Human blood groups classify blood based on specific inherited substances called antigens on the surface of red blood cells. These antigens are combinations of sugars and proteins. Understanding these classifications is important for human health, especially in medical procedures like blood transfusions. The presence or absence of these antigens dictates how an individual’s immune system reacts to blood from another person, helping prevent life-threatening immune responses.
The ABO Blood Group System
The ABO blood group system categorizes blood into four main types: A, B, AB, and O. These types are determined by the presence or absence of A and B antigens on the surface of red blood cells, with corresponding antibodies found in the blood plasma.
Individuals with Type A blood have A antigens on their red blood cells and anti-B antibodies in their plasma. Those with Type B blood possess B antigens and anti-A antibodies. Type AB blood contains both A and B antigens but neither anti-A nor anti-B antibodies. Type O blood has neither A nor B antigens but both anti-A and anti-B antibodies. These antibodies are IgM antibodies, often produced early in life due to environmental exposure.
The Rh Blood Group System
The Rh blood group system is another important classification, focusing on the D antigen. If the D antigen is present on the red blood cells, the individual is Rh-positive; if it is absent, they are Rh-negative.
The Rh system is important in pregnancy, where Rh incompatibility between a mother and her fetus can lead to hemolytic disease of the newborn (HDN). This occurs when an Rh-negative mother carries an Rh-positive fetus. Fetal red blood cells carrying the D antigen can enter the mother’s bloodstream, causing her immune system to produce anti-D antibodies.
These anti-D antibodies are of the IgG type, which can cross the placenta into the fetal circulation in subsequent pregnancies. Once in the fetus, these antibodies attach to the RhD antigens on the fetal red blood cells, leading to their destruction and causing anemia. While ABO incompatibility can also cause HDN, Rh incompatibility leads to more severe outcomes. Administering RhD immune globulin to Rh-negative mothers during pregnancy and after delivery can reduce the risk of sensitization and prevent HDN in future pregnancies.
Blood Transfusions and Compatibility
Matching blood types is important for safe blood transfusions to prevent severe immune reactions. When incompatible blood is transfused, the recipient’s antibodies can attack the donor’s red blood cells, leading to a hemolytic transfusion reaction. This reaction can cause symptoms such as fever, chills, back pain, and can be life-threatening.
Type O-negative blood is considered the “universal donor” for red blood cells because it lacks A, B, and RhD antigens, allowing it to be given to individuals of any ABO and Rh type without triggering an immune response based on these major systems. Individuals with AB-positive blood are known as “universal recipients” for red blood cells. Their red blood cells possess A, B, and RhD antigens, and their plasma lacks corresponding antibodies, allowing them to receive red blood cells from any ABO and Rh type.
Before any transfusion, thorough ABO and Rh typing of both donor and recipient blood is performed, along with cross-matching tests, to ensure compatibility and minimize the risk of adverse reactions. Monitoring the patient closely during the transfusion is also a standard safety measure.
Inheritance of Blood Groups
Blood types are inherited from parents to children. The ABO blood group is determined by a gene on chromosome 9 with three main alleles: A, B, and O. A and B alleles are co-dominant, meaning both are expressed if inherited together, as seen in AB blood type. The O allele is recessive, expressed only if two O alleles are inherited. For example, if one parent has a genotype of AO (blood type A) and the other has BO (blood type B), their children could inherit any of the four ABO blood types: A, B, AB, or O.
The Rh blood group system is also inherited, with the Rh-positive trait (D antigen present) being dominant over the Rh-negative trait (D antigen absent). An individual with at least one dominant Rh-positive allele (D) will be Rh-positive. Only individuals inheriting two recessive Rh-negative alleles (dd) will be Rh-negative. For instance, if one parent is Rh-positive (Dd) and the other is Rh-negative (dd), each child has a 50% chance of being Rh-positive (Dd) and a 50% chance of being Rh-negative (dd).
Other Blood Group Systems
Beyond the ABO and Rh systems, there are over 40 other recognized human blood group systems, including Kell, Duffy, Kidd, and MNS. While less frequently encountered in routine transfusions, these “minor” blood groups are important in specific medical contexts.
Antibodies against antigens in these other systems can cause transfusion reactions or hemolytic disease of the newborn, though often less severely than ABO or Rh incompatibilities. For example, Duffy-negative individuals exhibit resistance to certain strains of malaria, which highlights their additional biological significance. These additional blood group systems are considered in cases of repeat transfusions, organ transplants, or when investigating certain autoimmune conditions to ensure patient safety and compatibility.