Natural killer (NK) cells form a unique third group of lymphocytes that differs from T and B cells in surface phenotype, target cell recognition and function. NK cells have two relevant functions, related to the innate immune response against pathogens microorganisms. One is cytotoxicity, mediated by the recognition and lysis of target cells such as virus and bacteria infected-cells. The second NK cell function is to produce cytokines, mainly IFN-g, that can modulate innate and specific immune responses. Cytotoxicity and cytokine secretion contribute to host resistance against microorganisms and both functions are significantly altered in infectious diseases (Rev Méd Chile 2000; 128: 1361-70).