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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 - α1, α2 and α3. The heavy chain C - terminus is cytoplasmic.

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 - α1, α2 and α3. The heavy chain C - terminus is cytoplasmic.

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. This domain activates [[T cells|T helper cells]] through the interaction with the CD4⁺ on the cell surface of the TH cells.

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. [[Image:MHC I Structure.jpg|thumb|right|200px| Structure of MHC I molecule - Copyright Prof Dirk Werling DrMedVet PhD MRCVS]]

[[Image:MHC I Structure.jpg|thumb|right|200px| 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 and the α3 domain and β2m are like the constant domains.

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 and the α3 domain and β2m are like the constant domains.