A farmer previously sensitized to a bacterial vaccine receives an intradermal booster dose. Within 4 hours he develops marked localized swelling, erythema, and hemorrhage at the injection site. Biopsy shows necrotizing vasculitis with abundant neutrophils. This reaction differs from systemic serum sickness chiefly by:

• A Being mediated by IgE rather than IgG
• B Requiring complement activation exclusively via the lectin pathway
• C Involving CD8+ T cell killing of vascular endothelium
• D Having immune complexes form locally at the site of antigen entry rather than circulating and depositing systemically ✓

Explanation

This is an Arthus reaction, a local Type III response in which preformed IgG meets antigen introduced into the skin, forming complexes in situ that activate complement and recruit neutrophils within hours. Systemic serum sickness involves complexes formed in the circulation days after exposure, depositing in glomeruli, joints, and vessels. Both are IgG and complement dependent, so mediation type does not distinguish them.

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.