The smaller fragment called C3a serves to increase vascular permeability and promote extravasation of phagocytes, while the larger C3b fragment can be used as an opsonin or bind to either type of C3 convertase to form the trimolecular C5 convertase to activate C5 for the membrane attack complex.

Cleavage of complement C3 by a free floating convertase, thrombin, plasmin or even a bacterial enzyme leads to formation of C3a and C3b fragments.

C3b, the larger fragment, becomes covalently attached to the microbial surface or to the antibody molecules through the thioester domain at the site of complement activation.

[1][2] Since C3 convertases cleave C3 to produce C3b which can then form an additional C3 convertase through the alternative pathway, this is a potential mechanism of signal amplification in the complement cascade resulting in the deposition of large numbers of C3b molecules on the surface of activating particles, enabling opsonisation and acute local inflammation.

Thus it can form covalent amide or ester linkages with the plasma membrane of the pathogen and any associated antibodies, where it then behaves as an opsonin.

[1] Nevertheless, this positive feedback mechanism can be regulated by binding of the control protein, nonproteolytic glycoprotein β1H (factor H), to C3b, which prevents association of factor B, and facilitates the decay-dissociation of Bb in the C3bBb complex, in addition to enhancing proteolytic inactivation of C3b by C3b inactivator (C3bINA – endopeptidase).