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[[Image:TH1-2.jpg|thumb|right|200px|TH1 and 2 selection is influenced by infection - B. Catchpole, RVC 2008]]
[[Image:TH1-2.jpg|thumb|right|200px|TH1 and 2 selection is influenced by infection - B. Catchpole, RVC 2008]]
==Introduction==
==Introduction==
T cells are long lived and are involved in '''cell mediated immunity'''. Functionally they are divided by the expression of CD4<sup>+</sup> or CD8<sup>+</sup> markers. CD4<sup>+</sup> T helper cells recognise antigens bound to MHC II complexes and are involved with the control of intracellular and extracellular pathogens; they can interact with CD8+, NK and dendritic cells or with B cells. Cytotoxic CD8<sup>+</sup> T cells recognise the MHC I complex and destroy infected or neoplastic cells.
T cells are long lived and are involved in '''cell mediated immunity'''. Functionally they are divided by the expression of CD4<sup>+</sup> or CD8<sup>+</sup> markers. CD4<sup>+</sup> T helper cells recognise antigens bound to MHC II complexes and are involved with the control of intracellular and extracellular pathogens; they can interact with CD8<sup>+</sup>, NK and dendritic cells or with B cells. Cytotoxic CD8<sup>+</sup> T cells recognise the MHC I complex and destroy infected or neoplastic cells.
Within the blood and lymphoid organs the majority of T cells are antigen-naive T cells; only a small proportion are memory T cells. Naive T cells have yet to encounter antigen and can only be activated by antigen that is presented by dendritic cells. After initial antigenic activation, naïve T-cells develop into an intermediate stage cell called the TH<sub>0</sub> cell which can then be activated by any antigen-presenting cell, e.g. Dendritic cells, [[Macrophages|macrophages]] or [[B cells]].
Within the blood and lymphoid organs the majority of T cells are antigen-naive T cells; only a small proportion are memory T cells. Naive T cells have yet to encounter antigen and can only be activated by antigen that is presented by dendritic cells. After initial antigenic activation, naïve T-cells develop into an intermediate stage cell called the TH<sub>0</sub> cell which can then be activated by any antigen-presenting cell, e.g. Dendritic cells, [[Macrophages|macrophages]] or [[B cells]].