A 19-year-old man develops urticaria, bronchospasm, and hypotension within 10 minutes of receiving penicillin intravenously. He was treated with penicillin for a skin infection 3 months ago without incident. Which immunological mechanism is responsible for his current reaction?

• A Type II hypersensitivity — IgG-mediated complement fixation to drug-coated RBCs
• B Type III hypersensitivity — immune complex deposition and complement activation
• C Type I hypersensitivity — IgE-mediated mast cell and basophil degranulation ✓
• D Type IV hypersensitivity — CD8+ T cell cytotoxicity

Explanation

Anaphylaxis is the archetypal Type I (immediate) hypersensitivity reaction, occurring within minutes after re-exposure to an antigen in a sensitized individual. The first exposure induced IgE production that bound to Fc-epsilon-RI receptors on mast cells. Re-exposure causes IgE crosslinking and rapid mast cell degranulation, releasing histamine, tryptase, and leukotrienes, which mediate urticaria, bronchospasm, and vasodilatory hypotension. Type III reactions (serum sickness) occur 7–10 days later and cause fever, arthritis, and vasculitis.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.