The term complement refers to the ability of a system of some nonspecific proteins in normal human serum to complement, i.e., augment the effects of other components of immune system, such as antibody. The complement system, which is an important component of the human innate host defense system, consists of approximately 20 proteins that are present in normal human serum.
The complement system is an extremely powerful system comprising of rapidly acting glycoproteins, several proenzymes, and components, and it exists in an inactive state in the plasma. All normal individuals always have complement components in their blood.
Complement shows the following properties:
1. It is present in sera of all mammals including humans and in lower animals including birds, amphibians, and fishes.
2. These are heat-labile substances that are inactivated by heating serum at 56°C for 30 minutes.
3. These are glycoproteins and are synthesized primarily by liver cells and to a very less extent by macrophages and many other cell types. The rate of synthesis of the various complement glycoproteins increase when complement is activated and consumed.
4. The complement usually does not bind to the antigen or antibody but only to antigen–antibody complex.
5. The importance of the complement lies in the fact that it contributes to both the acquired and innate immunity of an individual.
Complement components are designated by numerals, viz., C1–9. These components circulate in plasma in the form of pro-enzymes that are functionally inactive. Activation involves cleav-age by proteolysis into peptide fragments. The fragments are designated with lowercase suffixes—for example, C3 is cleaved into two fragments, C3a and C3b. Normally, the large fragment is designated “b”, and the small fragment “a”. But for historical reasons, with respect to the fragments of C2, the large fragment is designated C2a and the small one is designated C2b.