ABO incompatibility is a mismatch between blood types that causes the immune system to attack and destroy red blood cells. This reaction occurs when an individual receives blood containing antigens their body recognizes as foreign. This immune reaction is important in medical scenarios like blood transfusions, where it can cause immediate complications, and in obstetrics, where it affects the health of a newborn.
Understanding ABO Blood Groups
The ABO blood group system classifies human blood into four main types: A, B, AB, and O. This classification depends on the presence or absence of specific markers, known as antigens, on the surface of red blood cells. Antigens are inherited molecules that act like a unique name tag, identifying the cell to the immune system.
Individuals with Type A blood have A antigens on their red cells and naturally occurring antibodies against the B antigen (anti-B) in their plasma. Conversely, those with Type B blood carry B antigens and produce anti-A antibodies. Type AB blood has both A and B antigens but contains neither anti-A nor anti-B antibodies.
Type O blood lacks both A and B antigens on its red cells. However, the plasma of a Type O individual contains both anti-A and anti-B antibodies. These antibodies are formed early in life, stimulated by exposure to similar antigens found in common environmental substances.
The Mechanism of Immune Reaction
An ABO incompatibility reaction begins the moment incompatible red blood cells are introduced into the bloodstream, most commonly due to a transfusion error. The recipient’s pre-existing plasma antibodies recognize the foreign antigens on the donor red blood cells as a threat.
For instance, if Type A blood is given to a Type B recipient, the recipient’s anti-A antibodies will bind to the donor’s A antigens. This binding causes the red blood cells to clump together, a process called agglutination. Following agglutination, the immune system rapidly destroys the foreign cells in a process known as hemolysis.
This rapid destruction of red blood cells releases hemoglobin and other components into the bloodstream. This acute hemolytic transfusion reaction can lead to life-threatening complications, including kidney failure, shock, and disseminated intravascular coagulation. The naturally occurring anti-A and anti-B antibodies are predominantly of the large IgM class, which efficiently triggers this destructive immune response.
ABO Incompatibility in Pregnancy
While the immune reaction is severe in transfusions, ABO incompatibility between a mother and fetus is usually milder. The mismatch occurs when a mother has a blood type, typically Type O, and the fetus inherits a different type, such as Type A or Type B. This scenario is present in about 15% of all pregnancies.
Type O mothers are most often affected because their anti-A and anti-B antibodies are more likely to be the smaller IgG subclass, which can cross the placenta. Antibodies from Type A or Type B mothers are mostly the larger IgM type that cannot pass through the placental barrier. When maternal IgG antibodies cross over, they enter the fetal circulation and target the baby’s incompatible red blood cells.
This process can cause Hemolytic Disease of the Newborn (HDN), although only a small fraction of incompatible pregnancies result in symptomatic disease. Unlike Rh incompatibility, ABO-HDN can affect a firstborn child because the maternal antibodies are naturally occurring, not requiring a previous exposure. The destruction of the newborn’s red cells results in anemia and a buildup of bilirubin, a yellowish waste product from the broken-down hemoglobin.
The main symptom of HDN is jaundice, a yellowing of the skin and eyes appearing shortly after birth. The severity of ABO-HDN is generally mild because the A and B antigens are less developed on fetal red blood cells compared to adult cells. If bilirubin levels become extremely high, there is a risk of kernicterus, which can cause permanent brain damage.
Detection and Clinical Management
Identifying the risk of ABO incompatibility begins with routine prenatal screening to determine the mother’s blood type. If the mother is Type O, the newborn is monitored closely after delivery. A common diagnostic tool used at birth is the Direct Antiglobulin Test (Direct Coombs test), performed on the baby’s cord blood.
A positive Direct Coombs test confirms that maternal antibodies have attached to the surface of the baby’s red blood cells. However, a positive result does not always mean the baby will develop symptoms, as most cases of ABO incompatibility are clinically insignificant. The primary concern for affected newborns is managing hyperbilirubinemia, or high bilirubin levels.
The standard treatment for jaundice caused by ABO-HDN is phototherapy. This involves placing the infant under special lights that convert bilirubin into a form the body can excrete more easily. In rare instances where bilirubin levels rise rapidly or severe anemia is present, an exchange transfusion may be necessary to replace the baby’s blood with compatible donor blood.