Complement control protein are proteins that interact with components of the complement system. The complement system is tightly regulated by a network of proteins known as "regulators of complement activation (RCA)" that help distinguish target cells as "self" or "non-self." A subset of this family of proteins, complement control proteins (CCP), are characterized by domains of conserved repeats that direct interaction with components of the complement system. These "Sushi" domains have been used to identify other putative members of the CCP family. There are many other RCA proteins that do not fall into this family. Most CCPs prevent activation of the complement system on the surface of host cells and protect host tissues against damage caused by autoimmunity. Because of this, these proteins play important roles in autoimmune disorders and cancers. Members Most of the well-studied proteins within this family can be categorized in two classes: Membrane-bound complement regulators Membrane Cofactor Protein, MCP (CD46) Decay Accelerating Factor, DAF (CD55) Protectin (CD59) Complement C3b/C4b Receptor 1, CR1 (CD35) Complement Regulator of the Immunoglobulin Superfamily, CRIg Soluble complement regulators Factor H C4-Binding Protein (C4bp) Other proteins with characteristic CCP domains have been identified including members of the sushi domain containing (SUSD) protein family and Human CUB and sushi multiple domains family (CSMD). Mechanisms of protection Every cell in the human body is protected by one or more of the membrane-associated RCA proteins, CR1, DAF or MCP. Factor H and C4BP circulate in the plasma and are recruited to self-surfaces through binding to host-specific polysaccharides such as the glycosaminoglycans. Most CCPs function by preventing convertase activity. Convertases, specifically the C3 convertases C3b.Bb and C4b.2a, are the enzymes that drive complement activation by activating C3b, a central component of the complement system. Some CCPs, such as CD46, recruit other RCAs to proteolytically inactivate developing convertases. CD55 and other CCPs promote the rapid dissociation of active enzymes. Other CCPs prevent the activity of terminal effectors of the complement system, CD59 for example blocks oligomerization of the complement peptide C9 stalling the formation of the Membrane Attack Complex (MAC). For example, C3b.Bb is an important convertase that is part of the alternative pathway, and it is formed when factor B binds C3b and is subsequently cleaved. To prevent this from happening, factor H competes with factor B to bind C3b; if it manages to bind, then the convertase is not formed. Factor H can bind C3b