NK Cells Figure (Figure 17.16). Reprinted from OpenStax Microbiology (p. 706), by N. Parker, M. Shneegurt, A.-h. Thi Tu, B. M. Forester, and P. Lister, 2016, Rice University. Copyright 2016 by Rice University. Available at: https://openstax.org/books/microbiology/pages/17-3-cellular-defenses

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Natural killer (NK) cells are large mononuclear lymphocytes that target abnormal cells, including virus-infected and cancerous cells, and lack either T cell receptors (TCRs) or surface immunoglobulin (Ig). Abnormal cells do not express the normal major histocompatibility complex (MHC). Normal MHC markers prevent NK cell activation, so abnormal MHC expression by abnormal cells recruits NK cells (signals cell distress). Binding of NK cells on target cells (and detect abnormal MHC levels) also leads to activation. After activation, NK cells can employ several mechanisms for target cell destruction.

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Natural Killer Cells

One way NK cells can cause cell destruction is by expressing cytotoxic membrane proteins and cytokines that makes the target cell undergo apoptosis. Another way NK cells acts is by undergoing perforin mediated cytotoxicity using perforin and granzymes, which pokes holes in the target cells and enter the cell to stimulate apoptosis, respectively. NK cells have toxic granules but are not considered granulocytes, instead coming from lymphoid (rather than myeloid stem) cells.

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