The complement pathway consists of a series of over thirty proteins in plasma that are part of the immune response. Activation of the complement system lyses bacterial cells, forms chemotactic peptides (C3a and C5a) attracting immune cells, and increases phagocytotic clearance of infecting cells. Complement can also increase the permeability of vascular walls and cause inflammation. Most complement proteins exist in plasma as inactive precursors that cleave and activate each other in a proteolytic cascade in response to three different mechanisms by which the complement system is activated, the classical pathway, the alternative pathway and the lectin-induced pathway. These three systems are distinct in the initiation of the proteolytic cascade but share most of their components and all three converge in the creation of a C3 convertase that cleaves the C3 complement protein, leading ultimately to the formation of the membrane attack complex, MAC, a pore causing lysis of cells. The classical pathway is activated by the recognition of foreign cells by antibodies bound to the surface of the cells. The alternative and lectin-induced pathways are both antibody independent. Proteolysis is triggered in the alternative pathway by the spontaneous activation of C3 convertase from C3 and is triggered in the lectin-induced pathway by the recognition of carbohydrates on the bacterial cell surface by mannan-binding protein, Mbp. In addition to providing a key part of the response to bacterial infection, the complement system can be involved in the response to fungi, viruses and protists. While activation of the complement system is a key part of the immune system, it must also be kept in check to prevent inappropriate or exaggerated responses. Twelve different proteins have been identified that inhibit complement activation to control the system, including Factor H, Factor I and C1 inhibitor. Deficiencies in components of the complement system have been identified in humans that cause a variety of immune related disorders. C3 deficiency is associated with recurrent bacterial infections, while a lack of C2 can cause antibody-antigen complexes to accumulate and cause the autoimmune disorder systemic lupus erythematosus. People lacking C1 inhibitor have also been identified and found to be prone to uncontrolled complement activation and dangerous swelling through production of C3a and C5a anaphylotoxins.
