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

The TH₀ cells have the capacity to differentiate into TH₁, TH₂ cells and a very recently described subtype TH₁₇ cells. The type of cell that develops depends on the antigen presenting cell type. [[Macrophages|Macrophages]] cause the TH₀ cell to develop into a TH₁ cell induced by IL-12 production following macrophage-antigen interaction. B cells cause the TH₀ cell to develop into a TH₂ cell induced by IL-10 production following B cell-antigen interaction. On antigenic stimulation the TH₁ or TH₂ cells become activated, undergo clonal expansion and secrete a range of different cytokines. The third most recently described subset, TH₁₇ cells, form in the presence of IL-6 and TGF-β which are produced in the prescence of infection.

The TH₀ cells have the capacity to differentiate into TH₁, TH₂ cells and a very recently described subtype TH₁₇ cells. The type of cell that develops depends on the antigen presenting cell type. [[Macrophages|Macrophages]] cause the TH₀ cell to develop into a TH₁ cell induced by IL-12 production following macrophage-antigen interaction. B cells cause the TH₀ cell to develop into a TH₂ cell induced by IL-10 production following B cell-antigen interaction. On antigenic stimulation the TH₁ or TH₂ cells become activated, undergo clonal expansion and secrete a range of different cytokines. The third most recently described subset, TH₁₇ cells, form in the presence of IL-6 and TGF-β which are produced in the prescence of infection, and by either of the Antigen Presenting Cells (APCs).

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.

==Common Functions of TH₁ and TH₂ Cells==

==Common Functions of TH₁ and TH₂ Cells==

Both TH₁ and TH₂ cells produce IL-3 and granulocyte-macrophage colony stimulating factor ([[Leukopoiesis|GM-CSF]]). These act to activate and induce proliferation of [[Neutrophils|neutrophils]] and [[Macrophages|macrophages]]. [[Neutrophils|Neutrophils]] are the major phagocytic cells in the blood and the principal cells in acute inflammatory lesions whose function is chiefly the body's defence against extracellular bacteria. One of the major biological functions therefore of the activation of either TH subset is '''cytokine-controlled reactive [[Haematopoiesis - Overview|haematopoiesis]]'''.

Both TH₁ and TH₂ cells produce IL-3 and granulocyte-macrophage colony stimulating factor ([[Leukopoiesis|GM-CSF]]). These act to activate and induce proliferation of [[Neutrophils|neutrophils]] and [[Macrophages|macrophages]]. [[Neutrophils|Neutrophils]] are the major phagocytic cells in the blood and the principal cells in acute inflammatory lesions whose function is chiefly the body's defence against extracellular bacteria. One of the major biological functions therefore of the activation of either TH subset is '''cytokine-controlled reactive [[Haematopoiesis - Overview|haematopoiesis]]'''.

==Cytotoxic T-Cells==

==Cytotoxic T-Cells==