Cooperation between IgG Fc receptors and complement receptors in host defence

Abstract

Successful host protection against bacterial infectious diseases requires recognition and destruction of potential invading pathogen by monocytes and polymorphonuclear neutrophils (PMN), the so-called ‘professional phagocytes’. Many pathogenic bacteria have evolved mechanisms to evade direct recognition by phagocytes. As a result, the most successful host strategy for phagocytic elimination of potentially virulent bacteria requires coating of the bacteria with serum components, which are in turn recognized by specific receptors on the professional phagocytes. The process of serum protein binding to potentially dangerous environmental agents, both infectious and noninfectious, is known as opsonization. It has long been known that the two most important opsonic activities in serum are antibody and complement. Patients with genetic deficiencies of either complement opsonization or antibody production have markedly increased susceptibility to bacterial infections, demonstrating that both antibody and complement are necessary for optimal host defence. Since the work of Ehlenberger and Nussenzweig in 1975 [1], it has been clear that antibody and complement cooperate in opsonization for phagocytosis. In the intervening two decades since their experiments, the molecular nature of antibody and complement receptors has been elucidated. This increased understanding has allowed many laboratories to begin to identify the molecular mechanisms involved in cooperation between these two important opsonins. The purpose of this chapter is to review current understanding of the mechanism, extent, and significance of this cooperation.