A compound attribute that is the ratio of amount of HLA class II histocompatibility antigen gamma chain (human) in blood to amount of tumor necrosis factor receptor superfamily member 21 (human) in blood. The trait is CD74/TNFRSF21 protein level ratio in blood (Ontology of Biological Attributes identifier OBA_2053368). Also known as: the ratio of amount of HLA class II histocompatibility antigen gamma chain (human) in blood to amount of tumor necrosis factor receptor superfamily member 21 (human) in blood.