The classical pathway of complement is activated by immune complexes. What is the correct sequence of events from C1q binding to opsonisation?

• A C1q → C3 → C4 → C5 → C3b
• B C3 → C1q → C4 → C2 → C3 convertase
• C C1q → C1s → C1r → C2 → C4 → C3b
• D C1q → C1r → C1s → C4 → C2 → C3 convertase (C4b2a) → C3b (opsonin) ✓

Explanation

Classical pathway: IgG/IgM immune complexes bind C1q, causing conformational change activating C1r (serine protease) which cleaves and activates C1s. C1s cleaves C4 into C4a + C4b (C4b binds surface), then cleaves C2 into C2a + C2b. C4b2a = classical C3 convertase, which cleaves C3 into C3a (anaphylatoxin) + C3b (opsonin). C3b accumulates on target surface and acts as the most important opsonin for phagocytosis via CR1 (complement receptor 1 = CD35) on neutrophils and macrophages. This sequence (C1q-r-s → C4 → C2 → C3 → C5) is tested frequently in NEET PG.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.