The classical complement pathway is primarily initiated by C1q binding to the Fc portion of IgM or IgG antibodies that are complexed with antigen. While C1q can bind directly to some pathogens, this is not its primary or defining mechanism for initiating the classical pathway. Direct binding to microbial surfaces is more characteristic of the lectin pathway (via MBL) or the alternative pathway (via C3b). C4b2a is indeed the C3 convertase of the classical pathway, and all complement pathways converge to form the MAC.
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