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. All act to disrupt the formation of the convertases or to shorten the life-span of any complexes that do manage to form. Their presence on self-surfaces, and their absence from the surfaces of foreign particles, means that these regulators perform the important task of targeting complement to where it is needed - on the invading bacterium for example - while preventing activation on host tissues.
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 much more easily in the presence of sialic acid, which is a component of most cells in the human body; conversely, in the absence of sialic acid, factor B can bind C3b more easily. This means that if C3b is bound to a "self" cell, the presence of sialic acid and the binding of factor H will prevent the complement cascade from activating; if C3b is bound to a bacterium, factor B will bind and the cascade will be set off as normal.
Read more about this topic: Complement Control Protein
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