Biologically active complement (C5)-derived peptides and their relevance to disease.

In man and in most mammals, activation of the complement system via either the classical or the alternative pathway results in the generation of biologically active peptides. The most active peptides are C5a and C5a des Arg generated by cleavage of the alpha-chain of native C5. C5a is a potent anaphylatoxin and can induce human polymorphonuclear leukocytes to migrate in a directed fashion, to degranulate, to undergo a burst of oxidative metabolism and to aggregate. Upon generation C5a is converted in serum and plasma to C5a des Arg with loss of the noxious anaphylatoxin activity. C5a/C5a des Arg play important roles in host defenses against bacterial infections and possibly in the mediation of some pathologic lesions such as the leukocyte infiltration seen in the lungs during acute respiratory distress syndrome.