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→Introduction
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]].
The TH<sub>0</sub> cells have the capacity to differentiate into TH<sub>1</sub>, TH<sub>2</sub> cells and a very recently described subtype TH<sub>17</sub> cells. The type of cell that develops depends on the antigen presenting cell type. [[Macrophages|Macrophages]] cause the TH<sub>0</sub> cell to develop into a TH<sub>1</sub> cell induced by IL-12 production following macrophage-antigen interaction. B cells cause the TH<sub>0</sub> cell to develop into a TH<sub>2</sub> cell induced by IL-10 production following B cell-antigen interaction. On antigenic stimulation the TH<sub>1</sub> or TH<sub>2</sub> cells become activated, undergo clonal expansion and secrete a range of different cytokines. The third, most recently described subset, TH<sub>17</sub> cells form in the presence of IL-6 and TGF-β which are produced in the prescence of infection.
The TH<sub>0</sub> cells have the capacity to differentiate into TH<sub>1</sub>, TH<sub>2</sub> cells and a very recently described subtype TH<sub>17</sub> cells. The type of cell that develops depends on the antigen presenting cell type. [[Macrophages|Macrophages]] cause the TH<sub>0</sub> cell to develop into a TH<sub>1</sub> cell induced by IL-12 production following macrophage-antigen interaction. B cells cause the TH<sub>0</sub> cell to develop into a TH<sub>2</sub> cell induced by IL-10 production following B cell-antigen interaction. On antigenic stimulation the TH<sub>1</sub> or TH<sub>2</sub> cells become activated, undergo clonal expansion and secrete a range of different cytokines. The third, most recently described, subset, TH<sub>17</sub> cells, form in the presence of IL-6 and TGF-β which are produced in the prescence of infection.
For any one cell the cytokine-secreting activation state is short-lived, lasting between 4 - 40 hours. After this time these cells either die or mature into the long-lived memory cells. The proliferation of [[T cells]] continues until the presentation of antigen ceases.
For any one cell the cytokine-secreting activation state is short-lived, lasting between 4 - 40 hours. After this time these cells either die or mature into the long-lived memory cells. The proliferation of [[T cells]] continues until the presentation of antigen ceases.