Since small molecules leak into and out of the cell around and through the complexes while macromolecules remain inside, the cells normal mechanism for controlling water balance is disrupted. Water is therefore drawn into the cell causing it to swell and burst. The process is so efficient that a very small number of membrane attack complexes can kill a red blood cell. Even an enveloped virus that does not have a large osmotic pressure gradient across its membrane and is therefore not susceptible to such osmotic lysis can still be destroyed by these complexes, presumably because they disorganise the viral membrane. The self-amplifying destructive properties of the complement cascade make it essential that key activated components be rapidly inactivated after they are generated to ensure that the attack does not spread to nearby host cells.
Deactivation is achieved in at least two ways.
First, specific inhibitor proteins in the blood terminate the cascade by either binding or cleaving certain components once they have been activated by proteolytic cleavage. Inhibitor proteins, for example, bind to the activated components of C1 complex and block their further action, while other inhibitor proteins cleave C3b and thereby inactivate it. Without these inhibitors, all of the serum C3 might be depleted by the positive feedback loop created by the alternative pathway. C4b and C3b provide especially dramatic examples. When either of these components is formed by cleavage, it undergoes a series of rapid conformational changes that create a short lived active form.