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Complement is a group of proteins in the blood that facilitate immunological and inflammatory responses. The "complement cascade" is a series of reactions that take place in the blood.
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There are 9 major components of complement, labeled C1 through C9. The cascade can be started in several ways, especially by antigen-antibody complexes. The end-product of the cascade is the "membrane attack unit" (also called terminal complement component), which creates holes in the membrane, or surface, of attacking bacteria, thereby killing them.
CH50 or CH100 are tests for the activity of the complement system. There are also a number of side products of the complement cascade that attract white blood cells and increase the efficiency of certain types of white blood cells to engulf and destroy bacteria.
Some bacteria don't need antibodies to set off the complement cascade. When they are exposed to fresh blood serum, complement is activated by an "alternative pathway." C3, one of the major components of the complement cascade, binds (attaches) to these bacteria directly to eliminate them.
Bound C3, or some modified form of C3, forms a stable unit with other members of the complement cascade to form the membrane attack unit.
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