Natural killer cells (NK) are a type of lymphocyte (a white blood cell) and a component of nonspecific immune defense. They share a common progenitor with T cells and have been described as large, granular, bone-marrow derived lymphocytes. 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 and the absence of CD3 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 deficient in NK cells prove to be highly susceptible 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 recognition 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' recognize MHC (Major histocompatibility complex) class I alleles, which could explain why NK cells kill cells with low levels of MHC class I molecules - a concept that is known as "Missing-self" recognition.
Different families of NK receptors exist and many of these receptors seem to have evolved recently.
See also
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