A laboratory worker repeatedly immunized with horse serum proteins develops painful localized edema, erythema, and necrosis at the injection site 6 hours after the latest booster. Biopsy shows neutrophilic infiltration and fibrinoid necrosis of vessel walls. This reaction is best classified as which type of hypersensitivity?
- A Type III (immune complex-mediated, local Arthus reaction) ✓
- B Type II (cytotoxic)
- C Type I (IgE-mediated immediate)
- D Type IV (delayed-type, T-cell mediated)
Correct answer: A. Type III (immune complex-mediated, local Arthus reaction)
Explanation
The Arthus reaction is a local Type III hypersensitivity reaction. Pre-existing IgG antibodies form immune complexes with injected antigen in vessel walls, activating complement and attracting neutrophils. The 4-8 hour timing and neutrophilic vasculitis distinguish it from Type I (minutes) and Type IV (48-72 hours). Serum sickness is the systemic counterpart.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
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