can be used to identify nk cells using methods well known in the art within the context of this invention u201c active u201d or u201c activated u201d nk cells designate biologically active nk cells more particularly nk cells having the capacity of lysing target cells for instance an u201c active u201d nk cell is able to kill cells that express an nk activating receptor ligand and fails to express u201c self u201d mhc hla antigens kir incompatible cells examples of suitable target cells for use in redirected killing assays are p815 and k562 cells but any of a number of cell types can be used and are well known in the art see e g sivori et al 1997 j exp med 186 1129 1136 vitale et al 1998 j exp med 187 2065 2072 pessino et al 1998 j exp med 188 953 960 neri et al 2001 clin diag lab immun 8 1131 1135 u201c active u201d or u201c activated u201d cells can also be identified by any other property or activity known in the art as associated with nk activity such as cytokine e g ifn gamma and tnf alpha production of increases in free intracellular calcium levels nk cells are negatively regulated by major histocompatibility complex mhc class i specific inhibitory receptors karre et al.