The ABO and Rh systems are the primary methods for classifying human blood based on inherited proteins, called antigens, on the surface of red blood cells. The specific combination of these antigens dictates a person’s blood type. This system is a foundational part of modern medicine, ensuring patient safety in many procedures.
The Science of Blood Groups
The ABO system categorizes blood into four main types: A, B, AB, and O. This is based on the presence or absence of two specific antigens, A and B, on the surface of red blood cells. Individuals with type A blood have the A antigen, type B have the B antigen, and type AB has both. People with type O blood have neither the A nor the B antigen on their red blood cells.
Correspondingly, the liquid portion of blood, called plasma, contains antibodies that work against the antigens an individual’s red blood cells lack. For instance, a person with type A blood has anti-B antibodies in their plasma, while someone with type B blood has anti-A antibodies. Type O individuals have both anti-A and anti-B antibodies, and type AB individuals have neither.
The Rh system adds another layer to blood typing, focusing on a different protein known as the D antigen. If the D antigen is present on red blood cells, a person is considered Rh-positive (+). If the D antigen is absent, the person is Rh-negative (-). This designation is combined with the ABO type to create the eight most common blood groups, such as A+ or B-.
Blood Transfusion Compatibility
The principles of blood typing are applied in transfusions to prevent the immune system from attacking donated blood. If a person receives blood with antigens their body recognizes as foreign, their antibodies will bind to the transfused red blood cells. This interaction causes the cells to clump together, a process called agglutination, which can lead to a severe and potentially fatal transfusion reaction. To ensure safety, the donor’s antigens must not trigger a reaction from the recipient’s antibodies. For example, a person with type A blood cannot receive type B blood because their anti-B antibodies would attack the B antigens.
This dynamic leads to the concepts of the “universal donor” and “universal recipient.” Individuals with type O-negative blood are considered universal red blood cell donors because their red cells lack A, B, and D antigens. Conversely, individuals with type AB-positive blood are considered universal recipients because they lack anti-A, anti-B, and anti-D antibodies, allowing them to receive red blood cells from any ABO and Rh type.
Significance During Pregnancy
The Rh factor holds particular importance during pregnancy when an Rh-negative mother carries an Rh-positive fetus. This situation, known as Rh incompatibility, does not typically pose a risk during a first pregnancy because the mother’s and baby’s blood supplies are usually separate. However, during delivery, a small amount of the baby’s Rh-positive blood can mix with the mother’s Rh-negative blood.
This exposure can trigger the mother’s immune system to recognize the Rh protein as a foreign substance and begin producing anti-D antibodies in a process called sensitization. While this sensitization does not affect the first baby, it creates a risk for any subsequent pregnancies with an Rh-positive fetus. In these later pregnancies, the mother’s newly formed antibodies can cross the placenta and attack the fetus’s red blood cells.
This immune attack can lead to a condition called hemolytic disease of the newborn (HDN), causing the baby’s red blood cells to break down. HDN can result in severe anemia, jaundice, and other serious health problems. To prevent this, Rh-negative mothers are given an injection of Rh immunoglobulin around the 28th week of pregnancy and again after delivery if the baby is confirmed to be Rh-positive. The Rh immunoglobulin prevents the mother’s immune system from producing its own antibodies against the Rh factor, protecting future pregnancies.