The term 'complement' (C) refers to a system of factors that occur in normal serum and are activated characteristically by antigen-antibody interaction and subsequently mediate a number of biologically significant consequences.

Towards the end of the nineteenth century, it was noticed that bactericidal, bacteriolytic and hemolytic actions of the appropriate antibodies required the participation of a heat labile components present in the normal sera of human beings and animals. Buchner

(1889) was the first to observe that the bactericidal effect of serum was destroyed by heating at 55 °C for one hour. Pfieffer (1894) discovered the cholera vibrios were lysed when injected intraperitoneally into specifically immunised guinea pigs (bacteriolysis in vivo or Pfieffer's phenomenon). Bordet (1895) extended these observations and established that immunebacteriolysis and hemolysis required two factors the heat stable antibody and a heat labile factor, which

was called alexine. This term has been replaced by the present name complement which was coined by Ehrlich, because this factor complemented the action

of the antibody.