**1. Introduction**

Natural killer (NK) cells constitute a minor subset of lymphocytes that are crucial components of the innate immune system and play critical roles in host immunity against malignant cells and virus-infected cells but also in bacterial, fungal, and parasite immune responses [1]. NK cells represent 10% of the lymphocytes in human peripheral blood, and they comprise the third largest population of lymphocytes following B and T cells.

© 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.

Natural killer cells have diverse biological functions including killing pathogen-infected cells and cancer cells as well as an immunoregulatory role [2]. Natural killer cells can discriminate between normal cells and cells that do not express adequate amounts of major histocompatibility complex (MHC) class I molecules.

NK cell cytotoxicity is regulated by a balance between activating and inhibitory signals delivered by receptors expressed at the cell surface. These cells are known to directly recognize and kill malignant cells or induce apoptosis. However, tumor cells have the ability to evade immunosurveillance by using multiple mechanisms. Furthermore, tumors harbor a population of cancer stem cells (CSC), which is responsible of tumor progression and therapeutical resistance.

Therapeutic applications of NK cells in cancer and NK cells targeting cancer stem cells (CSCs) represent a promising strategy for cancer immunotherapy.
