<prompt> NK cells expressCD 15;CD55CD 16; CD 56CD16;CD 57CD21;CD66  <response>: Ref Robbins 8e p188 ;9e p192 NK cells belonging to the group of innate lymphoid cells are defined as large granular lymphocytes LGL and constitute the third kind of cells differentiated from the common lymphoid progenitor-generating B and T lymphocytes. NK cells are known to differentiate and mature in the bone marrow, lymph nodes, spleen, tonsils, and thymus, where they then enter into the circulation. NK cells differ from natural killer T cells NKTs phenotypically, by origin and by respective effector functions; often, NKT cell activity promotes NK cell activity by secreting interferon gamma. In contrast to NKT cells, NK cells do not express T-cell antigen receptorsTCR or pan T marker CD3 or surface 