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The Rh factor is an antigen (a protein) that is found on the surface of the red blood cell. Along with the A and B antigens it comprises the major scheme we use for "typing" or classifying human blood for compatibility for transfusion.
The term "Rh factor" is short for Rhesus factor. It seems we share this antigen with our relatives in the primate world, the rhesus monkeys.
The antigen is either there, or it is not. This is determined by the presence or absence of a single gene. We can test for the presence of the antigen by adding artificially produced antibodies to a mixture of the patient's red cells. If they form clumps, we know that the antibody is present on the red cells. We term this blood Rh positive. If the antigen is not present (and the cells don't clump), we designate the blood type as Rh negative.