RH factor is a protein of the red blood cell plasma membrane that behaves as antigen in blood transfusions triggering a humoral (antibody-based) immune response. Most people present the protein in their red blood cells and are part of the Rh+ group. People that do not have the protein classify as Rh-. The origin of the name Rh factor is related to the first researches that discovered this blood antigen was in rhesus monkeys (“Macaca mulatta”).   _{Your Comment}