NK cells vs. Tc cells
Posted 19 November 2009 - 04:13 AM
Posted 7 September 2010 - 09:43 PM
Can someone explain what the main differences are in functions of cytotoxic T cells versus natural killer cells (...and are there also "natural killer T cells")? Is it right that NK cells recognize virally infected cells by reduced MHC I expression because the infecting agent takes over host machinery so the host makes less proteins? And do cytotoxic T cells kill cells presenting viral peptides in MHC I? I am confused...also do they both kill by releasing cytotoxic granules into the infected cell, or what is the mechanism?
NK cells form part of the innate immune system which means that they are always "switched on" if you like. NK cells form an early line of defence against viruses and they do this by killing virally-infected cells. This is achieved by the release of two agents contained within granules in the NK cell. The first agent is called Perforin which forms pores in the virally-infected host cell. The second agent are called Granzymes which cause the apoptosis (programmed cell death) of the virally-infected host cell. NK cells recognise the virally-infected cells due to the lack of MHC Class I on the surface of the cells. Viruses are capable of redirecting MHC Class I away from the surface of host cells so that it is not expressed. When MHC Class I is present on the surface of host cells it interacts with an inhibitory receptor on the NK cell to prevent the NK cell from releasing Perforin and Granzymes. When the MHC Class I is absent it cannot interact with the NK cells inhibitory receptor and so the NK cell is stimulated to release the Perforin and Granzymes.
Cytotoxic T-cells form part of the adaptive immune response and this means that they are only "switched on" to deal with persistent viruses e.g. HIV, Hepatitis. Similarly to NK cells, cytotoxic T-cells kill virally-infected host cells using perforin and granzymes. Therefore, the killing mechanism used by NK cells and cytotoxic T-cells is the same. Cytotoxic T-cells recognise virally-infected cells due to the presence of MHC Class I on the surface of that cell. This MHC Class I presents peptide fragments of the virus infecting the cell to the Cytotoxic T-cell. The Cytotoxic T-cell recognises the MHC Class I and viral peptide fragment and then releases the Perforin and Granzymes to kill the virally-infected host cell.
Hope this helps.
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