The RH system

There are 5 main Rhesus (Rh) antigens on red cells – C, c, D, E, e.

The most important of these is the Rh D. Having the D antigen on the red cell gives you the positive (+) and lacking it gives you the negative (-) after the letter A, B, AB or O.

The D antigen is the most immunogenic, meaning it provokes an immune response that makes it most likely to cause a transfusion reaction in the recipient.

To prevent unwanted transfusion reactions and production of alloantibody, Rh D negative girls and women of child bearing potential should not be transfused with Rh D positive red cells except in an emergency.

Alloantibody produced by transfusion of Rh D positive blood to Rh D negative women, or as a result of specific events during pregnancy, can cause Haemolytic Disease of the Fetus and Newborn (HDFN).

HDFN occurs when, either during a previous transfusion or a previous pregnancy, alloantibody in the mother’s blood crosses the placenta, binding to the fetal red cells. This breaks down the fetal red blood cells causing anaemia, referred to as haemolytic anaemia.

77% of our donors have the D antigen on their red blood cells and are D positive.

The remaining 23% lack the D antigen and are D negative. (Source – NHS Blood and Transplant; Active Donor Base December 2016).

As well as D the Rh blood group system also has CE genes.

CE produces four combinations; ce, Ce, cE or CE

The most frequently occurring phenotypes of Rh CE and Rh D are Dce, cde, DCe, dCe, DcE, dcE, DCE and dCE.

Sometimes we will communicate specifically with groups of donors who have specific phenotypes such as those who have the Dce or the ‘Ro’ subtype or phenotype. While Ro blood is rare among white European people, it is much more common among black African and Afro-Caribbean people.

The Ro phenotype is over 10 times more likely to be found in individuals from a black background than individuals from a white background. It is very frequently found in patients with sickle cell disease.

Ro blood is needed for patients who have the Ro subtype. This ensures the closest possible match for patients and reduces the likelihood of them developing alloantibodies.

Alloimmunisation is exacerbated by differences in blood group distribution between patients with sickle cell disease (SCD) and the predominantly white European blood donor population.

A significant proportion of SCD and thalassaemia patients have the Ro phenotype cDe which is rare in donors of European origin. These groups of patients require ongoing blood transfusions making it important to group and type donors more extensively (extended phenotype). Transfusion of red cells for these patients should be matched for ABO, D, C, E c, e and Kell.
With demand for Ro donations increasing it is important that we continue our work with the black community to ensure the number of black donors registering and donating increases.