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→Helper CD4+
==Helper CD4<sup>+</sup>==
==Helper CD4<sup>+</sup>==
<p>These T cells express the marker CD4 and are categorised into three groups, TH<sub>1</sub> and TH<sub>2</sub>, with a third lineage, TH<sub>17</sub> being recently described. These lineages are distinguished by the cytokines they produce. T helper cells recognise antigens bound to MHC II complexes.
<p>These T cells express the marker CD4 and are categorised into three groups, T<sub>H</sub>1 and T<sub>H</sub>2, with a third lineage, T<sub>H</sub>17 being recently described. These lineages are distinguished by the cytokines they produce. T helper cells recognise antigens bound to MHC II complexes.
*TH<sub>1</sub> cells produce IL-2, IFN-γ and TNF-α
*T<sub>H</sub>1 cells produce IL-2, IFN-γ and TNF-α
*TH<sub>2</sub> cells produce IL-4, IL-5, IL-10 and IL-13
*T<sub>H</sub>2 cells produce IL-4, IL-5, IL-10 and IL-13
*TH<sub>17</sub> cells produce IL-17, IL-17F, IL-21
*T<sub>H</sub>17 cells produce IL-17, IL-17F, IL-21
T<sub>H</sub>1 cells interact with CD8<sup>+</sup>, NK and dendritic cells and T<sub>H</sub>2 cells interact with B cells. T<sub>H</sub>1 cells are involved with the control of intracellular pathogens and T<sub>H</sub>2 cells extracellular pathogens. IL-2 produced by T<sub>H</sub>1 cells stimulates further proliferation of CD4<sup>+</sup> cells. T<sub>H</sub>17 cells are produced to enhance innate immunity, with the cytokines produced increasing the extravasation of neutrophils.</p>
<p>TH<sub>0</sub> populations are CD4<sup>+</sup> cells that have yet to differentiate into TH<sub>1</sub>, TH<sub>2</sub> or TH<sub>17</sub> cells and they secrete IL-2, IL-4, IL-5, IFN-γ. In the presence of IL-4 they develop into TH<sub>2</sub> cells while in the presence of IL-2 they develop into TH<sub>1</sub> cells. In the abscence of IL-2 TH<sub>1</sub> cells will change into TH<sub>2</sub> cells. TH<sub>17</sub> cells develop in the presence of TGF-β and IL-6 and can further develop into TH<sub>1</sub> cells depending on various conditions. </p>
<p>T<sub>H</sub>0 populations are CD4<sup>+</sup> cells that have yet to differentiate into T<sub>H</sub>1, T<sub>H</sub>2 or T<sub>H</sub>17 cells and they secrete IL-2, IL-4, IL-5, IFN-γ. In the presence of IL-4 they develop into T<sub>H</sub>2 cells while in the presence of IL-2 they develop into T<sub>H</sub>1 cells. In the abscence of IL-2 T<sub>H</sub>1 cells will change into T<sub>H</sub>2 cells. T<sub>H</sub>17 cells develop in the presence of TGF-β and IL-6 and can further develop into T<sub>H</sub>1 cells depending on various conditions. </p>
<p>TH<sub>1</sub> cells have two populations; one that secretes IFN-γ and is short lived, and the other that doesn’t secrete IFN-γ and is long lived. The cells that do not secrete IFN-γ are termed '''memory T cells'''</p>
<p>T<sub>H</sub>1 cells have two populations; one that secretes IFN-γ and is short lived, and the other that doesn’t secrete IFN-γ and is long lived. The cells that do not secrete IFN-γ are termed '''memory T cells'''</p>
<p> T cell differentiation is a field of continual change, with discoveries made on a regular basis. </p>
<p> T cell differentiation is a field of continual change, with discoveries made on a regular basis. </p>