A 40-year-old man develops fever, arthralgia, urticarial rash, and palpable purpura eight days after starting a sulfonamide antibiotic. Serum C3 and C4 levels are low. Which hypersensitivity mechanism explains this presentation?

• A IgE-mediated mast cell degranulation
• B IgG antibody binding to fixed tissue antigens
• C Circulating immune complex deposition activating complement ✓
• D CD8+ T cell killing of drug-modified keratinocytes

Explanation

Serum sickness is a systemic Type III hypersensitivity reaction. Antibodies formed against the drug after about a week form circulating immune complexes that deposit in vessel walls, glomeruli, and joints, activate complement (hence low C3 and C4), and attract neutrophils. The 7 to 10 day latency reflects the time needed for antibody production, distinguishing it from IgE-mediated anaphylaxis which occurs within minutes. Fixed-tissue antigen binding describes Type II reactions such as penicillin-induced hemolytic anemia.

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.