Leukemia and lymphoma of natural killer lineage cells.

Natural killer (NK) cells are lymphocytes with a large granular lymphocyte morphology, a CD3- CD56+ phenotype, a non-major histocompatibility complex-restricted cytotoxicity, and germline configuration T-cell receptor genes. NK cell lineage tumors originate from either precursor NK cells or mature NK cells. Tumors originating conceivably from precursor NK cells include myeloid/NK cell precursor acute leukemia, precursor NK cell acute lymphoblastic leukemia, and blastic NK cell lymphoma. However, because the developmental pathway of normal NK cells and the characteristics of these NK precursors are not fully understood, the definition and characterization of the tumors are only provisional. Tumors of mature NK cell origin include aggressive NK cell leukemia/lymphoma, nasal-type NK cell lymphoma, and chronic NK lymphocytosis, but the last disorder seems to be reactive in most cases. Because NK cell tumors are rare and difficult to manage, vigorous studies are required for their understanding and management.