Editing Immune system (section)

=== Humoral defenses ===
The complement system is a [[biochemical cascade]] that attacks the surfaces of foreign cells. It contains over 20 different proteins and is named for its ability to "complement" the killing of pathogens by [[antibody|antibodies]]. Complement is the major humoral component of the innate immune response.<ref name=Rus>{{cite journal | vauthors = Rus H, Cudrici C, Niculescu F | title = The role of the complement system in innate immunity | journal = Immunologic Research | volume = 33 | issue = 2 | pages = 103–12 | year = 2005 | pmid = 16234578 | doi = 10.1385/IR:33:2:103 | s2cid = 46096567 }}</ref><ref name=Degn_2013>{{cite journal | vauthors = Degn SE, Thiel S | title = Humoral pattern recognition and the complement system | journal = Scandinavian Journal of Immunology | volume = 78 | issue = 2 | pages = 181–93 | date = August 2013 | pmid = 23672641 | doi = 10.1111/sji.12070 | doi-access = free }}</ref> Many species have complement systems, including non-[[mammal]]s like plants, fish, and some [[invertebrate]]s.{{sfn|Bertok|Chow|2005|pp= [https://books.google.com/books?id=DW3V2Uc-m8EC&q=%22complement+system%22&pg=PA17 112–113]}} In humans, this response is activated by complement binding to antibodies that have attached to these microbes or the binding of complement proteins to [[carbohydrate]]s on the surfaces of [[microbe]]s. This recognition [[cell signaling|signal]] triggers a rapid killing response.<ref>{{cite book | vauthors = Liszewski MK, Farries TC, Lublin DM, Rooney IA, Atkinson JP | title = Control of the Complement System | volume = 61 | pages = 201–283 | year = 1996 | pmid = 8834497 | doi = 10.1016/S0065-2776(08)60868-8 | isbn = 978-0-12-022461-6 | series = Advances in Immunology | url-access = registration | url = https://archive.org/details/advancesinimmuno61dixo/page/201 }}</ref> The speed of the response is a result of signal amplification that occurs after sequential [[proteolysis|proteolytic]] activation of complement molecules, which are also proteases. After complement proteins initially bind to the microbe, they activate their protease activity, which in turn activates other complement proteases, and so on. This produces a [[catalysis|catalytic]] cascade that amplifies the initial signal by controlled [[positive feedback]].<ref>{{cite journal | vauthors = Sim RB, Tsiftsoglou SA | s2cid = 24505041 | title = Proteases of the complement system | journal = Biochemical Society Transactions | volume = 32 | issue = Pt 1 | pages = 21–27 | date = Feb 2004 | pmid = 14748705 | doi = 10.1042/BST0320021 | url = http://pdfs.semanticscholar.org/9df4/e40fdcd4e1ba21a7047dca82ddf683c11d61.pdf | archive-url = https://web.archive.org/web/20190302045456/http://pdfs.semanticscholar.org/9df4/e40fdcd4e1ba21a7047dca82ddf683c11d61.pdf | url-status = dead | archive-date = 2019-03-02 }}</ref> The cascade results in the production of peptides that attract immune cells, increase [[vascular permeability]], and [[opsonin|opsonize]] (coat) the surface of a pathogen, marking it for destruction. This deposition of complement can also kill cells directly by disrupting their [[cell membrane|plasma membrane]] via the formation of a [[Complement membrane attack complex|membrane attack complex]].<ref name=Rus />