The membrane attack complex (MAC), frequently referred to as the complement membrane attack complex, is one of the methods used by the immune system to attack threats to the body. It is closely related to the complement system, which exists to aid antibodies and other aspects of the immune system in clearing pathogens from a person's body. The complementary system and the attack complex both involve many varieties of proteins that are found in the blood. The proteins bind to the membranes of pathogenic cells and form a circular pore that allows extracellular substances into the cell. When enough of these pores form, the integrity of the cell is severely compromised and cellular death is almost inevitable.
There are two primary stages through which the different proteins in the membrane attack complex act to destroy pathogenic cells. The first stage, usually referred to as initiation, involves the proteins C5, C6, and C7. Through a procedure involving the cleaving and binding of these proteins, the C7 protein is able to penetrate the pathogenic cell's membrane. Proteins C6 and C5 are bound to protein C7; this initiation stage and insertion into the membrane are necessary for the attack complex to proceed.
The second stage is referred to as the polymerization stage, which involves the proteins C8 and C9 and has the goal of actually forming the pore that will eventually destroy the pathogenic cell. C8 is able to insert itself into the pathogenic cell's membrane because of traits relating to the polarity of the molecules that make up the membrane and the protein. It is then able to induce many C9 proteins to form into a porous structure that penetrates the pathogenic cell's membrane. The structure is connected to the C5, C6, C7, and C8 proteins.
In any immune system response, it is important that the immune system be able to distinguish the body's cells from foreign cells so that it only targets pathogenic cells. This is also true in the membrane attack complex. The protein CD59, also referred to as protectin, is present on normal non-pathogenic cells; it prevents the process from acting on these healthy cells. Many viruses, such as HIV, are able to incorporate parts of host cells, including CD59, into their own viral forms, so they are unaffected.