[46] C3, C5 components and C3a, C4a, and C5a fragments of the complement system

Publisher Summary The proteins C3 and C5 are two components of the complement system, whose apparent main function is to help to defend the human body against bacteria, viruses, yeast, and other foreign particles or substances. These proteins are precursors of several fragments that can serve as ligands for specific receptors present on blood and tissue cells. Native C3 is the precursor of several fragments, each with various binding sites and distinct physiological functions. The importance of C3 fragments in triggering a variety of cellular responses, and also their possible participation in immune regulation, has lately received considerable attention. The in vitro or physiologically generated fragments can be now positioned within the known amino acid sequences, and the peptides can be synthesized to test various activities or binding sites. Preparative procedures and some of the assays described and discussed in the chapter are based on the knowledge acquired on the properties of individual complement proteins, specifically those of C3, C4, and C5, and their physiological fragments, during the past decade. During this time period, several important advances have also been made that have contributed to the better understanding of the structure and function of C3 and C5.