proteins in the blood that play a part in immune
defense against invading organisms. Complement proteins, in conjunction
with an antibody-antigen complex, initiate a sequence of events -- the
complement cascade -- that results in the destruction of the membrane
of the organism or cell under attack.
A group of proteins in normal blood serum and plasma that, in combination with antibodies, cause
the destruction of antigens, particularly bacteria and foreign blood corpuscles. See also Antibodies; Antigen.