level of MHC class II histocompatibility antigen beta chain DRB3 in blood serum (Ontology of Biological Attributes term OBA_2040365) is the amount of a MHC class II histocompatibility antigen beta chain DRB3 when measured in blood serum. Also known as: blood serum MHC class II histocompatibility antigen beta chain DRB3 amount.