The classical complement pathway is activated by which of the following?

• A. Most viruses.
• B. Antigen-antibody complexes.
• C. Fungal cell walls.
• D. Mannose in bacterial cell walls.

Answer: (B)

Antigen-antibody complexes trigger the classical complement pathway. When an antibody (usually IgM or IgG) binds its antigen, the Fc portion of the antibody is recognized by C1q in the C1 complex, activating C1r and C1s. This starts the cascade that cleaves C4 and C2 to form the C4b2a C3 convertase, which then splits C3 into C3a and C3b. C3b promotes opsonization and further drives inflammation and the formation of the membrane attack complex. Other options point to lectin or alternative pathway activations (for example, mannose-containing patterns recognized by lectin pathway), not the antibody-driven classical trigger.