A 25-year-old woman develops urticaria and bronchoconstriction 15 minutes after a penicillin injection. Positive skin testing to penicillin was noted previously. The mechanism is:
- A Type II hypersensitivity — IgG antibodies binding penicillin-coated RBCs causing complement-mediated hemolysis
- B Type I hypersensitivity — IgE cross-linking on mast cells triggering degranulation and release of histamine and leukotrienes ✓
- C Type III hypersensitivity — immune complex deposition in vessel walls activating complement
- D Type IV hypersensitivity — sensitized T cells releasing cytokines causing inflammation
Correct answer: B. Type I hypersensitivity — IgE cross-linking on mast cells triggering degranulation and release of histamine and leukotrienes
Explanation
Anaphylaxis and immediate hypersensitivity (urticaria, bronchoconstriction within minutes) are Type I IgE-mediated reactions. Prior sensitization induces IgE antibodies that bind Fc epsilon RI on mast cells; re-exposure cross-links IgE, triggering mast cell degranulation and release of histamine, leukotrienes, and PGD2. Type II involves cytotoxic antibodies; Type III involves immune complexes; Type IV is delayed (48–72 h), as in contact dermatitis.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.