Natural killer cells.

Natural killer (NK) cells are a subpopulation of lymphocytes, with azurophilic granules in their cytoplasm, that have spontaneous cytotoxicity against a variety of tumor cells, virus-infected cells, and some normal cells in the bone marrow and thymus. When lymphocytes are cultured in the presence of interleukin 2 (IL 2), strong cytotoxic reactivity develops and it has been difficult to determine clearly the relationship of these effector cells to NK cells. Much recent insight has been gained regarding the mechanism of lysis by NK cells; a central role is played by a cytolytic protein contained within their cytoplasmic granules. NK cells have been shown to be involved in host resistance against the development of tumor metastases and against certain virus infections, which provides a rationale for therapeutic strategies involving augmentation of NK activity.