The complement system is a part of the innate immune system that promotes the ability of antibodies and phagocytic cells to clear microbes, induces inflammation, and sometime directly attacks a pathogen. Complement activation includes proteolytic cascades where a zymogen is activated to its protease that cleaves and thus induces the next complement protein in the cascade. Three different pathways recognize molecules on microbial surfaces differently and can be distinctly activated. Each pathway converges on a common pathway at the formation of the C3 convertase complex. This portion of the cascade culminates in the formation of the C5b-9 complex, or the membrane attack complex.
While complement is important to innate immunity, it also interacts with and is affected by the activated coagulation system. Notably, complement is a highly regulated system controlled by both membrane bound and soluble inhibitors. Hyperactivation of complement due to loss of control mechanisms is often associated with thrombotic disease.
Int. J. Mol. 2019, 20(14), 3550
https://www.mdpi.com/1422-0067/20/14/3550