**1. Introduction**

Natural killer (NK) cells have been used in clinical studies in order to treat various malignan‐ cies. Missing‐self is one of the mechanisms of the NK cell response that works by the detection of the loss of autologous major histocompatibility complex (MHC) class I expression.

We will mention four elements of NK cells in this chapter: (1) mechanisms of NK cells; (2) activation of NK cells; (3) inclusion criteria based on KIR ligand mismatches; and (4) genetic modifications *ex vivo*.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

NK cells, which are thought to have emerged much later than B cells and T cells based on the evolutionary convergence of variable receptors, rarely cause autoimmune diseases. We will introduce the applicability of the NK cell to cancer treatment.
