2-10. THE MEMBRANE ATTACK COMPLEX (MAC)
The membrane attack complex (MAC), beginning with the cleavage and
activation of C5, is common to both complement pathways. The activation of C5 by
either C4b2a3b or C3b, Bb, 3b results in two fragments. The smaller C5a fragment is
released into the fluid phase as an anaphylatoxin or chemotactic factor. The larger C5b
fragment binds directly to the activating surface, followed by the binding of C6 and C7
(Figure 2-7). The C5b67 complex provides a binding site for C8 which initiates some
membrane damage. However, the subsequent binding of multiple molecules of C9 is
required for efficient cell lysis.
Figure 2-7. Membrane attack complex.