Rhesus factor

The second common blood grouping uses the Rhesus factor (Rh factor), so called because it was first discovered in Rhesus monkeys. Rhesus factor is in addition to the ABO groups. This factor is simply present or absent on the redcells. Unlike the ABO system there are no naturally occurring antibodies in the plasma, but antibodies may develop in a Rhesus negative individual if they are exposed to Rhesus positive blood.

Recipients who are Rhesus negative should therefore only receive Rhesus negative blood. If they were to be given Rhesus positive blood there would beno reaction on the first occasion, but the introduction of Rhesus antigens would cause the recipient to produce Rhesus factor antibodies. This means if the patient were to be given Rhesus positive blood on a second occasion, the new Rhesus factor antibodies would bind to the donated Rhesus factor antigen, leading to agglutination. Therefore Rhesus negative recipients may only receive Rhesus negative blood. However, in theory, Rhesus positive patients may receive Rhesus negative blood, as the red cells contain no antigens.

Once the Rhesus factor is taken into account it means that O negative is the universal donor and AB positive the universal recipient. The Rhesus factor is sometimes referred to as the D factor. This is because the most active component of the Rhesus factor antigen is termed the D factor.