Measurement of NK activity by the microcytotoxicity assay (MCA): a new application for an old assay.

Natural killer (NK) cells are spontaneously cytotoxic immune effector cells with the ability to selectively destroy tumor cells without harming normal cells. To perform this function, NK cells utilize two main cytotoxicity pathways, the well known perforin/granzyme-mediated secretory/necrotic killing and the recently defined TNF family ligand-mediated non-secretory/apoptotic killing. The former mechanism is manifested mainly against a few cultured leukemia cell targets, while the latter mediates killing against a large variety of tumor cell targets.