Before their phenotypic characterization, NK cells were functionally defined as cells freshly isolated from blood with the ability to lyse tumor cell targets. Activation of NK cells with IL-2 induces cytolytic machinery and the capacity to lyse a broad range of tumor targets not lysed by resting NK cells. Although there has been much progress in understanding the signaling machinery of NK cells, the recognition events may be more complex. It is well known that β 2 integrins expressed on the surface of NK cells recognize intracellular adhesion molecule-1 (ICAM-1) on target cells.15,16 However, this may be one of several receptor ligand pairs that are involved in the recognition process. Another adhesion ligand pair, CD2 on NK cells, recognizes LFA-3 on target cells.17 Antibodies against β 2 integrins and CD2 variability interrupt cytotoxicity in a target-specific manner.18 Activating and inhibitory NK cell receptors (table 1) have now been shown to recognize a wide range of self and non-self antigens.