Page 589 - Veterinary Immunology, 10th Edition

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FIG. 19.7 The activation of NK cells is triggered by two situations.
Target cells may fail to express MHC class I molecules. As a result,
NK cells lose their inhibitions about attacking such cells.
Alternatively, NK cells may be activated by the expression of stress-
related proteins on target cells.

Receptors

The most important NK cell receptors belong to three multigene
families. One is the killer cell immunoglobulin (Ig)-like receptor

(KIR or CD158) family expressed in humans and cattle. The other
two families consist of C-type lectins; one, called Ly49, is expressed
in rodents and horses, and the other, called NKG2D, is expressed in
rodents and primates. All three receptor families include both

inhibitory and activating receptors (Fig. 19.8). Other important NK
cell receptors include CD2, CD16 (FcγRIII), CD178, CD95L, CD40L
(CD154), toll-like receptors (TLR3 and TLR9), and leukocyte
function-associated antigen-1 (LFA-1) (Fig. 19.9). NK cells do not
express conventional antigen receptors such as the B cell antigen

receptors (BCRs) or T cell antigen receptors (TCRs), nor do they
express a CD3 complex (Chapter 14).

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