Changes

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

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

===Structure===

===Structure===

* MHC class I is expressed on virtually all nucleated cells

MHC class I is expressed on virtually all nucleated cells and consists of a membrane-associated heavy chain bound non-covalently with a secreted light chain. The heavy chain is made up of three distinct extracellular protein domains - &alpha1, &alpha2 and &alpha3. The heavy chain C - terminus is cytoplasmic.

* MHC class I consists of a membrane-associated heavy chain bound non-covalently with a secreted light chain

 

** Heavy chain:

The light chain is known as β2-microglobulin and is similar in structure to one of the heavy chain domains. It is not membrane associated but binds to the α3-domain of the heavy chain

*** Made up of three distinct extracellular protein domains

[[Image:MHC I Structure.jpg|thumb|right|175px| Structure of MHC I molecule - Copyright Prof Dirk Werling DrMedVet PhD MRCVS]]

**** α1, α2 and α3

 

*** The C- terminus is cytoplasmic

The MHC class I domains are structurally and genetically related to immunoglobulin and TcR domains; the outer domains (&alpha1 and &alpha2) are like the variable domains and the &alpha3 domain and &beta2m are like the constant domains.

** Light chain:

 

*** Known as β2-microglobulin

MHC class I molecules are folded to form specific 3-dimensional structures. The α1 and α2 domains are folded to produce an antigen-binding groove which can bind molecules of a limited size only (8-10 amino acids). This limits the size of epitope seen by the T-cell receptors.

*** Similar in structure to one of the heavy chain domains

*** Not membrane associated

**** But binds to the α3-domain of the heavy chain

[[Image:MHC I Structure.jpg|thumb|right|150px| Structure of MHC I molecule - Copyright Prof Dirk Werling DrMedVet PhD MRCVS]]

* The MHC class I domains are structurally and genetically related to immunoglobulin and TcR domains

** The outer domains (α1 and α2) are like the variable domains

** The α3 domain and β2m are like thrconstant domains

* MHC class I molecules are folded to form specific 3-dimensional structures

** The α1 and α2 domains are folded to produce an antigen-binding groove

*** This groove can bind molecules of a limited size only (8-10 amino acids)

**** This limits the size of epitope seen by the T-cell receptors

===Presentation Pathway===

===Presentation Pathway===