A lymphoid cell that recognizes and destroys tissue cells infected with pathogenic organisms and also certain tumour cells. NK cells are important as part of the innate immune response against infection, before the more antigen-specific mechanisms of the adaptive immune response are mobilized (see cytotoxic t cell), and they play a significant role in combating infections with, for example, herpesviruses. They become activated in response to interferons or the release of cytokines by macrophages, bind to target cells, and release cytotoxic granules onto the surface of their target. The toxic effector molecules penetrate the target’s plasma membrane and induce programmed cell death (see apoptosis). Receptors on NK cells recognize changes in the expression of surface markers on infected cells, enabling them to distinguish ‘stressed’ (i.e. infected) cells from uninfected self tissue. By means of various cytokines and chemokines NK cells also undergo adaptive changes, interact with other types of immune cells, and even have a capacity for immunological memory.