transferrin, protein (beta1 globulin) in blood plasma that transports iron from the tissues and bloodstream to the bone marrow, where it is reused in the formation of hemoglobin. Found fixed to the surface of developing red blood cells, transferrin frees iron directly into the cell. Human beings have 14 different types of transferrin, but all are believed to be determined at a single genetic locus. They are named by letters of the alphabet; the commonest is called C.