Changes

[[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==

Within the blood and lymphoid organs the majority of [[T_cells#Helper_CD4.2B|CD4+ T cells]] are antigen-naive T cells. Only a small proportion are memory T 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. T cells are long lived and are involved in '''cell mediated immunity'''. Functionally they are divided into three subsets that are distinguished by the expression of CD4⁺ or CD8⁺ markers. CD4⁺ T helper cells recognise antigens bound to MHC II complexes, whereas cytotoxic CD8⁺ T cells recognise the MHC I complex.  

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 TH₀ cell which can then be activated by any antigen-presenting cell, e.g. Dendritic cells, [[Macrophages|macrophages]] or [[B 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 TH₀ cell which can then be activated by any antigen-presenting cell, e.g. Dendritic cells, [[Macrophages|macrophages]] or [[B cells]].