hypersensitivity reaction (immune system disorder) to therapeutic agents that occasionally occurs on subsequent exposure to a drug against which an individual has already produced antibodies (antibody). Some drugs rarely cause allergic reactions (e.g., tetracyclines, digitalis), while others frequently provoke allergy (e.g., penicillin). Symptoms vary with the drug and the sensitivity of the affected person but include, as separate reactions, hives (urticaria), serum sickness, and, rarely, anaphylaxis (collapse of the circulatory system with accompanying respiratory symptoms). Several drugs can successfully counteract these allergic symptoms; after drug allergy has been established, lifelong avoidance of the offending drug, and often of its derivatives, must be observed.