Microbiology, 2021

17.3 • Cellular Defenses 707
Figure 17.17 Natural killer cell with perforin-containing granules. (credit: modification of work by Rolstad B)
Monocytes
The largest of the white blood cells, monocytes have a nucleus that lacks lobes, and they also lack granules in
the cytoplasm (Figure 17.18). Nevertheless, they are effective phagocytes, engulfing pathogens and apoptotic
cells to help fight infection.
When monocytes leave the bloodstream and enter a specific body tissue, they differentiate into tissue-specific
phagocytes called macrophages and dendritic cells. They are particularly important residents of lymphoid
tissue, as well as nonlymphoid sites and organs. Macrophages and dendritic cells can reside in body tissues for
significant lengths of time. Macrophages in specific body tissues develop characteristics suited to the
particular tissue. Not only do they provide immune protection for the tissue in which they reside but they also
support normal function of their neighboring tissue cells through the production of cytokines. Macrophages
are given tissue-specific names, and a few examples of tissue-specific macrophages are listed in Table 17.6.
Dendritic cells are important sentinels residing in the skin and mucous membranes, which are portals of entry
for many pathogens. Monocytes, macrophages, and dendritic cells are all highly phagocytic and important
promoters of the immune response through their production and release of cytokines. These cells provide an
essential bridge between innate and adaptive immune responses, as discussed in the next section as well as
the next chapter.
Figure 17.18 Monocytes are large, agranular white blood cells with a nucleus that lacks lobes. When monocytes leave the bloodstream,
they differentiate and become macrophages with tissue-specific properties. (credit left: modification of work by Armed Forces Institute of
Pathology; credit right: modification of work by Centers for Disease Control and Prevention)