The third component of complement: new functions for an old friend.

The present review has evaluated four biologic processes in which the participation of C3 as a primary mediator deserves more extensive investigation. These novel functions of C3 derive in part from the biochemistry ascribable to a thiolester-containing protein and are underscored by its remarkably high concentration in plasma and widespread presence in tissues. Such physiologic characteristics argue that we have only begun to understand the functions of this most versatile complement protein. The availability of full-length cDNA and the ability to manipulate the C3 gene in tissue culture or embryonic stem cells should help to illuminate the pluripotent nature of this fascinating protein.