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16th February 2024
Summary
Hypersensitivity refers to an exaggerated systemic response from an otherwise normal immune system. Gell and Coomb’s classification categorises hypersensitivity reactions based on the cellular mechanism.
| Type | Mechanism | Pathology |
|---|---|---|
| I
Anaphylactic |
Immediate reaction
Antigen binds to IgE receptors on a mast cell, causing it to degranulate and release histamine and other vasoactive substances. |
Anaphylaxis
Atopy Drug reactions |
| II
Cytotoxic |
Antigen binds to the surface of IgG and IgM and activates complement, cell lysis and killer cells. | Autoimmune haemolytic anaemia
Goodpasture Syndrome Grave’s disease Blood transfusion reactions Idiopathic thrombocytopenic purpura Myasthenia gravis |
| III
Immune-complex |
Antigen and antibody combine to form a free-moving complex, which then migrates to the tissues where complement is activated. | SLE
Rheumatoid arthritis Serum sickness Post-streptococcal glomerulonephritis Syphilis Reactive arthritis Hypersensitivity pneumonitis |
| IV
Delayed hypersensitivity |
Antigens stimulating T lymphocytes causing lymphokine release | Contact dermatitis
Tuberculin skin reactions Graft rejection Graft vs. host disease Tuberculoid leprosy |
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