Cells that have specialized phagocytic functions, primarily macrophages and neutrophils, are the first line of defense against microbes that breach epithelial barriers.

We have introduced these cell types in Chapter 2, and we will discuss many other details of their functions later in this chapter and in other chapters. For now, it is important to know that these phagocytic cells perform two general types of functions in defense against microbes. First, they are able to internalize and kill microbes. Neu-trophils and macrophages are particularly good at this function. Second, phagocytes respond to microbes by producing various cytokines that promote inflammation and also enhance the antimicrobial function of host cells at the site of infection. Among the "professional phagocytes," macrophages are particularly good at this second function. Macrophages are also involved in the repair of damaged tissues, which is another function important in host defense. The essential role that phagocytes play in innate immune defense against microbes is demonstrated by the high rate of lethal bacterial and fungal infections in patients with low blood neutrophil counts caused by bone marrow cancers or cancer therapy and in patients with inherited deficiencies in the functions of phagocytes.

Intraepithelial lymphocyte

FIGURE 4-5 Epithelial barriers. Epithelia at the portals of entry of microbes provide physical barriers, produce antimicrobial substances, and harbor intraepithelial lymphocytes that are believed to kill microbes and infected cells.

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