Natural killer cells (NK) are a type of lymphocyte (a white blood cell) and a component of nonspecific immune defense. These cells do not destroy the attacking microorganisms directly; they attack infected cells and cells that appear that they might cause cancer.

NK cells are not phagocytic; they weaken the target cell's plasma membrane, causing water and ions to diffuse into the cell and expanding it. Under this large pressure, the target cell lyses.

NK cells are characterised immunohistochemically by the presence of CD56 on the cell membrane.

NK cells are activated in response to interferons or macrophage-derived cytokines. They serve to contain virus infections while the adaptive immune response is generating antigen specific cytotoxic T cells that can clear the infection. Patients with deficient in NK cells prove to be highly suceptible to early phases of herpes virus infection.

If NK cells defend the body against viruses and other pathogens they must possess mechanisms to decide if a cell is infected or not. The exact mechanisms are as yet unknown, but recogniton of 'altered self' is thought to be involved. To control their cytotoxic activity they possess two types of surface receptors known as 'activating receptors' and ' inhibitory receptors'.

These 'inhibitory receptors' recognice MHC (Major histocompatibility complex) class I allels, wich could explain why NK cells kill cells with low level of of MHC class I molecules.

Different families of NK receptors exist and many of these receptors seem to have been evolved recently.

See also

• Immune system
• Granzymes
• Major histocompatibility complex

Literature

Immunobiology The Immune System In Health And Disease by Janeway, Travers, Walport & Shlomchik Churchchill Livingstone Copyright 2005

Cellular and Molekular Immunology by Abbul K. Abbas & Andrew Lichtman Saunders Copyright 2003