There are more than 50 antigens known to exist within the Rh blood group system.
One of those antigens is the D antigen.
When the D antigen is lacking, you are Rh- negative, also often written as Rh- or Rh(D) negative.
When all of the antigens of the Rh blood group system are missing, you are Rh null.
Rhnull phenotype is a rare blood group with a frequency of approximately 1 in 6 million individuals, transmitted via an autosomal recessive mode. It is characterized by the weak (Rhmod) or lack (Rhnull) of expression of all Rh antigens on the red cells.