Which complement pathway is activated by antibodies bound to antigen?

• A. Alternative pathway
• B. Lectin pathway
• C. Classical pathway
• D. Terminal pathway

Answer: (C)

Antibody-bound antigen activates the classical complement pathway. When antibodies such as IgM (which is especially effective) or IgG bind to an antigen, the Fc portion of these antibodies is recognized by the C1 complex (C1q with associated serine proteases C1r and C1s). This engagement triggers activation of C1r and C1s, leading to cleavage of C4 and C2 to form the C3 convertase of the classical pathway (C4b2a). This sets off the cascade that cleaves C3, promotes opsonization, and ultimately leads to the membrane attack complex via the terminal steps. The alternative pathway is antibody-independent and begins with spontaneous C3 hydrolysis, while the lectin pathway is activated by mannose-binding lectin binding to sugars on pathogens; the terminal pathway refers to the final formation of the MAC, which can be reached through any pathway after C5 activation.