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Polymorphonuclear (PMN) leukocytes are a population of cells also referred to as granulocytes. These include the basophils, eosinophils, and neutrophils. Granulocytes are short‐lived phagocytic cells that contain the enzyme‐rich lysosomes, which can facilitate destruction of infectious microorganisms. They also produce peroxide, superoxide radicals, and nitric oxide, which are toxic to many microorganisms. Some lysosomes also contain bactericidal proteins, such as lactoferrin. PMN leukocytes play a major role in protection against infection. Defects in PMN cell function are accompanied by chronic or recurrent infection.

Figure 2.11. CTL killing of target cells. CTLs interact in a cognate fashion via antigen‐specific TCRs in conjunction with CD8. Antigenic peptides expressed by class I MHC molecules on target cells bind to antigen‐specific TCRs in a process called antigen presentation (see Chapter 8). Following direct cell contact, pore‐forming perforin and granzymes are released by the CTL to promote destruction of the target cell.