A 32-year-old man develops acute onset of fever, maculopapular rash, and polyarthritis 10 days after receiving equine anti-tetanus antiserum for a contaminated wound. Serum complement levels are low. The rash and joint symptoms are most likely caused by deposition of immune complexes in small vessels. Which complement component is consumed in the highest quantity in this condition?

• A C1q
• B C2
• C C4
• D C3 ✓

Explanation

Serum sickness is a Type III hypersensitivity reaction caused by immune complex formation between heterologous (equine) proteins and host antibodies. Immune complexes activate complement via the classical pathway, and D3 is consumed in the highest quantity because it is the convergence point of classical, alternative, and lectin pathways, being continuously cleaved by D3 convertases.

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.