NK cells (Natural killer cells) are defined as large granular lymphocytes that are essential in the innate immune system. NK cells are derived from the common lymphoid progenitor generating B and T lymphocytes.
In humans, there are several stages of NK development in the bone marrow, which are based on the expression levels of CD34, CD117, CD56 and CD94. These phenotypes are restricted to the bone marrow and are not found in lymph nodes, spleen or peripheral blood. NK maturation begins with the expression of CD56 followed by simultaneous expression of CD94/NKG2A. Also crucial to the development of NK cells from common precursors is the expression of CD122 (IL 2/15 receptor beta).
In mouse, NK cell developmental in bone marrow starts with the CD122+ and transitions through to sequential acquisition of the NK cell receptors CD314 (NKG2D), Ly49, and CD117 (c-kit).
NK cells are important components of the innate immunity and play a key role in host defense by virtue of their ability to release cytokines and to mediate cytolytic activity against tumor cells and virus-infected cells.