Any physiologic process in which a protease directs the cleavage of complement components C4, C2, or factor B which then form the C3 and C5 convertases for the classical and lectin pathways, respectively. This process is involved in direct lysis of target cells, immune adherence and phagocytosis of pathogens, and recruitment and activation of immunocompetent cells.

Any process involved in the activation of any of the steps of the complement cascade, which allows for the direct killing of microbes, the disposal of immune complexes, and the regulation of other immune processes; the initial steps of complement activation involve one of three pathways, the classical pathway, the alternative pathway, and the lectin pathway, all of which lead to the terminal complement pathway.

The sequential activation of serum COMPLEMENT PROTEINS to create the COMPLEMENT MEMBRANE ATTACK COMPLEX. Factors initiating complement activation include ANTIGEN-ANTIBODY COMPLEXES, microbial ANTIGENS, or cell surface POLYSACCHARIDES.

(complement pathway regulation) Factors which act in the complement sequence to activate, modulate or prevent the progression of the reaction.