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[[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]]

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 (so, for example, it is not expressed on erythrocytes), although quantity varies betweeen cell types, 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.  

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.