Changes

==Introduction==

==Introduction==

[[Image:NK healthy.jpg|thumb|right|175px|Healthy cells present MHC to NK cells to suppress their activation - B. Catchpole, RVC 2008]]

[[Image:NK healthy.jpg|thumb|right|175px|Healthy cells present MHC to NK cells to suppress their activation - B. Catchpole, RVC 2008]]

NK cells do not develop in the thymus and represent 5-10% of the circulating lymphocytes. They recognise and kill transformed cells by releasing perforins and granzymes which create channels in the target cell membrane causing lysis.  They express the markers CD16, CD56 and CD94.

NK cells do not develop in the thymus and represent 5-10% of the circulating lymphocytes. They recognise and kill transformed cells by releasing perforins and granzymes which create channels in the target cell membrane causing lysis.  They express the markers CD16, CD56 and CD94.

Natural Killer cells also play a role in [[Immunity to Viruses|'''antibody-dependent cell-mediated cytotoxicity''']]

Natural Killer cells also play a role in [[Immune Responses to Viral Infections|'''antibody-dependent cell-mediated cytotoxicity''']]

==NK Receptors==

==NK Receptors==

Some viruses are able to down-regulate MHC expression of the infected cell; this mecganism is also used as a protection against the host immune system as a lack of MHC inhibits normal T-cell activity. NK cells can counteract the down-regulation tactic and in this regard are mainly associated with activity against virus-infected cells and tumour cells, which can also have lowered MHC expression.

Some viruses are able to down-regulate MHC expression of the infected cell; this mechanism is also used as a protection against the host immune system as a lack of MHC inhibits normal T-cell activity. NK cells can counteract the down-regulation tactic and in this regard are mainly associated with activity against virus-infected cells and tumour cells, which can also have lowered MHC expression.

The receptors on NK cells do not act like antigen-specific receptors because although they trigger functional activity within the cell, they do not stimulate proliferation and there is no clonal expansion of NK cells. NK cells work through two different types of receptors:

The receptors on NK cells do not act like antigen-specific receptors because although they trigger functional activity within the cell, they do not stimulate proliferation and there is no clonal expansion of NK cells. NK cells work through two different types of receptors:

*'''Interferon gamma'''; (IFNγ) which stimulates macrophages to be active against the target cell and stimulates target cell expression of MHC.  

*'''Interferon gamma'''; (IFNγ) which stimulates macrophages to be active against the target cell and stimulates target cell expression of MHC.  

[[Category:Lymphocytes|G]]

[[Category:Lymphocytes|G]]