A farmer previously sensitized to a fungal antigen receives an intradermal injection of the same antigen. Within 4 hours he develops localized swelling, erythema, and hemorrhage at the site. Biopsy shows necrotizing vasculitis with abundant neutrophils infiltrating vessel walls and fibrinoid necrosis. The underlying mechanism is:

• A Deposition of preformed antigen-antibody complexes in tissue vessels activating complement ✓
• B IgE-mediated mast cell degranulation releasing vasoactive amines
• C CD4 T cell mediated delayed-type inflammation peaking at 48 hours
• D Direct antibody binding to fixed tissue antigens with complement lysis of cells

Explanation

The Arthus reaction is a localized type III hypersensitivity response. Preformed circulating IgG complexes with injected antigen in vessel walls, activates complement, and generates C5a, which recruits neutrophils that cause the vasculitis and hemorrhage within hours. Option C describes the tuberculin-type response, which peaks at 48 to 72 hours and lacks fibrinoid vasculitis. Option D is type II cytotoxic hypersensitivity against fixed cellular antigens, not soluble deposited complexes.

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.