With the active development of molecular-biological science in the last 3-5 years, the complement system became vastly recognized as one of the most important regulators of adaptive immunity. It has been shown that interaction between complement components, included in immune complexes, and complement receptors on a cellular membrane is necessary condition for efficient localization of antigen (Ag) in secondary lymphoid compartment. Transport, trapping and retention of immune complexes on the surface of cells to a large degree depend on functionally active complement system. The important signaling significance of complement receptors and their ligands on different cell types has been recently shown. The complement system plays one of the leading roles in the pathogenesis of autoimmune diseases, wherein, not, as was earlier considered, because of the absence of an adequate clearance of immune complexes, but as a result of maintaining immunologic tolerance. At the present time, complementology is experiencing a new rise, which opens wider possibilities of solving vitally important questions relating to pathogenesis of many diseases, and consequently of providing grounds for new therapeutic approaches.