There are two forms of C5-convertase, both derived from the two C3-convertases. In both cases, the main difference between C3 and C5-convertase is the presence of C3b. Thus, the form derived from the classical complement pathway consists of surface bound C4b, C2a, and C3b while the derivation of the alternative complement pathway consists of two C3b and one Bb (and thus is referred to as C3b2Bb). C5 is then converted by either of these into C5a and C5b. C5b complexes with C6 and C7 to settle on the cell surface, serving to recruit C8 to insert into the cell membrane and trigger the binding and polymerization of C9, thus forming the membrane attack complex or MAC, which can create a pore in cell membranes in order to kill pathogens.