Hypersensitivity describes an exaggerated or inappropriate immune response that can cause tissue injury. The classic Gell–Coombs framework groups reactions into four types according to their main immune mechanism. Real diseases can involve more than one pathway, so the categories are useful learning models rather than perfect boxes.
Type I: Immediate, IgE-mediated
After sensitisation, allergen-specific IgE binds to Fc receptors on mast cells and basophils. On re-exposure, allergen cross-linking can trigger mediator release, including histamine. Examples include allergic rhinitis, some forms of asthma and anaphylaxis. The timing is often rapid, although late-phase inflammation can follow.
Type II: Antibody-mediated against cells or tissues
IgG or IgM antibodies recognise antigens on cell surfaces or extracellular structures. Injury may involve complement, phagocytes or antibody-dependent cellular mechanisms; some antibodies alter receptor function without directly destroying cells. Examples commonly taught include autoimmune haemolytic anaemia and Graves disease.
Type III: Immune-complex mediated
Antigen–antibody complexes form and deposit in tissues, where complement activation and inflammatory cells can cause injury. Examples include serum sickness and aspects of systemic lupus erythematosus. The location of deposits and the inflammatory response help explain the affected tissues.
Type IV: T-cell mediated, delayed
Type IV reactions depend on T cells rather than antibodies as the main effectors. Cytokine-producing T cells can recruit and activate other cells, while cytotoxic T cells can injure target cells. Contact dermatitis and the tuberculin skin test are classic examples. The response commonly develops over many hours to days.
Quick comparison
| Type | Main mechanism | Memory cue |
|---|---|---|
| I | IgE, mast cells | Immediate allergy |
| II | IgG/IgM against cell or tissue targets | Antibody targets |
| III | Deposited immune complexes | Complexes settle |
| IV | T cells | Delayed cellular response |
For revision, draw a four-column diagram showing trigger, main immune component, tissue effect and one example. Then test yourself by explaining why each example fits the mechanism. Browse related life-science topics on the SciMantra Resources page.
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