<prompt> NK cell marker is?CD 34CD 56CD 1CD 45  <response>: Ans. is b i.e., CD 56 Virus infected cells and tumor cancer cells are killed by NK cells and cytotoxic T-cells Natural killer cells NK cells These cells possess cytotoxic activity against virus infected cells, tumor cells and transplanted foreign cells. Cytotoxicity of NK cells is neither antibody dependent nor MHC restricted. Activity is nonimmune as it does not require antigenic stimulation. NK-cells are positive for CD 16 and CD56. NK cells are usually negative for CD3, but a subset is positive for CD3 called NKT-cells. IL-2 acts as a growth factor for NK cells. NK cells activity is augmented by interferon. NK cells secrete peroforin resembles complement C9 which causes transmembrane pores through which cytotoxic factors e.g. TNF-3 enters the cells and induce apoptosis. 