Rhnull

Rhnull is an exceedingly rare blood type characterized by the complete absence of all red blood cell Rh antigens. In Rhnull individuals, the RhD, C, c, E, and e antigens are not expressed on the surface of red blood cells. The phenotype is sometimes referred to as true Rhnull, and the term "golden blood" reflects both its rarity and potential utility for patients who have developed antibodies to nearly all Rh antigens.

Two genetic mechanisms can produce the Rhnull phenotype. The true Rhnull type arises from mutations or deletions affecting the RHD and RHCE genes, eliminating the Rh antigen complex from the red cell membrane. A rarer regulator type results from mutations in RHAG, an accessory protein required for normal expression of Rh antigens; red cells lack Rh antigens but other blood group determinants may be preserved. Both forms produce identical serologic results: absence of Rh antigens on red cells.

Laboratory testing shows negative reactions for all standard Rh antigens, and red cells often exhibit membrane instability with increased osmotic fragility and other morphological changes. Clinically, most Rhnull individuals are otherwise healthy but may have mild chronic hemolytic anemia and reticulocytosis; some have varying degrees of RBC fragility.

Transfusion is the major clinical issue: Rhnull blood can only be safely given to other Rhnull recipients, since transfusing ordinary Rh-positive or Rh-negative blood can provoke alloimmunization against Rh antigens. Donor supply is extremely limited; patients may require lifelong management and rare donor matching.

Rhnull is extremely rare, with only a handful of cases reported worldwide. It was first described in the 1960s, and since then additional families have been identified.