Phagocytes, including neutrophils and macrophages, are cells whose primary function is to identify, ingest, and destroy microbes. The functional responses of phagocytes in host defense consist of sequential steps: recruitment of the cells to the sites of infection, recognition of and activation by microbes, ingestion of the microbes by the process of phagocytosis, and destruction of ingested microbes. In addition, through direct contact and by secreting proteins, phagocytes communicate with other cells in ways that promote or regulate immune responses. The effector functions of phagocytes are important in innate immunity, discussed in Chapter 4, and also in the effector phase of some adaptive immune responses, as we will discuss in Chapter 10. As a prelude to more detailed discussions of the role of phagocytes in immune responses in later chapters, we will now describe their morphologic features and briefly introduce the functional responses of neutrophils and macrophages.
Neutrophils, also called polymorphonuclear leukocytes, are the most abundant population of circulating white blood cells and mediate the earliest phases of inflammatory reactions. Neutrophils circulate as spherical cells about 12 to 15 |jm in diameter with numerous membranous projections. The nucleus of a neutrophil is segmented into three to five connected lobules, hence the synonym polymorphonuclear leukocyte (Fig. 2-1A). The
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