Complement is normally present in the body in an inactive form but when its activity is induced

by antigen-antibody combination or other stimuli, components react in a specific sequence as a cascade.

Basically, the C cascade is a series of reactions in which the preceding components act as enzymes on the succeeding components, cleaving them into dissimilar fragments. The larger fragments usually join the cascade. The smaller fragments which are released

often possess biological effects which contribute to defence mechanisms by amplifying the inflammatory process, increasing vascular permeability, inducing

smooth muscle contraction, causing chemotaxis of leucocytes, promoting virus neutralisation, detoxifying endotoxins and effecting the release of histamine from

mast cells.

The C cascade can be triggered off by two parallel but independent mechanisms or pathways which differ only in the initial steps. Once C3 activation occurs, the subsequent steps are common in both pathways

which have been called the classical C pathway and the alternative or properdin pathway.

The classical pathway is so called because it was the first one identified. But actually it is a more recently evolved mechanism of specific active immunity, while the alternative pathway represents a more primitive system of nonspecific innate immunity.