Changes

[[Image:MHC.jpg|thumb|right|250px|Major Histocompatibility Complexes - B. Catchpole, RVC 2008]]

[[Image:MHC.jpg|thumb|right|250px|Major Histocompatibility Complexes - B. Catchpole, RVC 2008]]

==Introduction==

==Introduction==

===Presentation Pathway===

===Presentation Pathway===

[[Image:MHC I processing.jpg|thumb|200px|left|'''MHC I presentation pathway, courtesy of B. Catchpole, 2008''']]

[[Image:MHC I processing.jpg|thumb|200px|left|'''MHC I presentation pathway, courtesy of B. Catchpole, 2008''']]

MHC I presents '''endogenous''' (intracellular) peptides. Viral proteins are broken down to peptides by the proteasome and transferred to the endoplasmic reticulum (ER) via transporters associated with antigen processing (TAP) molecules. In the ER peptides are processed with empty MHC I molecules and exported to the cell surface for presentation to the T-cell receptors of [[T_cells#Cytotoxic_CD8.2B|CD8⁺ T-cells]]

MHC I presents '''endogenous''' (intracellular) peptides. Viral proteins are broken down to peptides by the proteasome and transferred to the endoplasmic reticulum (ER) via transporters associated with antigen processing (TAP) molecules. In the ER peptides are processed with empty MHC I molecules and exported to the cell surface for presentation to the T-cell receptors of [[T_cells#Cytotoxic_CD8.2B|CD8⁺ T-cells]]. The purpose of presentation in this manner is to kill the cell to prevent viral replication. Activation of the cell with '''Interferon''' enhances expression of MHC class I molecules as part of the [[Innate Immunity to Viruses|innate immunity]] to viruses.

==MHC II==

==MHC II==

==TCR-MHC Interaction==

==TCR-MHC Interaction==

[[Image:MHC T cell Interaction.jpg|thumb|right|200px|Molecules of T lymphocyte recognition - Copyright Prof Dirk Werling DrMedVet PhD MRCVS]]

[[Image:MHC T cell Interaction.jpg|thumb|right|200px|Molecules of T lymphocyte recognition - Copyright Prof Dirk Werling DrMedVet PhD MRCVS]]

Only peptide associated with self-MHC will interact with and activate [[T cells]], which cannot be activated by a peptide on a foreign cell.

Only peptide associated with MHC will interact with and activate [[T cells]]. T cells therefore cannot be directly activated by a peptide on a foreign organism.

T cells will react against foreign MHC molecules and this is the basis of graft rejection

T cells will react against foreign MHC molecules (e.g. from a different person) and this is the basis of graft rejection, for example in Heart transplants.

==MHC Genetics (Polymorphism)==

==MHC Genetics (Polymorphism)==

===MHC Polymorphism In People===

===MHC Polymorphism In People===

* There are three types (loci) of MHC class I molecules known as human leukocyte antigen (HLA)- A, B, and C

* There are three types (loci) of MHC class I molecules known as human leukocyte antigen (HLA) - A, B, and C

* Equally there are three loci of MHC class II molecules - HLAs DP, DQ and DR

* Equally there are three loci of MHC class II molecules - HLAs DP, DQ and DR

In the entire human population there are only approximately 50 different variants (alleles) at each MHC class I and class II locus.

In the entire human population there are only approximately 50 different variants (alleles) at each MHC class I and class II locus.

The variation within MHC class I is entirely based on the class I heavy chain (the β2m is invariant). The variation within MHC class II is mainly within the β chains. Every individual has two alleles at each MHC locus inherited one from each parent. Any individual will therefore express two variants at most at each locus.

The variation within MHC class I is entirely based on the class I heavy chain (the β2m is invariant). The variation within MHC class II is mainly within the β chains. Every individual has two alleles at each MHC locus inherited one from each parent. Any individual will therefore express two variants at most at each locus.

This gives a maximum variability for an individual of:

This gives a maximum variability for an individual of:

* 6 different variants of MHC class I (2 each of HLA- A, B and C)

* 6 different variants of MHC class I (2 each of HLA - A, B and C)

* 6 different variants of MHC class II (2 each of HLA- DP, DQ and DR)

* 6 different variants of MHC class II (2 each of HLA - DP, DQ and DR)

Many animal species have fewer loci than the human e.g. ruminants have no MHC class II DP.

Many animal species have fewer loci than the human e.g. ruminants have no MHC class II DP.

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{{Jim Bee 2007}}

{{Jim Bee 2007}}

[[Category:Adaptive Immune System]]

[[Category:Adaptive Immune System]]

[[Category:Lymphocytes]]

[[Category:Lymphocytes]]

[[Category:Image Review]]

[[Category:Image Review]]