granulomatous reaction in tuberculosis, leprosy.
- Reaction against virally infected cells.
- Reaction against malignant cells in the body.
- Reaction against organ transplantation e.g. transplant rejection, graft versus host reaction
Summary
