The blood types exist in accordance with the presence, on the surface of the red blood cells, of antigens which may be of varied biochemical nature.
Each person has a different set of antigens in the red blood cells, and it is unlikely (or even impossible) to find two individuals with the same antigenic combination. Therefore, specific antibodies (directed against the antigens that the individual does not possess) can be found in serum or plasma, causing agglutination or hemolysis when an incompatible transfusion occurs.
The most important antigens are the ABO system and the Rh system. According to the ABO system, there are the following types of blood: A, B, AB and O. The Rh System divides blood types into Rh positive or negative.
Individuals with blood A have circulating anti-B antibodies; those with B blood have anti-A antibodies; those with AB blood do not have antibodies and those with O blood have anti-A and anti-B antibodies. Thus, individuals with blood A may receive A or O blood transfusions; those with B blood, B or O transfusions; those with AB blood, transfusions of any blood type, and those with blood O can only receive blood type O. In other words, AB type is the universal receptor and type O, universal donor.
Individuals with Rh positive do not have antibodies, and those with Rh negative have anti-Rh antibodies only if they are exposed to Rh positive blood. Therefore, individuals with Rh negative blood can only receive Rh negative (or Rh positive, but only once, since there will be antibody production later). Individuals with Rh positive can receive Rh positive or negative transfusions. Complementing the above information, then, the universal donor is the type O negative, and the universal receptor, the positive type AB.
In addition to the importance of blood types in transfusion cases, it is also important to gestation, because a disease called fetal erythroblastosis may occur, or hemolytic disease of the newborn. This disease is found when a mother with Rh + who has had a child with Rh + (or who has had contact with Rh + blood in a transfusion) gives birth to a child with Rh +. After the first birth, or accidental transfusion, the mother's blood comes in contact with the blood of the fetus and creates anti-Rh + antibodies. During the second pregnancy, these antibodies can cross the placenta and cause hemolysis of the second child's blood.
There is now treatment for this condition, and mothers with Rh negative, with positive indirect blood coombs, should receive anti-D immunoglobulin (RhoGAM®) a few weeks before delivery or in the first 72 hours after delivery in order to prevent the formation of antibodies that could create complications in subsequent pregnancies. Blood type research (ABO and Rh), in addition to indirect coombs (if Rh negative), should be done in the prenatal of all pregnant women.
