A 28-year-old woman with SLE has serum complement levels C3 and C4 both markedly reduced. Which pathway of complement activation is predominantly responsible for this pattern of consumption?

• A Alternative pathway activated by bacterial LPS
• B Lectin pathway activated by mannose-binding lectin
• C Terminal complement complex (MAC) formation only
• D Classical pathway activated by immune complexes ✓

Explanation

In SLE, circulating immune complexes (DNA-anti-DNA) activate the classical complement pathway, consuming both C1q, C4, C2, and C3 — hence both C3 and C4 are low. The alternative pathway bypasses C1, C4, and C2, leaving C4 relatively normal. Low C4 with low C3 is the hallmark of classical pathway activation, seen in SLE, post-streptococcal GN, and type I cryoglobulinemia.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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