The ABO compatibility system classifies human blood based on the presence or absence of specific inherited substances located on the surface of red blood cells. These substances, called antigens, are genetically determined markers that function as identification tags for the body’s immune system. This classification is important for medical procedures because a blood type mismatch can trigger a harmful immune response.
Understanding the ABO System: Antigens and Antibodies
The ABO system classifies blood into four major types based on which specific antigens (A and B) are present on the red blood cell surface. Antigens A and B are carbohydrate molecules anchored to the cell membrane. An individual’s immune system will naturally produce corresponding antibodies against any ABO antigen not present on their own red blood cells.
These antibodies are proteins found in the blood plasma that are designed to recognize and attack foreign antigens. For example, a person with Type A blood has A antigens on their red cells but carries anti-B antibodies in their plasma. Conversely, a Type B person has B antigens and anti-A antibodies.
Type AB blood possesses both A and B antigens but develops neither anti-A nor anti-B antibodies. Type O blood lacks both A and B antigens but produces both anti-A and anti-B antibodies in the plasma. The inheritance of these types is governed by a single gene locus, where the A and B alleles are codominant over the O allele.
The Critical Role of ABO in Blood Transfusions
ABO compatibility is the most important factor in blood transfusions because mixing incompatible blood types triggers an immediate and severe immune reaction. If a recipient is given red blood cells with a foreign antigen, their plasma antibodies will bind to the donor cells. This binding causes a reaction called agglutination, where the red blood cells clump together and are destroyed in a process known as hemolysis.
This immune attack, known as an acute hemolytic transfusion reaction, can lead to severe complications such as kidney failure, shock, and potentially death. The rules of compatibility are designed to prevent the recipient’s antibodies from attacking the donor’s red blood cells. For instance, a Type A person can only safely receive blood from Type A or Type O donors, as neither carries the B antigen that would be targeted by the recipient’s anti-B antibodies.
Type O negative blood is known as the universal red cell donor because O cells lack both A and B antigens, meaning they will not provoke an antibody attack in any recipient. Conversely, a person with Type AB positive blood is considered the universal recipient because they possess all major ABO and Rh antigens and have no anti-A or anti-B antibodies to reject donor cells. The positive or negative sign associated with a blood type refers to the separate Rhesus (Rh) factor, which is another set of antigens that must also be compatible to ensure a safe transfusion. In emergency situations where a patient’s blood type cannot be quickly determined, O negative blood is the immediate choice for transfusion to minimize risk.
ABO Compatibility in Organ Donation and Pregnancy
The ABO system is also a significant consideration in solid organ transplantation, particularly for kidneys, hearts, and lungs. Antigens are not exclusive to red blood cells; they are also expressed on the surface of endothelial cells lining blood vessels within the transplanted organ. If the donor organ is ABO-incompatible with the recipient, the recipient’s antibodies can immediately attack the organ’s blood vessels, leading to a rapid hyperacute rejection.
To circumvent this, standard transplant protocol requires ABO-identical or ABO-compatible matching, following rules similar to blood transfusions. However, special procedures, such as plasma exchange and immunosuppressive therapy, can sometimes allow for ABO-incompatible transplantation, most commonly in very young children who have not yet developed strong anti-A and anti-B antibodies.
In the context of pregnancy, ABO incompatibility between a mother and her fetus is a possibility, but it is typically a much milder concern than Rh incompatibility. This condition occurs almost exclusively when a mother has Type O blood and the fetus has Type A or Type B blood. The anti-A and anti-B antibodies produced by a Type O mother are often the smaller IgG type, which can cross the placenta and target the fetal red blood cells. While this can cause a mild form of hemolytic disease of the newborn, it is rarely severe enough to require intervention.