The coreceptor CD8. This dimeric protein extends from the surface of a cytotoxic T cell and binds to class I MHC molecules that are expressed on the surface of the cell that is bound to the T cell. The dashed lines represent extended polypeptide chains that link the immunoglobulin domains of CD8 to the membrane. Notice that the coreceptor interacts primarily with the constant α3 domain of the class I MHC domain.
[Drawn from 1AKJ.pdb.]