complement (kom´ple-ment)

Ehrlich's term for the thermolabile substance, normally present in serum, that is destructive to certain bacteria and other cells sensitized by a specific complement-fixing antibody. C. is a serum protein complex comprising at least 20 distinct proteins, the activity of which is affected by a series of interactions resulting in enzymatic cleavages and which can follow one or the other of at least two pathways. In the case of immune hemolysis (classical pathway), the complex comprises nine components (designated C1 through C9) which react in a definite sequence and the activation of which is effected by the antigen-antibody complex; only the first seven components are involved in chemotaxis, and only the first four are involved in immune adherence or phagocytosis or are fixed by conglutinins. An alternative pathway (see properdin system) is activated by factors other than antigen-antibody complexes and involves components other than C1, C4, and C2 in the activation of C3. See also component of complement. [L. complementum, that which completes, fr. com-pleo, to fill up]
heparin c. the protein component of heparin in blood.


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