The Arthus reaction is the prototypical example of which type of hypersensitivity, and what is the key pathological mediator?

• A Type III (immune complex), complement activation and neutrophil influx ✓
• B Type I (IgE-mediated), mast cell degranulation
• C Type II (cytotoxic), complement-mediated cell lysis
• D Type IV (cell-mediated), T lymphocyte cytotoxicity

Explanation

The Arthus reaction is the classic Type III (immune complex-mediated) hypersensitivity reaction where locally injected antigen forms immune complexes with pre-existing circulating antibodies. These complexes deposit in vessel walls, fix complement, and recruit neutrophils, producing localized vasculitis with edema, hemorrhage, and necrosis. Farmer's lung and serum sickness are systemic equivalents. Type I is immediate hypersensitivity; Type IV is delayed-type hypersensitivity.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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