A farmer receives a second injection of tetanus toxoid into the same arm as previous doses. Four hours later he develops intense localized swelling, erythema, and hemorrhagic exudate at the injection site. Biopsy shows necrotizing vasculitis with abundant neutrophils. This reaction results from:

• A Preformed IgE antibodies cross-linking mast cell Fc receptors
• B IgG antibodies binding antigen to form immune complexes that activate complement locally ✓
• C Cytotoxic CD8 T cells killing antigen-presenting cells at the site
• D Direct activation of the alternative complement pathway by the toxoid

Explanation

The Arthus reaction is a localized type III hypersensitivity response occurring hours after antigen exposure in a previously sensitized individual. Preformed circulating IgG complexes with locally injected antigen, deposit immune complexes in vessel walls, fix complement, and attract neutrophils, causing edema, hemorrhage, and vasculitis. It differs from serum sickness by being confined to the injection site rather than systemic. Option A describes anaphylaxis, which develops within minutes rather than hours.

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.