NK cells distinguish infected and stressed cells from healthy cells, and NK cell activation is regulated by a balance between signals that are generated from activating receptors and inhibitory receptors. There are several families of these receptors (Fig. 4-6), some members of which we will discuss later. These receptors recognize molecules on the surface of other cells and generate activating or inhibitory signals that promote or inhibit NK responses. In general, the activating receptors recognize ligands on infected and injured cells, and the inhibitory receptors recognize healthy normal cells. When an NK cell interacts with another cell, the outcome is determined by the integration of signals generated from the array of inhibitory and activating receptors that are expressed by the NK cell and that interact with ligands on the other cell. Because of the stochastic nature of their expression, there is significant diversity in the array of activating and inhibitory receptors that different NK cells express in any one individual. The result of this is that an individual's NK cells will respond to different types of microbes or infected cells. Furthermore, the genes encoding many of these receptors are polymorphic, meaning that there are several variants of the genes in the population, so that one person will express a slightly different form of the receptors than another person.
Most NK cells express inhibitory receptors that recognize class I major histocompatibility complex (MHC) molecules, which are cell surface proteins normally expressed on almost all healthy cells in the body (Fig. 4-7). A major function of class I MHC molecules, distinct from their role in regulating NK cell activation, is to display peptides derived from cytoplasmic proteins, including microbial proteins, on the cell surface for recognition by CD8+ T lymphocytes. We will describe the structure and function of MHC molecules in relation to CD8+ T cell antigen recognition in Chapter 6. For now, it is important to understand that NK cells use fundamentally different types of receptors than do T cells to recognize class I MHC molecules. Unlike T cells, many of the NK receptors for class I MHC respond by inhibiting NK activation. This is useful because normal cells express class I MHC molecules, and many viruses and other causes of cell stress lead to a loss of cell surface expression of class I MHC. Thus, NK cells interpret the presence of class I MHC molecules as markers of normal, healthy self, and their absence is an indication of infection or damage. Conversely, NK cells will not receive inhibitory signals from infected or stressed cells. At the same time, the NK cells are likely to receive activating signals from the same
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