A farm worker develops intense erythema, edema, and hemorrhagic vesicles at a site of repeated tetanus toxoid injection within 4 hours of the dose. Biopsy shows necrotizing vasculitis with abundant neutrophils and deposits of antigen, IgG, and complement in vessel walls. This localized lesion represents:

• A Type I hypersensitivity mediated by IgE cross-linking on cutaneous mast cells
• B Type II hypersensitivity with complement-mediated lysis of fixed tissue cells
• C An Arthus reaction, a localized type III immune complex deposit in vessel walls ✓
• D A delayed-type hypersensitivity response peaking at 48 hours via CD4 T cells

Explanation

The Arthus reaction is the experimental model of localized type III hypersensitivity: preformed circulating IgG meets injected antigen in the vessel wall, forming immune complexes that activate complement, attract neutrophils via C5a, and produce acute vasculitis within hours. Distinguishing features versus type I are the 4-hour latency rather than minutes, the neutrophil-predominant vasculitic infiltrate rather than edematous mast-cell degranulation, and demonstrable antigen-IgG-complement deposits in vessels.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.