The RH blood grouping system is a human blood grouping system that has a negative and a positive classification. When a person has a blood group D substance, called an antigen, on their red blood cells, they are said to be RH-positive. When the D antigen is lacking, the person is RH negative and most people are positive. the RH system is probably the most complex of the red blood cell blood groups and is second in importance only to the A, B, and O blood group systems. an RH negative person cannot receive blood from an RH positive person because the antigens in the RH positive blood will stimulate the RH negative body and produce RH antibodies. If RH-positive blood is re-introduced, it can lead to a hemolytic transfusion reaction.