Clinical Immunology Vol. 95, No. 1, April, pp. S14 âS18, 2000 doi:10.1006/clim.1999.4816, available online at http://www.idealibrary.com on Lewis L. Lanier 1 Immunobiology Department, DNAX Research Institute of Molecular and Cellular Biology, 901 California Avenue, Palo Alto, California 94304 NK CELLSâDEFINITION AND ORIGINS NK cells are a population of lymphocytes involved in innate immune responses against certain viruses, intracellular bacteria, and parasites (1â3). Like other hematopoietic cells, these lymphocytes arise from bone marrow progenitor cells, but are distinct from T and B cells because they do not require RAG genes for development and unlike T cells do not require a thymus for maturation (4). In humans, NK cells are present in fetal liver and spleen as early as 6 â 8 weeks of gestation, prior to the organization and colonization of the fetal thymus (5, 6). Five to 20% of lymphocytes circulating in the blood and 5% of splenic lymphocytes in children and adults are NK cells, whereas they are rarely present in normal, uninfected lymphatic tissues (7). Human NK cells are identiï¬ed by their expression of CD56 and lack of CD3 on the cell surface and the majority also express CD16, a low-afï¬nity receptor for the Fc region
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