15-15 Natural killer cells attack pathogens, but are not inducible

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• Natural killer cells are part of innate immunity, but respond in a fashion similar to T cells. They attack cells coated with IgG and attempt to kill them.

We end this section with a set of cells that do not fit neatly into either innate or adaptive immunity and these are the natural killer cells. They are a group of nonphagocytic granular lymphocytes present in the body at low populations and containing small amounts of immunoglobulin and MHC I molecules on their surface. Natural killer cells are not T cells, but their behavior is similar to that of cytotoxic T cells. However, natural killer cells are found in animals never exposed to relevant antigens, which suggests that they are not inducible. This is in contrast to T cells that increase in number when exposed to activating antigen. Natural killer cells have Fc receptors on their surface that are capable of binding IgG, which have been shown to trigger antibody-mediated cytotoxicity. While they tend to defy easy classification, it is clear that natural killer cells are responsible for recognizing and destroying virally infected cells, bacteria, fungi, protozoa, helminths and tumors and are, therefore, an important defense against pathogens.

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