The amount of a alpha-1,3-mannosyl-glycoprotein 4-beta-N-acetylglucosaminyltransferase B when measured in blood serum. The trait is level of alpha-1,3-mannosyl-glycoprotein 4-beta-N-acetylglucosaminyltransferase B in blood serum (Ontology of Biological Attributes id OBA_2042360). Also known as: blood serum alpha-1,3-mannosyl-glycoprotein 4-beta-N-acetylglucosaminyltransferase B amount.