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[[Image:TCR2.jpg|thumb|right|300px|T-cell receptor binds antigen fragments presented by MHC on the cell surface - B. Catchpole, RVC 2008]]

[[Image:TCR2.jpg|thumb|right|300px|T-cell receptor binds antigen fragments presented by MHC on the cell surface - B. Catchpole, RVC 2008]]

Also known as '''T lymphocytes

Also known as '''T lymphocytes'''<br />

 

==Introduction==

==Introduction==

<p>T cells are 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 major histocompatability complex (MHC) II.</p><p>Functionally they are divided into three subsets that are distinguished by the 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>T cells are 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 major histocompatability complex (MHC) II.</p><p>Functionally they are divided into three subsets that are distinguished by the 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>

==Helper CD4⁺==

==Helper CD4⁺==

<p>These T cells express the marker CD4 and are categorised into three groups, TH₁ and TH₂, with a third lineage, TH₁₇ 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_H1 and T_H2, with a third lineage, T_H17 being recently described. These lineages are distinguished by the cytokines they produce. T helper cells recognise antigens bound to MHC II complexes.

*TH₁ cells produce IL-2, IFN-γ and TNF-α  

*T_H1 cells produce IL-2, IFN-γ and TNF-α  

*TH₂ cells produce IL-4, IL-5, IL-10 and IL-13

*T_H2 cells produce IL-4, IL-5, IL-10 and IL-13

*TH₁₇ cells produce IL-17, IL-17F, IL-21  

*T_H17 cells produce IL-17, IL-17F, IL-21  

TH₁ cells interact with CD8⁺, NK and dendritic cells and TH₂ cells interact with B cells. TH₁ cells are involved with the control of intracellular pathogens and TH₂ cells extracellular pathogens. IL-2 produced by TH₁ cells stimulates further proliferation of CD4⁺ cells. TH₁₇ cells are produced to enhance innate immunity, with the cytokines produced increasing the extravasation of neutrophils.</p>

T_H1 cells interact with CD8⁺, NK and dendritic cells and T_H2 cells interact with B cells. T_H1 cells are involved with the control of intracellular pathogens and T_H2 cells extracellular pathogens. IL-2 produced by T_H1 cells stimulates further proliferation of CD4⁺ cells. T_H17 cells are produced to enhance innate immunity, with the cytokines produced increasing the extravasation of neutrophils.</p>

<p>TH₀ populations are CD4⁺ cells that have yet to differentiate into TH₁, TH₂ or TH₁₇ cells and they secrete IL-2, IL-4, IL-5, IFN-γ. In the presence of IL-4 they develop into TH₂ cells while in the presence of IL-2 they develop into TH₁ cells. In the abscence of IL-2 TH₁ cells will change into TH₂ cells. TH₁₇ cells develop in the presence of TGF-β and IL-6 and can further develop into TH₁ cells depending on various conditions. </p>

<p>T_H0 populations are CD4⁺ cells that have yet to differentiate into T_H1, T_H2 or T_H17 cells and they secrete IL-2, IL-4, IL-5, IFN-γ. In the presence of IL-4 they develop into T_H2 cells while in the presence of IL-2 they develop into T_H1 cells. In the abscence of IL-2 T_H1 cells will change into T_H2 cells. T_H17 cells develop in the presence of TGF-β and IL-6 and can further develop into T_H1 cells depending on various conditions. </p>

<p>TH₁ 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_H1 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>

**Perforins are structurally related to [[Complement#Membrane Attack Complex|complement factor C9]]

**Perforins are structurally related to [[Complement#Membrane Attack Complex|complement factor C9]]

**Granzymes are proteolytic enzymes that target cell nucleases and cause apoptosis  

**Granzymes are proteolytic enzymes that target cell nucleases and cause apoptosis  

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 T_H1 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==

*One with restricted antigen binding, that act as first line defence against invading organisms and recognises antigens bound to MHC I complex

*One with restricted antigen binding, that act as first line defence against invading organisms and recognises antigens bound to MHC I complex

*The other subset doesn’t require the MHC complex and this subset has two further divisions

*The other subset doesn’t require the MHC complex and this subset has two further divisions

** One producing cytokines and chemokines  (TH₁ and TH₂)  

** One producing cytokines and chemokines  (T_H1 and T_H2)  

**The other having cytotoxic effects.</p>

**The other having cytotoxic effects.</p>

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{{Jim Bee 2007}}

{{Jim Bee 2007}}

[[Category:Lymphocytes|D]]

[[Category:Lymphocytes|D]]