Some of the C3b binds directly to the molecules of C4b . C5 Cleavage of C5 by this C5 convertase initiates the assembly of a set of complement proteins that make up the membrane attack complex. The resulting complex C5b . 8 guides the polymerization of as many as 18 molecules of C9 into a tube inserted into the lipid bilayer of the plasma membrane. This tube forms a channel allowing the passage of ions and small molecules.
Water enters the cell by osmosis and the cell lyses.
The complement system acts in several ways to mobilize defense mechanisms. Opsonization by C3b targets foreign particles for phagocytosis. Chemotaxis by C5a attracts phagocytic cells to the site of damage. This is aided by the increased permeability of the capillary beds mediated by C3a and C5a . The early complement components are also important for solubilizing antigen-antibody complexes assisting in their catabolism and elimination from the body.