Natural killer cell

A natural killer cell, or NK cell, is a lymphocyte of the innate immune system. It arises in the bone marrow, like B cells and T cells, but carries no antigen-specific receptor, so it does not need to be primed by an earlier encounter. NK cells are fairly large lymphocytes with visible granules in their cytoplasm, which is why they are also called large granular lymphocytes, and they make up a small share of the lymphocytes in blood.

An NK cell decides whether to kill by weighing signals from its inhibitory and activating receptors. Healthy cells display MHC class I molecules, which act as a signal to hold back. Many viruses and tumors cut down MHC class I to hide from cytotoxic T cells, and stressed or infected cells also show new surface molecules that activate NK cells. When activation wins, the NK cell releases perforin, which punches pores in the target's membrane, and granzymes, enzymes that enter and trigger apoptosis. NK cells can also kill antibody-coated cells, a process called antibody-dependent cellular cytotoxicity, and they secrete interferon-gamma, which activates macrophages.

Because they act within days, NK cells hold infections in check while the slower adaptive response develops, and they are being studied for cancer immunotherapy.