**2. Lineage of NK cells**

NK cells constitute a third lymphoid line derived from a common T-cell and B-cell bone marrow precursor. Unlike T cells, NK cells do not have a specialized differentiating organ, are matured inside of bone marrow, and can develop even in athymic mice. The acquisition of their functions

© 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.

does not use recombination-activating gene (RAG) enzymes for rearrangement of their receptor genes or complete V(D)J recombination as is the case for T cells or B cells [10] and is considered as the only lymphocytes without a clonally specific receptor [11]. Additionally, in contrast to the conventional αβ T cells, the genesis of NK cells appears to be independent of the self-major histocompatibility complex (MHC), although they have different recognition specificities of allogeneic MHC molecules.

Mature NK cells are able to self-renew and possibly persist in the host for months or years. Nevertheless, unlike long-lived CD8+ and CD4+ T cells that retain a "memory-like" phenotype and function after homeostatic proliferation, expanded NK cells return to a quiescent phenotype and respond with comparable kinetics against viral challenge [10].
