Classification of allergic reactions (Gell and Coombs)

Type	Description	Mechanism	Clinical features
I Immediate reaction (30 to 60 min) Accelerated reaction (1 to 72 hours)	Anaphylactic, immediate-type hypersensitivity	Antigen exposure causes release of vasoactive substances, such as histamine, prostaglandins, and leukotrienes from mast cells or basophils. This response is usually, but not always, IgE-dependent.	Anaphylaxis Angioedema Bronchospasm Urticaria (hives)
II	Antibody-dependent cytotoxicity	An antigen or hapten that is intimately associated with a cell binds to antibody, leading to cell or tissue injury.	Hemolytic anemia Interstitial nephritis
III	Immune complex disease	Damage is caused by formation or deposition of antigen-antibody complexes in vessels or tissue.	Serum sickness
IV	Cell-mediated or delayed hypersensitivity	Antigen exposure sensitizes T cells, which then mediate tissue injury.	Contact dermatitis
V (>72 hours)		Uncertain, but probably involving T cell cytotoxicity.	Maculopapular rash

IgE: immunoglobulin E.

Adapted from: Weiss ME, Adkinson NF. Immediate hypersensitivity reactions to penicillin and related antibiotics. Clin Allergy 1988; 18:515.

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