Changes

=Immune Tolerance=

=Immune Tolerance=

Immunological tolerance occurs when there is an unresponsiveness towards particular antigens, so any further immune responses are prevented or suppressed. Tolerance is required to prevent:

Immunological tolerance occurs when there is unresponsiveness towards particular antigens, so any further immune responses are prevented or suppressed. Tolerance is required to prevent:

* Potentially harmful inflammatory responses towards innocuous substances, such as air-borne or food molecules

* Potentially harmful inflammatory responses towards innocuous substances, such as air-borne or food molecules

* To prevent an immune attack against host tissue - this is known as '''self-tolerance'''

* To prevent an immune attack against host tissue - this is known as '''self-tolerance'''

===Peripheral Tolerance===

===Peripheral Tolerance===

It is inevitable that some self-reactive T cells will evade the thymic selection process and enter the peripheral circulation, because some self-antigens are not expressed in the thymus, and others will not show sufficient affinity to MHC to form the MHC:self-peptide complex required for negative selection in the thymus. Conversely some T cell receptors will not have enough affinity for their respective self-antigen to induce apoptosis. Peripheral tolerance describes an unresponsiveness towards self-antigen which is developed outside the primary lymphoid organs. There are four ways this may be achieved in T cells:

It is inevitable that some self-reactive T cells will evade the thymic selection process and enter the peripheral circulation, because some self-antigens are not expressed in the thymus, and others will not show sufficient affinity to MHC to form the MHC: self-peptide complex required for negative selection in the thymus. Conversely some T cell receptors will not have enough affinity for their respective self-antigen to induce apoptosis. Peripheral tolerance describes unresponsiveness towards self-antigen which is developed outside the primary lymphoid organs. There are four ways this may be achieved in T cells:

* Ignorance

* Ignorance

* Anergy

* Anergy

==Regulatory T Cells==

==Regulatory T Cells==

A number of cell populations identified during studies on autoimmunity and organ transplantation have shown the capacity to suppress responses to self-antigen and therefore can be utelised to regulate rejection. Although once considered a tentative theory, this form of tolerance is now considered a major mechanism in the protection of host tissue from immune attack.  

A number of cell populations identified during studies on autoimmunity and organ transplantation have shown the capacity to suppress responses to self-antigen and therefore can be utilised to regulate rejection. Although once considered a tentative theory, this form of tolerance is now considered a major mechanism in the protection of host tissue from immune attack.  

[[Image:T reg cells.JPG|thumb|right|200px|Regulatory T cells- copyright Brian Catchpole]]

[[Image:T reg cells.JPG|thumb|right|200px|Regulatory T cells- copyright Brian Catchpole]]