Beyond the well-known ABO system, another layer of identification on our red blood cells is the Rhesus (Rh) blood group system. Within this system, one marker is particularly significant for its medical role: the Rhesus D antigen, or RhD. Understanding your RhD status is a routine part of modern healthcare, playing a role in blood transfusions and pregnancy by preventing immune system reactions to foreign cells.
Defining the RhD Antigen
An antigen is a molecule, often a protein, found on the surface of cells that can trigger an immune response. The Rh blood group system is composed of over 50 different antigens, but the D antigen is the most likely to provoke a strong immune response when introduced into someone who lacks it. This is why it holds a special status in blood typing.
Your RhD status is straightforward: you are either RhD positive or RhD negative. If the D antigen is present on your red blood cells, you are considered RhD positive; if it is absent, you are RhD negative. In the United Kingdom and Australia, approximately 85% of the population is RhD positive, while the remaining 15% is RhD negative.
How RhD Status is Inherited
A person’s RhD status is determined by genes inherited from their parents. The presence or absence of the D antigen is controlled by the RHD gene, which provides instructions for building the D antigen protein. Everyone inherits two copies of this gene, one from each parent.
If you inherit at least one functional RHD gene, your body will produce the D antigen, and you will be RhD positive. To be RhD negative, an individual must inherit two non-functional or deleted RHD genes, one from each parent. Consequently, an RhD negative mother can have an RhD positive baby if the father is RhD positive.
Importance of RhD in Blood Transfusions
Knowledge of RhD status is necessary for safe blood transfusions. If an RhD negative individual receives a transfusion of RhD positive blood, their immune system recognizes the D antigen as foreign. This triggers the production of antibodies, which are proteins designed to destroy the foreign cells. This initial exposure and antibody production is a process called sensitization.
While the first such transfusion might not cause an immediate, severe reaction, it primes the immune system. A later exposure to RhD positive blood will mount a much faster and more aggressive antibody response. This can lead to a serious transfusion reaction where the donated red blood cells are rapidly destroyed. To prevent this, blood banks carefully type both donors and recipients for their RhD status.
RhD Factor and Pregnancy
Rh incompatibility can arise when an RhD negative mother carries an RhD positive fetus, which is possible if the father is RhD positive. The issue begins if a small amount of the fetus’s RhD positive blood enters the mother’s bloodstream, an event called a feto-maternal hemorrhage. This exposure, most common during childbirth, can cause the mother’s immune system to become sensitized by producing anti-D antibodies. Because this process takes time, it rarely affects the first RhD positive baby.
Once a mother is sensitized, her immune system is prepared to react to the D antigen. In any subsequent pregnancy with an RhD positive fetus, her anti-D antibodies can cross the placenta and enter the baby’s circulation. These antibodies then attack and destroy the fetus’s red blood cells, a condition called Hemolytic Disease of the Fetus and Newborn (HDFN). This can lead to anemia, jaundice, and in severe cases, brain damage or death for the baby.
To prevent this, RhD negative pregnant women are monitored. A preventative treatment, an injection of anti-D immunoglobulin, is administered during pregnancy and after birth if the baby is confirmed to be RhD positive. The anti-D immunoglobulin works by destroying any fetal RhD positive cells in the mother’s bloodstream. This action occurs before her immune system can notice them and produce its own antibodies, effectively preventing sensitization.