CDC classical pathway (IgG1, IgG3, IgM). The binding of the C1q component to Fc portions of IgG or IgM triggers a proteolytic cascade, resulting in the generation of large amounts of C3b. C3b molecules act as opsonins but also bind to the C3 convertase to form a C5 convertase, leading to the generation of the membrane attack complex (MAC), which kills the target.