NK cell

Natural killer cells (or NK cells) are a type of cytotoxic lymphocyte which constitute a major component of the innate immune system. NK cells play a major role in the rejection of tumors and cells infected by viruses. The cells kill by releasing small cytoplasmic granules of proteins called perforin and granzyme that cause the target cell to die by apoptosis.
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Natural killer cell. A type of white blood cell that contains granules with enzymes that can kill tumor cells or microbial cells. Also called a large granular lymphocyte.

Bone marrow-derived, mononuclear white blood cells (large granular lymphocytes) that are able to kill invading microorganisms without activation by cells of the immune system. They are, therefore, part of the innate immune system. They are specialized in killing virus-infected cells and cells transformed to develop cancer.