[Natural killers and their physiological importance].

Natural killer cells (NK cells) are a distinct subset of mononuclear cells that rapidly kill certain tumor cell lines and even normal cells without a prior sensibilization to the targets. Probably, NK cells are the "first line of defence" in the body against foreign agents. A hypothesis about the role of NK cells in the natural antitumor resistance and in the control of cell proliferation and differentiation has recently been confirmed.