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[[Image:NK activated.jpg|thumb|right|150px|Natural Killer cells release enzymes to kill cells with inadequate MHC presentation - B. Catchpole, RVC 2008]]
[[Image:NK activated.jpg|thumb|right|150px|Natural Killer cells release enzymes to kill cells with inadequate MHC presentation - B. Catchpole, RVC 2008]]
[[Image:NK kill.jpg|thumb|right|150px|Infected cells are destroyed before replication - B. Catchpole, RVC 2008]]
[[Image:NK kill.jpg|thumb|right|150px|Infected cells are destroyed before replication - B. Catchpole, RVC 2008]]
NK cells can be classified as lymphocytes because they are capable of recognising antigen, however they are more often associated with the [[Innate Immune System - WikiBlood|innate]] immune response. They target cells by monitoring [[MHC - WikiBlood|MHC]] production, which is expressed by healthy cells to present antigen to T-cells. Low MHC levels can be used as a marker for a cell whose machinery is compromised by a replicating virus. When MHC levels drop, it acts as a danger signal to the NK cells, which then release enzymes to kill the infected cells.
NK cells can be classified as lymphocytes because they are capable of recognising antigen, however they are more often associated with the [[Innate Immune System|innate]] immune response. They target cells by monitoring [[MHC - WikiBlood|MHC]] production, which is expressed by healthy cells to present antigen to T-cells. Low MHC levels can be used as a marker for a cell whose machinery is compromised by a replicating virus. When MHC levels drop, it acts as a danger signal to the NK cells, which then release enzymes to kill the infected cells.
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.