Summary

We have now concluded our discussion of the complement system - a biochemical cascade which complements other activities of the immune system. As part of the cascade, proteins are produced which perform these complementary roles and/or go on to react with other proteins to continue the cascade. The complement system produces chemotactic factors, which effectively "summon" immunocompetent cells - which can destroy the initiating pathogens - to the site of the infection. It can also mark pathogens as targets for macrophages and it produces the membrane attack complex, which destroys pathogens through lysis. Finally, it cleans up any neutralised antigen:antibody complexes.

This is all achieved through three different pathways: the classical pathway, the alternative pathway and the lectin pathway. All three of them are different ways of cleaving the important protein C3, which produces C3a and C3b. C3b - which is, additionally, an important opsonin - goes on to form C5-convertase, which cleaves C5 into C5a and C5b. C5b - through a series of intermediate complexes - forms the membrane attack complex, in conjunction with other complement components. This destroys cells through lysis. As a reminder, many of the intermediate products of these pathways, such as C4a, are important anaphylatoxins, which stimulate the release of histamine from mast cells. C5a is, additionally, a chemotactic factor and C3a may be involved in the production of the cytokine ASP.

Due to the dangers that the complement system can pose to the body, it needs to be highly regulated and this need underlies the reality of numerous, complex pathways - for it is important that the complement system only be activated when necessary, but it is equally important for it to be able to be activated quickly.