At present, there are 55 antigens in the Rh system, however new antigens continue to be discovered.
The D antigen is the most immunogenic and important Rh antigen, followed by the c antigen and the E antigen.
Routine Rh typing of donors and patients only test for the presence/absence of the D antigen. The presence of the D antigen on red cells means that the individual will be D positive, while the absence of the D antigen means that the individual will be D negative.
If a D negative individual is exposed to even a small amount of D positive cells either through transfusion or pregnancy, they can form an antibody to the D antigen (anti-D).
- All Rh alloantibodies should be considered potentially capable of causing clinically significant transfusion reactions: severe haemolytic transfusion reactions (anti-D, anti-C, anti-e and anti-c).
- Haemolytic disease of the fetus and newborn (severe cases with anti-D and anti-c and mild to moderate cases with anti-C, anti-E and anti-e).
Rh inheritance patterns
The Rh blood group system is attributable to two genes, RHD and RHCE, which are located on chromosome 1.
The RHD gene is dominant, so the expression of the D antigen depends upon whether an RHD gene has been inherited from one or both parents.
A person is considered to be D positive whenever the RHD gene is present, even though the gene may have only been inherited from one parent. Conversely, a person will be D negative if no RHD gene is inherited.
Slight differences, also known as polymorphisms, of the RHD gene exist. These polymorphisms result in D variant phenotypes, and somewhat complicate the inheritance pattern of the D antigen.
RHD gene inheritance patterns
Below are the possible D phenotypes that children may express according to the combination of parental D phenotypes determined by the RHD gene inheritance.
|Parental D phenotype||Child's D phenotype|
|Positive and Positive||Positive or Negative|
|Positive and Negative||Positive or Negative|
|Negative and Negative||Negative|