Important modulator in the HLA class II restricted antigen presentation pathway by interaction with the HLA-DM molecule in B-cells. Modifies peptide exchange activity of HLA-DM. Heterodimer of an alpha chain (DOA) and a beta chain (DOB). Forms a heterotetrameric complex with an HLA-DM molecule during intracellular transport in endosomal/lysosomal compartments in B-cells. Complexes with HLA-DM molecule during intracellular transport and in endosomal/lysosomal compartments. Heterotetramerization is necessary to exit the ER. The following alleles of DOB are known: DOB*01:01, DOB*01:02, DOB*01:03 and DOB*01:04. The sequence shown is that of DOB*01:01. Belongs to the MHC class II family.