The phagocytes: neutrophils and monocytes

The production and deployment of phago-cytes are central functions of the hemato-poietic system. In the 1950s, radioiso-topic studies demonstrated the high prodution rate and short lifespan of neu-trophils and allowed researchers to fol-low the monocytes as they moved from the marrow through the blood to become tissue macrophages, histiocytes, and den-dritic cells. Subsequently, the discovery of the colony-stimulating factors greatly improved understanding the regulation of phagocyte production. The discovery of the microbicidal myeloperoxidase-H2O2-halide system and the importance of NADPH oxidase to the generation of H2O2 also stimulated intense interest in phagocyte disorders. More recent re-search has focused on membrane recep-tors and the dynamics of the responses of phagocytes to external factors includ-ing immunoglobulins, complement pro-teins, cytokines, chemokines, integrins, and selectins. Phagocytes express toll-like receptors that aid in the clearance of a wide range of microbial pathogens and their products. Phagocytes are also important sources of pro- and anti-inflammatory cytokines, thus participat-ing in host defenses through a variety of mechanisms. Over the last 50 years, many genetic and molecular disorders of phago-cytes have been identified, leading to improved diagnosis and treatment of con-ditions which predispose patients to the risk of recurrent fevers and infectious diseases. (Blood. 2008;112:935-945