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(A) The α chain of the class I molecule has three extracellular domains, α₁, α₂ and α₃, encoded by separate exons. It is noncovalently associated with a smaller polypeptide chain, β₂-microglobulin, which is not encoded within the MHC. The α₃ domain and β₂-microglobulin are Ig-like. While β₂-microglobulin is invariant, the α chain is extremely polymorphic, mainly in the α₁ and α₂ domains. (B) In class II MHC proteins, both chains are polymorphic, mainly in the α₁ and β₁ domains; the α₂ and β₂ domains are Ig-like. Thus, there are striking similarities between class I and class II MHC proteins. In both, the two outermost domains (shaded in blue) interact to form a groove that binds peptide fragments of foreign proteins and presents them to T cells