ATURAL KILLER (NK) cells comprise 10% to 15% N of human peripheral blood lymphocytes (PBL) and most have the morphology of large granular lymphocytes (LGL).’s2 NK cells are defined functionally by their ability to lyse target cells without deliberate prior sensitization and without restriction by major histocompatibility (MHC) antigen^.^.^ NK activity was first described in the 1970s, when it was observed that lymphocytes freshly isolated from normal unimmunized hosts could lyse allogeneic tumor cell Subsequent studies showed that such NK activity was mediated by “null cells,” lymphocytes devoid of characteristic T-cell and B-cell surface antigens.’*’’Use of monoclonal antibodies (MoAbs) and techniques of molecular biology have allowed the unequivocal identification of NK cells as a discrete population of cytolytic effectors (Table 1). NK cells have been implicated in several activities in vivo, including destruction of tumor cells, resistance to viral infections, and regulation of hemat~ poiesis.~.’’Although NK activity was originally described in the mouse and in humans, effectors resembling murine and human NK cells have been isolated from many vertebrates, including monkeys, rats, hamsters, cats, dogs, birds, and fish.” Furthermore, NK-like effectors have been described in animals as primitive as starfish and Because the latter do not possess cells analogous to vertebrate T and B lymphocytes, the development of NK-like activity probably preceded evolution of the adaptive immune~ ystem.”.’~ Although NK cells can be isolated from spleen, bone marrow, liver, lung, and the present review will focus on peripheral blood NK cells. A summary of all aspects of NK cell biology is beyond the scope of this article; other recent reviews discuss areas not considered here. 4, 5,246