Evolutionary conservation of MHC class I and class II molecules—different yet the same

Despite the underlying similarities, class I molecules seem to be more heterogeneous in structure and function than class II molecules. Many features which are shared between classical class I, certain nonclassical class I and classical class II molecules (including the patterns of dilsufide bonds and particular glycosylation sites) are also conserved in vertebrate evolution; some clearly reflect structural requirements but others may be due to similarities in function. In contrast, other features (for example, the residues involved in binding the mainchain atoms of the antigenic peptide) are different in classical class I and class II molecules, but nevertheless are highly conserved in vertebrate evolution. The residues implicated in interaction with the co-receptors CD4 and CD8 are not very well conserved in vertebrate evolution, and may reflect co-evolution of the MHC molecules with their particular co-receptors.