Komplementtijärjestelmän

The Komplementtijärjestelmän, or complement system, is a part of the innate immune system composed of a series of plasma proteins that cooperate to defend the body against pathogens. These proteins circulate in an inactive form and become activated in a cascade of proteolytic cleavages.

When a pathogen enters the bloodstream, factors in the complement system are triggered by three main pathways: the classical pathway, which is antibody-dependent; the lectin pathway, initiated by mannose-binding lectin recognizing carbohydrate patterns on microbes; and the alternative pathway, which is constantly active at low levels and amplifies the response. All three converging opportunities lead to the formation of the C3 convertase enzyme that cleaves complement component C3 into C3a and C3b.

C3b acts as an opsonin, coating the surface of foreign cells and marking them for phagocytosis by macrophages and neutrophils. The split products C3a and C5a are potent inflammatory mediators that recruit immune cells, cause vasodilation, and increase vascular permeability. The final part of the cascade generates the membrane attack complex (MAC) from C5b, C6, C7, C8, and C9, which inserts into the pathogen cell membrane and creates pores that lyse the organism.

Regulation of the complement system is crucial to prevent host damage. A number of regulatory proteins, such as factor H, CD55 (DAF), and CD59 (MIR), ensure that complement activation is confined to foreign surfaces and that tissue is spared. Deficiencies or dysregulations of complement proteins can result in increased susceptibility to infections or autoimmune diseases such as systemic lupus erythematosus.