Changes

[[Image:NK healthy.jpg|thumb|right|150px|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]]

[[Image:NK infected.jpg|thumb|right|150px|Infected cells lack MHC, which means they cannot switch off the NK activation signal - B. Catchpole, RVC 2008]]

[[Image:NK infected.jpg|thumb|right|175px|Infected cells lack MHC, which means they cannot switch off the NK activation signal - 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 activated.jpg|thumb|right|175px|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 - Introduction|'''innate immune response''']].  They target cells by monitoring [[Major Histocompatability Complexes|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 - Introduction|'''innate immune response''']].  They target cells by monitoring [[Major Histocompatability Complexes|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.  

Natural Killer cells also play a role in [[Adaptive Immunity to Viruses|'''Antibody-Dependent Cell-mediated Cytotoxicity''']]

Natural Killer cells also play a role in [[Adaptive Immunity to Viruses|'''Antibody-Dependent Cell-mediated Cytotoxicity''']]

===NK Receptors===

==NK Receptors==

* Some viruses down-regulate MHC expression of the infected cell

* Some viruses down-regulate MHC expression of the infected cell

** This is used as a protection against the host immune system

** This is used as a protection against the host immune system