11 - The Complement System

Abstract

The complement system is comprised of multiple soluble plasma and other body fluid proteins, which function either as enzymes or as binding proteins, together with cellular receptors for many of them, and regulatory membrane proteins found on blood and other tissue cells. These proteins play a critical role in facilitating phagocytosis of immune complexes. There are numerous biological activities associated with complement besides immune lysis. Complement proteins circulate in the blood in an inactivate form. These molecules and their fragments, resulting from the activation process, are significant in the regulation of immune responsiveness. The complement system can be activated by three different pathways: the classical, alternative, and lectin pathways. Under rigid controls, activation of the complement cascade is finely tuned by complement regulatory proteins. Protein inhibitors that occur naturally and block the action of complement components include factor H, factor I, Cl inhibitor, and C4-binding protein (C4BP). Most genes encoding complement component-related proteins in humans have been sequenced and assigned to chromosomal loci. Structurally and/or functionally similar components are organized in major gene clusters in the human genome. Deficiencies of the complement components, although rare, have been reported for most of the constituents. C2 deficiency is the most common complement deficiency disorder. These deficiencies can be inherited or acquired. Complement deficiency has been associated with autoimmune diseases or increased susceptibility to infections.