The complement system is part of the immune system. It assists antibodies and phagocytes in clearing pathogens and foreign proteins (antigens) from the body. Unlike the antibody-mediated aspect of the immune system, the complement system is inadaptable and unchanging. It represents the innate component of the immune system.
The complement system acts through a complex interaction of proteins and protease enzymes. The complement reaction begins with a stimulus by any number of triggers, causing protease enzymes to cleave proteins to release cytokines. The cytokines, through a series of added cleavages and amplifying reactions, mount a cell-killing attack on foreign cell membranes.
Here are the functions involved in the complement system
1. opsonization: the enhancing of phagocytosis by cells of antigens
2. Chemotaxis: attraction of WBC (leukocytes) and macrophages to the invading organism
3. Lysis: rupture of membranes of foreign cells
4. Clumping or agglutination: clumping of antigen-bearing organisms
Here are two ways the complement system can be activated, and the results:
1. The classical complement pathway: The antigen-antibody response from the non-complement part of the immune system activates the complement system. The complement cascade referred to above yields increased phagocytic activity, recruits WBC to the area of inflammation and activates the cello-membrane destroying pathway.
2. An alternate complement pathway: This does not require a specific antigen-antibody reaction. The foreign protein or foreign cells themselves trigger it. It is a non-specific immune response. The result is the same as in the classical pathway, however, ultimately promoting phagocytosis, chemotaxis and cell lysis.
Please see the reference for details of the complement cascade and the proteins involved.