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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>

TCRs are linked to the cell membrane; structurally, they look like a single arm of an antibody molecule consisting of a distal domain and a membrane proximal constant domain

TCRs are linked to the cell membrane; structurally, they look like a single arm of an antibody molecule consisting of a distal domain and a membrane proximal constant domain

==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 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

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.</p>

<p>Th0 populations are CD4⁺ cells that have yet to differentiate into Th1 or Th2 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.</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>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 lacking IFN-γ are termed '''memory T cells'''</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>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>

==Cytotoxic CD8⁺==

==Cytotoxic CD8⁺==

*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)

*The other pathway uses perforins and granzymes which form pores in the target cell membrane causing cell lysis

*The other pathway uses perforins and granzymes which form pores in the target cell membrane causing cell lysis

**Perforins are structurally related to 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>

[[Category:Lymphocytes|D]]

[[Category:Lymphocytes|D]]