The complement system in blood plasma is a major mediator of innate immune defence. The function of complement is to recognize, then opsonize or lyse, particulate materials, including bacteria, yeasts and other microrganisms, host cell debris and altered host cells.
There are three ways in which the complement system protects against infection. First, it generates large numbers of activated complement proteins that bind covalently to pathogens, opsonizing them for engulfment by phagocytes bearing receptors for complement. Second, the small fragments of some complement proteins act as chemoattractants to recruit more phagocytes to the site of complement activation, and also to activate these phagocytes. Third, the terminal complement components damage certain bacteria by creating pores in the bacterial membrane.
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