Two hours after receiving a tetanus toxoid booster, a previously sensitized patient develops intense local swelling, erythema, and hemorrhagic necrosis at the injection site. Biopsy shows dense neutrophilic infiltrate around small vessels with fibrinoid necrosis. Which hypersensitivity type and dominant effector cell are responsible?

• A Type I, mast cell
• B Type II, macrophage
• C Type IV, CD4 positive T cell
• D Type III, neutrophil ✓

Explanation

The Arthus reaction is the prototype local type III hypersensitivity lesion. Preformed IgG meets deposited antigen in vessel walls, immune complexes activate complement, and D5a recruits neutrophils that release lysosomal enzymes causing vasculitis, edema, and hemorrhagic necrosis within hours. Type I reactions peak within minutes and are dominated by mast cell mediators, while the tuberculin reaction of type IV takes 24 to 72 hours and is populated by T cells and macrophages rather than neutrophils.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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