Natural killer cell
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Natural killer cells (or NK cells) are a type of cytotoxic lymphocyte that 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.
NK-cells are defined as large granular lymphocytes that do not express T-cell antigen receptors (TCR) or Pan T marker CD3 or surface immunoglobulins (Ig) B cell receptor but that usually express the surface markers CD16 (Fc?RIII) and CD56 in humans, and NK1.1/NK1.2 in certain strains of mice.
They were named "natural killers" because of the initial notion that they do not require activation in order to kill cells that are missing "self" markers of major histocompatibility complex (MHC) class I. However, it is now known that the cells are activated.
