A regulatory role for complement in innate immunity and autoimmunity.

The complement system comprises a strong defense against various pathogens and is a major component of our innate immune system. While earlier studies have established a crucial role of complement in recognition, opsonization and enhanced phagocytosis of microorganisms by professional phagocytes such as polymorphonuclear leukocytes and macrophages, recent studies delineate an additional role of complement in initiation and maintenance of the acquired immune response. In addition, it seems that opsonization of apoptotic cells by complement may lead to polarization of the response of professional antigen-presenting cells to a more inflammatory or tolerogenic response. The present review summarizes these different contributions of complement to the shaping of the immune balance.