The Antibody‐Complement System and Properdin: A Review

P was originally conceived as a unique serum substance, which in the assumed absence of Ab, inactivated C and, in conjunction with C, participated in the killing of bacteria, the inactivation of viruses, and the hemolysis of abnormal red blood cells. More recent evidence has indicated, on the contrary, that Ab is an essential requirement for these reactions. In this revised orientation of the relationship of P and normal serum activities, the part played by P is uncertain and remains to be determined. There is no rigorous evidence at this time that P is involved in nonspecific resistance to infection or in neoplastic growth. The introduction of P as a serological entity nonetheless contributed a great deal to scientific advance by stimulating and revitalizing research efforts in many neglected areas, particularly those related to non‐specific resistance to infection.