Expansion of MHC-IIB Has Constrained the Evolution of MHC-IIA in Passerines
The major histocompatibility complex (MHC) is central in adaptive immunity, with the highly polymorphic MHC genes encoding antigen-presenting molecules. Two MHC class II (MHC-II) loci, DA1 and DA2, predate the radiation of extant birds and persist throughout much of the avian phylogeny. Within each locus, the MHC-II molecules are encoded by A-genes (DAA) and B-genes (DAB), which are arranged in A–
