In the late 19th century, serum was found to contain a nonspecific heat-labile complementary principle that interacted with antibodies to induce bacteriolysis. Ehrlich and Morgan termed this factor complement.
The complement system as understood today is a multimolecular system composed of more than 32 proteins and consisting of serum proteins, serosal proteins, and cell membrane receptors that bind to complement fragments. They constitute 10% of the globulin fraction of serum. Many of these proteins are designated by the letter C (C1, 4, 2, 3, 5, 6, 7, 8, and 9) and are assigned numbers in the order of their discovery. See tables 1-6 in Pathophysiology for more information.