hypersensitivity reaction

A hypersensitivity reaction is the immune system overreacting to a drug — treating a medicine as if it were a dangerous invader. The damage comes not from the drug poisoning tissue directly, but from the body's own defensive response, which can range from a mild itchy rash to sudden, life-threatening anaphylaxis.

Immunologists classify hypersensitivity into four types. Type I is immediate, IgE-mediated, and underlies hives, wheezing, and anaphylaxis within minutes (the classic penicillin allergy). Type II involves antibodies attacking drug-coated cells (some drug-induced anaemias). Type III is immune-complex deposition causing serum-sickness-like illness. Type IV is delayed, T-cell-mediated, and appears days later as contact dermatitis or severe skin reactions such as Stevens-Johnson syndrome.

A key feature is that the immune system must usually be 'sensitised' first: the initial exposure may pass quietly while the body learns to recognise the drug, so the dramatic reaction often strikes on a later exposure. This is why a drug tolerated once can suddenly cause a severe reaction the next time.

Severity is not dose-dependent in the ordinary sense — a tiny amount can trigger anaphylaxis in a sensitised person — which makes a documented hypersensitivity an important contraindication. The most feared form, anaphylaxis, is a medical emergency treated first and foremost with intramuscular adrenaline.