Editing Complement system (section)

{{Short description|Part of the immune system that enhances the ability of antibodies and phagocytic cells}}
{{About|an aspect of the immune system||Complement (disambiguation){{!}}Complement}}
[[File:Complement pathway.svg|thumb|lang=en|upright=1.4|Scheme of the complement system]]

The '''complement system''', also known as '''complement cascade''', is a part of the [[humoral]], [[innate immune system]] and enhances (complements) the ability of [[antibodies]] and [[phagocytic cell]]s to clear [[microbes]] and damaged cells from an organism, promote [[inflammation]], and attack the [[pathogen]]'s [[cell membrane]].<ref name=":1">{{Cite book |title=Immunobiology: The Immune System in Health and Disease |vauthors=Janeway Jr CA, Travers P, Walport M, Shlomchik MJ |publisher=Garland Science |year=2001 |location=New York |chapter=The complement system and innate immunity |access-date=25 February 2013 |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK27100}}</ref> Despite being part of the innate immune system, the complement system can be recruited and brought into action by antibodies generated by the [[adaptive immune system]].

The complement system consists of a number of small, inactive, liver synthesized [[protein precursor]]s circulating in the [[blood]]. When stimulated by one of several triggers, [[protease]]s in the system [[protease|cleave specific proteins]] to release [[cytokine]]s and initiate an amplifying cascade of further cleavages. The end result of this ''complement activation'' or ''complement fixation'' cascade is stimulation of [[phagocyte]]s to clear foreign and damaged material, [[inflammation]] to attract additional phagocytes, and [[immunologic activation|activation]] of the cell-killing [[membrane attack complex]]. About 50 proteins and protein fragments make up the complement system, including [[plasma protein]]s, and [[cell membrane receptor]]s. They account for about 10% of the [[globulin]] fraction of blood serum.<ref>{{Cite journal |vauthors=Glovsky MM |date=9 November 2019 |title=Complement-Related Disorders: Background, Pathophysiology, Activation |url=https://emedicine.medscape.com/article/136368-overview |journal=Medscape |veditors=Talavera F, Dreskin SC, Kaliner MA}}</ref>

Three biochemical pathways activate the complement system: the [[classical complement pathway]], the [[alternative complement pathway]], and the [[lectin pathway]].<ref name="Abbas_2010">{{Cite book |url=https://archive.org/details/cellularmolecula00abba_1/page/272 |title=Cellular and Molecular Immunology |vauthors=Abbas AK, Lichtman AH, Pillai S |publisher=Elsevier |year=2010 |isbn=978-1-4160-3123-9 |edition=6th |pages=[https://archive.org/details/cellularmolecula00abba_1/page/272 272–288]}}</ref> The alternative pathway accounts for the majority of terminal pathway activation and so therapeutic efforts in disease have revolved around its inhibition.<ref name=":0">{{Cite journal |vauthors=Tzoumas N, Hallam D, Harris CL, Lako M, Kavanagh D, Steel DH |date=November 2020 |title=Revisiting the role of factor H in age-related macular degeneration: Insights from complement-mediated renal disease and rare genetic variants |journal=Survey of Ophthalmology |volume=66 |issue=2 |pages=378–401 |doi=10.1016/j.survophthal.2020.10.008 |pmid=33157112 |s2cid=226274874}}</ref>