C5 is involved in the activation of the lythic pathway within the complement system which is an important factor in innate immunity. The complement pathways can be divided in the activation pathways and lytic pathway. The activation pathways lead via C3 to the cleavage of the fifth complement component C5 into C5a and C5b. C5a induces smooth muscle contraction, increases vascular permeability, causes degranulation of mast cells and basophils, and release of lysosomal enzymes. In addition C5a stimulates the directed migration of neutrophils, eosinophils, basophils and monocytes. Studies indicate the modulation of the acute-phase response in liver and an overall immune response by inducing synthesis of cytokines such as TNF-alpha, IL-1beta, IL-6 and IL-8. C5b initiates the assembly of the membrane attack complex (MAC) that mediates cytolysis.
