Natural killer (NK) cells are lymphocytes distinct from T and B cells that play important roles in innate immune responses mainly against intracellular viruses and bacteria. The term natural killer derives from the fact that these cells are capable of performing their killing function without a need for clonal expansion and differentiation, which is required for effector responses of the immune system's other killer cells, the cytotoxic T lymphocytes (CTLs). NK cells constitute 5% to 15% of the mononu-clear cells in the blood and spleen. They are rare in other lymphoid organs but are concentrated in certain organs such as the liver and gravid uterus. NK cells arise from bone marrow precursors and appear as large lymphocytes with numerous cytoplasmic granules. NK cells do not express highly diverse, clonally distributed antigen receptors typical of B and T cells. Rather, they use germline DNA-encoded receptors, discussed later, to distinguish pathogen-infected from healthy cells. They can be identified in the blood by expression of CD56 and the absence of CD3, two membrane proteins often found together on activated CTLs.
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