The complement system is made up of a large number of distinct plasma proteins that react with one another to opsonize pathogens and induce a series of inflammatory responses that help to fight infection. A number of complement proteins are proteases that are themselves activated by proteolytic cleavage. Such enzymes are called zymogens and were first found in the gut. The digestive enzyme pepsin, for example, is stored inside cells and secreted as an inactive precursor enzyme, pepsinogen, which is only cleaved to pepsin in the acid environment of the stomach. The advantage to the host of not being autodigested is obvious.
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1. Janeway Jr C A, et al. (2001). The complement system and innate immunity.