Antibodies manage to protect us against germs principally by two ways: by enveloping their surfaces (opsonisation), what makes their engulfment and killing by phagocytes easier and more effective or by cell lysis. However, in this case, they need a help from the complement system.
The complement system, consists of more than 35 proteins, which are synthesised at various sites throughout the body. Most of them are produced in the liver and by macrophages what makes them readily available for defence at sites of inflammation where macrophages accumulate. The complement proteins in the plasma represent 10% of its globulin fraction.
The complement proteins are present in the body in an inactive state and their activation proceeds stepwise: once a complement component is active, it subsequently activates the next one, etc.: “a cascade“ reaction. Depending on activation agents, there are three pathways of the complement activation: classical, lectin, and alternative. Except target cells lysis, the complement mediates also other biological functions: chemotaxis, opsonisation, pro-inflammatory activity, and clearance of immune complexes. To ensure that the complement mediated reactions proceed very rapidly yet in a controlled fashion, they are very closely controlled: the complement components are present in limited quantities and in an inactive form, have a very short half-life and are easily inhibited by various inhibitory proteins, like C1 inhibitor, decay accelerating factor, factors I and H etc. Deficiencies of the complement proteins results in severe pathological conditions, like angioneurotic oedema, nocturnal haemoglubinuria, susceptibility to infections, especially those induced by Neisseria species, etc.
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citation: Buc Milan: The complent system and its biological significance. Multimedia support in the education of clinical and health care disciplines :: Portal of Faculty of Medicine, Comenius University [online] , [cit. 05. 12. 2020]. Available from WWW: https://portal.fmed.uniba.sk/articles.php?aid=170. ISSN 1337-9577.