Ten days after receiving horse-derived antivenom for a snakebite, a farmer develops fever, urticaria, arthralgia, proteinuria, and cervical lymphadenopathy. Serum C3 and C4 levels are reduced. What is the underlying mechanism?

• A IgE-mediated activation of basophils releasing mediators systemically
• B Antibodies against host cell surface receptors stimulating target cells
• C Deposition of circulating antigen-antibody complexes in tissues with complement activation ✓
• D Delayed-type hypersensitivity mediated by CD4+ TH1 cells against the foreign protein

Explanation

This is classic serum sickness, the prototype systemic Type III reaction. Antibodies formed against the foreign (horse) protein bind residual circulating antigen, forming immune complexes that deposit in glomeruli, joints, skin, and vessel walls, activating complement and recruiting neutrophils. The 7 to 14 day latency reflects the time needed for antibody synthesis, and the fall in B3 and B4 indicates complement consumption. Option A describes immediate anaphylaxis, which occurs within minutes, not days, and lacks hypocomplementemia.

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.