The Alternative Pathway of Complement — A System for Host Resistance to Microbial Infection

THE alternative pathway of complement activation is that part of the complement system that can provide natural, nonimmune defense against microbial infection. In the early 1950's, Pillemer and his associates found that if a novel protein termed properdin (derived from the Latin verb perdere, "to destroy") was removed from human serum, certain serum bactericidal, virus-neutralizing, and hemolytic activities that appeared not to require antibody were impaired. These observations attracted great interest in the "properdin system" as a nonimmune mechanism for activating complement that was distinct from the antibody-dependent activation of the classical complement components Cl, C4, and C2. However, an . . .