ABO compatibility refers to matching blood types for medical procedures, particularly blood transfusions and organ transplants. This matching prevents severe immune reactions when incompatible blood components are mixed. Understanding ABO compatibility ensures patient safety during blood or tissue exchange.
Understanding ABO Blood Types
Blood types are categorized based on the presence or absence of specific antigens on the surface of red blood cells and corresponding antibodies in the blood plasma. The ABO system classifies blood into four main types: A, B, AB, and O. Type A blood has A antigens and anti-B antibodies, while Type B blood has B antigens and anti-A antibodies. Type AB blood has both A and B antigens but no anti-A or anti-B antibodies, and Type O blood has neither A nor B antigens but contains both anti-A and anti-B antibodies in the plasma.
Beyond the ABO system, the Rh factor, the RhD antigen, is another important component in blood typing. Individuals are classified as Rh-positive if they have the RhD antigen on their red blood cells, and Rh-negative if they do not. Unlike ABO antibodies, Rh antibodies develop only after exposure to Rh-positive blood, such as during a transfusion or pregnancy. This Rh status, combined with the ABO type, creates the eight common blood types (e.g., A+, O-), which are inherited from parents.
ABO Compatibility in Blood Transfusions
Ensuring ABO compatibility is important for safe blood transfusions to prevent harmful reactions. Type O-negative blood is considered the universal donor for red blood cells because it lacks A, B, and RhD antigens, allowing transfusion to any ABO and Rh type without an immune response. Conversely, AB-positive individuals are universal recipients for red blood cells, as their blood contains A, B, and RhD antigens and lacks corresponding antibodies, enabling them to receive blood from any ABO and Rh type.
Before a transfusion, cross-matching confirms compatibility between the donor’s red blood cells and the recipient’s plasma. This involves mixing a small sample of the recipient’s serum with donor red blood cells to check for agglutination, or clumping, which indicates incompatibility. This process detects antibodies in the recipient that might react with donor antigens, ensuring a safe transfusion. In emergencies, O-negative blood is often used when there is no time for full cross-matching.
ABO Compatibility in Pregnancy
ABO and Rh compatibility between a pregnant person and their fetus is an important consideration, particularly for Rh incompatibility. Rh incompatibility occurs when an Rh-negative mother carries an Rh-positive fetus, as her immune system may recognize fetal RhD antigens as foreign. This can lead to the mother producing anti-D antibodies, which may cross the placenta and attack the fetus’s red blood cells. This condition, known as Hemolytic Disease of the Newborn (HDN), can cause the fetus to develop anemia and jaundice.
While Rh incompatibility affects subsequent pregnancies because sensitization occurs during the first birth, ABO incompatibility can affect the first pregnancy since anti-A and anti-B antibodies are naturally occurring. However, ABO incompatibility in pregnancy is less severe than Rh incompatibility, as fetal red blood cells express fewer ABO antigens. To prevent Rh sensitization, Rh-negative mothers receive RhoGAM, an immune globulin injection, during the second trimester and again shortly after birth if the baby is Rh-positive.
Consequences of Incompatibility
When ABO or Rh incompatibility goes unmanaged, the consequences can be severe. In blood transfusions, an incompatible transfusion can trigger an acute hemolytic transfusion reaction (AHTR), where the recipient’s antibodies rapidly destroy the transfused red blood cells. Symptoms begin shortly after the transfusion starts and can include fever, chills, chest pain, back pain, difficulty breathing, and a sense of dread. This reaction can quickly progress to kidney failure, shock, and disseminated intravascular coagulation (DIC).
Unmanaged Rh or ABO incompatibility in pregnancy can lead to Hemolytic Disease of the Newborn (HDN). In HDN, maternal antibodies cross the placenta and destroy the fetal or newborn’s red blood cells, leading to anemia and elevated bilirubin levels. Symptoms in newborns can range from mild jaundice and lethargy to more severe conditions like an enlarged liver or spleen, fluid accumulation in body tissues (hydrops fetalis), and potential brain damage from high bilirubin levels (kernicterus). Severe cases may require immediate medical interventions, including phototherapy or exchange transfusions, to manage the newborn’s condition.