Changes

[[Image:TCR2.jpg|thumb|right|150px|T-cell receptor binds antigen fragments presented by MHC on the cell surface - B. Catchpole, RVC 2008]]

[[Image:TCR2.jpg|thumb|right|150px|T-cell receptor binds antigen fragments presented by MHC on the cell surface - B. Catchpole, RVC 2008]]

[[Image:T Cell diagram 2.jpg|thumb|right|150px|T Cell - Copyright Prof Dirk Werling DrMedVet PhD MRCVS]]

[[Image:T Cell diagram 2.jpg|thumb|right|150px|T Cell - Copyright Prof Dirk Werling DrMedVet PhD MRCVS]]

''Also called T lymphocytes''

Also known as '''T lymphocytes

<p>So named as they differentiate in the [[Thymus - Anatomy & Physiology|thymus]]. They are long lived and are involved in cell mediated immunity. They represent 60-80% of the circulating lymphocytes and all express the markers CD2, CD3 and CD7 as well as having T cell receptors (TCR). Each T cell has 30,000 TCRs each of which is identical and recognises antigens and MHC II.</p><p>Functionally they are divided into three subsets that are distinguished by presence or absence of CD4 or CD8 markers. CD4 and CD8 cells have α/β antigen receptors while the γδ cells have the γ/δ antigens receptors.</p>

<p>So named as they differentiate in the [[Thymus - Anatomy & Physiology|thymus]]. They are long lived and are involved in cell mediated immunity. They represent 60-80% of the circulating lymphocytes and all express the markers CD2, CD3 and CD7 as well as having T cell receptors (TCR). Each T cell has 30,000 TCRs each of which is identical and recognises antigens and MHC II.</p><p>Functionally they are divided into three subsets that are distinguished by presence or absence of CD4 or CD8 markers. CD4 and CD8 cells have α/β antigen receptors while the γδ cells have the γ/δ antigens receptors.</p>

* T-cell receptors are the antigen-specific receptors for T-lymphocytes

* T-cell receptors are the antigen-specific receptors for T-lymphocytes

** Structurally, they look like a single arm of an antibody molecule

** Structurally, they look like a single arm of an antibody molecule

===Helper CD4+===

==Helper CD4+==

<p>These T cells express the marker CD4 and are categorised into two groups, Th1 and Th2, which are distinguished by the cytokines they produce. T helper cells recognise antigens bound to MHC II complex.

<p>These T cells express the marker CD4 and are categorised into two groups, Th1 and Th2, which are distinguished by the cytokines they produce. T helper cells recognise antigens bound to MHC II complex.

*Th1 cells produce Il-2, IFN-γ and TNF-α  

*Th1 cells produce Il-2, IFN-γ and TNF-α  

<p>For more information on T helper cells click [[T cell differentiation#TH1 Cells|here]]

<p>For more information on T helper cells click [[T cell differentiation#TH1 Cells|here]]

===Cytotoxic CD8⁺===

==Cytotoxic CD8⁺==

<p>These T cells express the marker CD8 and once fully mature seek and destroy target cells (infected or cancer forming cells). When the cytotoxic T cell recognises the MHC I complex on the target cell (MHC I binds to TCR) the T cell kills that cell e.g. viral peptides associate with MHC I and the CD8⁺ T cell recognises this and binds to the cell. Only two pathways are used by cytotoxic T cells to kill cells:

<p>These T cells express the marker CD8 and once fully mature seek and destroy target cells (infected or cancer forming cells). When the cytotoxic T cell recognises the MHC I complex on the target cell (MHC I binds to TCR) the T cell kills that cell e.g. viral peptides associate with MHC I and the CD8⁺ T cell recognises this and binds to the cell. Only two pathways are used by cytotoxic T cells to kill cells:

*One pathway uses the CD95 (death receptor) which  triggers apoptosis in the target cell (usually other T cells)

*One pathway uses the CD95 (death receptor) which  triggers apoptosis in the target cell (usually other T cells)

In both cases direct contact is required between the T cell and target cell, and cell killing can take several minutes.</p><p> Cytotoxic T cells secrete a pattern of cytokines similar to that of TH1 cells i.e. IFN-γ but not IL-2. IFN-γ shifts the balance of the immune response in favour of TH1 cells giving an increased level of T cell proliferation. The initiation of the immune response via cytotoxic T cells leads to the selective proliferation of cytotoxic T cells enhancing the main mechanism of killing infected cells.</p>

In both cases direct contact is required between the T cell and target cell, and cell killing can take several minutes.</p><p> Cytotoxic T cells secrete a pattern of cytokines similar to that of TH1 cells i.e. IFN-γ but not IL-2. IFN-γ shifts the balance of the immune response in favour of TH1 cells giving an increased level of T cell proliferation. The initiation of the immune response via cytotoxic T cells leads to the selective proliferation of cytotoxic T cells enhancing the main mechanism of killing infected cells.</p>

===γδ cells===

==γδ cells==

<p>''Information on these cells is varied.''</p>

<p>''Information on these cells is varied.''</p>

<p>They do not express CD4 or CD8 and have γδ antigen receptors rather than α/β like other T cells.  They develop in the [[Thymus - Anatomy & Physiology|thymus]] and migrate to epithelial tissues where they remain. The number present in an individual varies greatly but is generally greatest in immature ruminants and pigs.</p>

<p>They do not express CD4 or CD8 and have γδ antigen receptors rather than α/β like other T cells.  They develop in the [[Thymus - Anatomy & Physiology|thymus]] and migrate to epithelial tissues where they remain. The number present in an individual varies greatly but is generally greatest in immature ruminants and pigs.</p>