The Rh blood group system consists of 49 defined blood group antigens. One of them is the D antigen. If you test negative for the presence of the D antigen, you are Rh Negative. This is why Rh Negative is also often listed as Rh(D) negative or D negative. If you test negative for the presence of ALL antigens of the Rh Blood Group System, you are RHnull.