Natural killer cell: A cell that can react against and destroy another cell without prior sensitization to it. Abbreviated NK cell. NK cells are part of our first line of defense against cancer cells and virus-infected cells. NK cells are small lymphocytes that originate in the bone marrow and develop without the influence of the thymus. An NK cell attaches to a target cell, releases chemicals that breach its cell wall, and causes it to lyse (break up).
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