The complement system is a major antimicrobial defence system in the human body. During inflammation, complement
activation products attract and activate leucocytes against their targets: bacteria, fungi and parasites. Target cells can become opsonized by complement components C3b and C4b either following deposition of antibodies (the classical pathway, CP), a carbohydrate-binding protein, mannose-binding lectin (MBL) (lectin pathway) or directly via the alternative pathway (AP). Lysis of the target is caused by the membrane attack complex of complement (MAC). The complement system also has a role in maintaining homeostasis of the host, for example by removing macromolecular aggregates and senescent endogenous cells.
Because of its strong potential for generating inflammation and causing tissue destruction, the complement system has to be kept strictly under control. Cells of the host need special protection against the products of complement lysis. Out of the 30 different glycoproteins belonging to the human complement system, 20 act in plasma and 10 are regulators or receptors on cell membranes. These molecules regulate complement activation at different steps of the complement cascade, participate in the clearance of complement-coated particles and/or protect host cells from damage. The importance of complement regulators of complement system becomes apparent in cases where their deficiency results in disease.
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