Serum complement cascade, a part of innate immunity required for host protection against invading pathogens, is also a mediator of various forms of disease and injury. It is activated by classical, lectin, and alternative pathways that lead to activation of C3 component by C3 convertases, release of C3b opsonin, C5 conversion and eventually membrane attack complex formation. The tightly regulated activation process yields also C3a and C5a anaphylatoxins, which target a broad spectrum of immune and non-immune cells.
The complement system, which emerged about 600-700 million years ago, constitutes an important part of the innate immune system that is designed to eliminate "harmful" substances from the body. This elimination is tightly regulated by complement cascade inhibitors. The complement cascade inhibitors include both plasma proteins factor H and C4 binding protein (C4BP) and membrane proteins primarily complement receptor type 1 (also known as CD35 or CR1), decay-accelerating factor (DAF) and membrane cofactor protein (MCP). In addition, control is also achieved through the activity of C1 inhibitor (C1-INH), carboxypeptidase N, and CD59.
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