Which complement pathway activation product directly cleaves C3 without involving C1q, MBL, or antibody?

• A C1s cleaving C4 and C2 to form the classical pathway C3 convertase
• B MASP-2 cleaving C4 and C2 to form the lectin pathway C3 convertase
• C C3b binding to factor B, forming the alternative pathway C3 convertase (C3bBb) ✓
• D Properdin stabilizing the C5 convertase

Explanation

The alternative pathway is initiated by spontaneous C3 hydrolysis (tick-over), which allows factor B binding; factor D then cleaves factor B to generate the initial C3 convertase C3bBb, which amplifies C3 activation independently of antibody, C1q, or MBL. The classical pathway relies on C1q-antibody interaction leading to C1s cleavage of C4 and C2. The lectin pathway uses MASP-2 in an analogous fashion after MBL/ficolins bind carbohydrate patterns. Properdin stabilizes the alternative pathway C3 convertase but does not independently cleave C3.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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