A patient is prescribed a penicillin and develops an immediate severe reaction. What is the mechanism behind this adverse reaction?

• A. IgE-mediated Type I hypersensitivity causing anaphylaxis
• B. T-cell mediated Type IV hypersensitivity
• C. Immune complex Type III hypersensitivity
• D. Autoimmune mechanism

Answer: (A)

Immediate severe reaction to penicillin is due to IgE-mediated Type I hypersensitivity causing anaphylaxis. In sensitized individuals, penicillin or its breakdown products form antigens that bind IgE on mast cells and basophils. Cross-linking of these IgE molecules triggers rapid degranulation and release of mediators like histamine, leukotrienes, and prostaglandins, leading to sudden vasodilation, increased vascular permeability, bronchoconstriction, and airway edema—the hallmark of anaphylaxis.

The other options involve different immune pathways that don’t explain the immediate reaction: a T-cell–mediated Type IV response is delayed and seen with contact dermatitis; a Type III immune complex reaction involves circulating immune complexes causing serum sickness-like symptoms over days; an autoimmune mechanism isn’t the typical driver of an acute penicillin reaction.