Your blood type is a fundamental piece of your biological identity, determined by the genes inherited from your parents. If you have an RhD positive blood type, it means your red blood cells have a specific protein, known as the D antigen, on their surface. This is the most common blood type configuration globally, with the majority of the world’s population carrying this RhD antigen. The “positive” designation is a component of your full blood type, which also includes your ABO group, creating classifications like O positive or A positive.
The Science of the Rh Factor
The Rh blood group system is one of the most complex, with over 50 different antigens identified. The most significant of these is the D antigen because it is the most likely to provoke an immune response. The presence or absence of this single protein is what defines a person’s Rh status. If the D antigen is present, the person is RhD positive; if it’s absent, they are RhD negative.
Your RhD status is a direct result of your genetics. It is determined by the RHD gene, which provides the instructions for making the D antigen. Because you inherit genes from both parents, their Rh statuses determine yours. The RhD positive trait is dominant, meaning a person only needs to inherit one copy of the RHD gene to be RhD positive.
The prevalence of RhD positive status is high across the globe, though it varies among different populations. For instance, about 85% of people in the United Kingdom are RhD positive.
Rh Factor in Pregnancy
For individuals who are RhD positive, pregnancy is typically uncomplicated by the Rh factor. Whether the fetus is RhD positive or RhD negative, the mother’s immune system will not have a negative reaction related to this specific blood protein. Her body already recognizes the D antigen, so a fetus with the same antigen presents no issue.
The medical significance of the Rh factor becomes apparent in cases of Rh incompatibility. This occurs when an RhD-negative mother carries an RhD-positive fetus. During pregnancy or childbirth, a small amount of the fetus’s blood can enter the mother’s bloodstream. Since the mother’s immune system does not recognize the D antigen, it may identify the fetal red blood cells as foreign and begin producing antibodies against them.
This initial sensitization during a first pregnancy usually doesn’t harm the first baby. However, these newly created anti-D antibodies are permanent. If the woman has a subsequent pregnancy with another RhD-positive fetus, her antibodies can cross the placenta and attack the fetus’s red blood cells. This can lead to a condition known as hemolytic disease of the newborn, which can cause serious health problems for the baby.
To prevent this immune response, RhD-negative mothers are given a medication called anti-D immunoglobulin, e.g., RhoGAM. This injection is administered around 28 weeks of pregnancy and again within 72 hours after delivery if the baby is confirmed to be RhD positive. The anti-D immunoglobulin works by destroying any RhD-positive fetal blood cells in the mother’s circulation before her immune system can make its own antibodies.
Blood Transfusions and Donations
When it comes to blood transfusions, the Rh factor is a main consideration alongside the ABO blood group to ensure compatibility. For an RhD-positive individual, receiving blood is relatively flexible. They can safely receive red blood cells from donors who are RhD positive or RhD negative, as long as the ABO type is compatible.
The rules for donating blood are more specific for an RhD-positive person. Their red blood cells can only be given to other individuals who are also RhD positive. Giving RhD-positive blood to an RhD-negative recipient could trigger an immune response, causing the production of anti-D antibodies. This sensitization could cause complications in future transfusions or, for females, during future pregnancies.
This is why RhD-negative blood, particularly O negative (O-), is valuable in emergency medicine. Since O- blood lacks A, B, and RhD antigens, it can be safely given to almost any patient, regardless of their blood type, without causing a major immune reaction. This makes it the universal red cell donor type, often used when a patient’s blood type is unknown or in emergencies.