While most people are familiar with the main blood types—A, B, AB, and O—a less-discussed characteristic is the Rh factor. This inherited trait determines whether your blood type is positive or negative. Being Rh-negative is not a medical condition or illness, but rather a normal, natural variation found in a portion of the global population. Understanding your Rh status is a routine part of modern healthcare.
Understanding the Rh Factor
The Rhesus (Rh) factor is a specific protein, known as the RhD antigen, that may be present on the surface of your red blood cells. If your cells have this protein, you are considered Rh-positive; if they do not, you are Rh-negative. The Rh system is complex, with over 50 different antigens, but the D antigen is the most significant because it is the most likely to provoke an immune response.
Your Rh status is determined by genetics. The gene for the Rh factor is a dominant trait, meaning you only need to inherit one copy of the Rh-positive gene from either parent to have Rh-positive blood. For example, if one parent is Rh-positive and the other is Rh-negative, their child can be Rh-positive.
A person can only be Rh-negative if they inherit two copies of the Rh-negative gene, one from each parent. This explains why two Rh-positive parents can have an Rh-negative child if both parents are heterozygous, meaning they each carry one positive and one negative gene. If both parents are Rh-negative, their children will always be Rh-negative.
Rh Incompatibility in Pregnancy
The most significant health consideration related to the Rh factor arises during pregnancy in cases of Rh incompatibility. This situation occurs when an Rh-negative mother carries a fetus with Rh-positive blood. Since the mother’s and baby’s circulatory systems are separate, this difference does not cause problems during the pregnancy itself. The primary risk emerges when fetal blood enters the mother’s bloodstream.
This mixing of blood most commonly happens during labor and delivery but can also occur following a miscarriage, an invasive prenatal test like amniocentesis, or physical trauma. When the Rh-positive fetal red blood cells enter the Rh-negative mother’s circulation, her immune system recognizes the RhD antigen as a foreign substance. Her body then begins to produce antibodies to destroy these foreign cells in a process called sensitization.
This initial sensitization process takes time, so the first Rh-positive baby is usually born before the mother can produce enough antibodies to cause harm. The danger is in subsequent pregnancies with another Rh-positive fetus. Now that the mother’s immune system is sensitized, it is primed to launch a faster antibody response that can cross the placenta and attack the red blood cells of the new fetus.
This attack leads to the breakdown of fetal red blood cells, a condition known as hemolytic disease of the fetus and newborn (HDFN). The destruction of red blood cells causes anemia, where the baby’s blood cannot carry enough oxygen. As red blood cells are broken down, bilirubin is released, leading to jaundice and, in severe cases, potential brain damage. The most serious instances of HDFN can cause severe anemia, heart failure, and swelling, a life-threatening condition called hydrops fetalis.
Prevention and Management of Rh Incompatibility
Fortunately, Rh incompatibility is a highly preventable condition. The management is an injection of a substance called Rh immune globulin, often known by the brand name RhoGAM. This is a blood product containing antibodies to the RhD antigen. These injected antibodies find and neutralize any Rh-positive fetal blood cells in the mother’s bloodstream before her own immune system can react and create its own permanent antibodies.
The administration of Rh immune globulin is a routine part of prenatal care for Rh-negative women. The first dose is given around the 28th week of pregnancy as a preventative measure. After the baby is born, its blood type is tested. If the baby is confirmed to be Rh-positive, the mother will receive a second dose of the injection, usually within 72 hours of delivery.
This preventative treatment is also administered to Rh-negative women after any event that could cause fetal and maternal blood to mix, as mentioned previously. Following this protocol for every pregnancy greatly reduces the risk of an Rh-negative mother becoming sensitized and endangering future babies.
Other Health Considerations for Rh-Negative Individuals
Outside of pregnancy, the primary health consideration for Rh-negative individuals involves blood transfusions. To prevent a harmful immune reaction, an Rh-negative person must receive Rh-negative blood. If an Rh-negative individual is given Rh-positive blood, their immune system will produce antibodies against it. A subsequent transfusion with Rh-positive blood would then trigger a serious transfusion reaction, which is why O-negative blood is considered the universal red cell donor.
The distribution of Rh-negative blood varies across global populations. It is most common in people of Caucasian descent, with about 15% of this population being Rh-negative, but much rarer in individuals of African or Asian ancestry. This rarity can sometimes present challenges in maintaining adequate supplies in blood banks.
Over the years, various myths and unsupported theories have emerged about Rh-negative blood, suggesting links to special abilities or non-human origins. These claims lack any scientific evidence. The existence of the Rh-negative blood type is a result of normal human genetic variation from the inheritance of a non-functional gene from both parents. It is a well-understood biological trait, not a marker of anything mysterious.