What chickens might tell us about the MHC class II system

•Chickens provide most knowledge of antigen presentation outside of mammals.•Chickens have most of the genes expected for the class II system from mammals.•Chickens have two class II B genes and two DMB genes.•One class II B/DMB gene pair is widely expressed.•A second class II B/DMB gene pair is expressed most strongly in spleen and intestine.

Almost all knowledge about the structure and function of MHC class II molecules outside of mammals comes from work with chickens. Most of the genes implicated in the class II system are present in chickens, so it is likely that the machinery of antigen processing and peptide-loading is similar to mammals. However, there is only one isotype (lineage) of classical class II genes, with one monomorphic DR-like BLA gene and two polymorphic BLB genes, located near one DMA and two DMB genes. The DMB2 and BLB2 genes are widely expressed at high levels, whereas the DMB1 and BLB1 genes are only expressed at highest levels in spleen and intestine, suggesting the possibility of two class II systems in chickens.