Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. HLA class II histocompatibility antigen, DRB3-1 beta chain is a protein that in humans is encoded by the HLA-DRB3 gene.The protein encoded by this gene belongs to the HLA class II beta chain paralogues. The class II molecule is a heterodimer consisting of an alpha (DRA) and a beta chain (DRB), both anchored in the membrane. It plays a central role in the immune system by presenting peptides derived from extracellular proteins.