Divergence between genes but limited allelic polymorphism in two MHC class II A genes in Leach's storm-petrels Oceanodroma leucorhoa.

The major histocompatibility complex (MHC) is critical to host-pathogen interactions. Class II MHC is a heterodimer, with α and β subunits encoded by different genes. The peptide-binding groove is formed by the first domain of both subunits (α and β), but studies of class II variation or natural selection focus primarily on the β subunit and II B genes.