Blood group antigens are molecules located on the surface of red blood cells. These antigens, which can be carbohydrates or proteins, are recognized by the immune system and define an individual’s unique blood type. Their presence or absence helps the body identify its own cells.
ABO and Rh Blood Groups
The ABO blood group system is widely recognized and classifies blood into four primary types: A, B, AB, and O. This classification depends on the presence or absence of specific antigens, A and B, on the red blood cell surface. Individuals with type A blood have A antigens and anti-B antibodies in their plasma, while those with type B blood possess B antigens and anti-A antibodies. Type AB blood features both A and B antigens but lacks anti-A or anti-B antibodies. Conversely, type O blood has neither A nor B antigens, yet contains both anti-A and anti-B antibodies in the plasma.
The Rh blood group system is another important classification, determining whether blood is positive or negative. This system primarily focuses on the presence or absence of the RhD antigen, a protein found on the surface of red blood cells. If the RhD antigen is present, the blood is considered Rh-positive; if absent, it is Rh-negative. The Rh system encompasses 50 defined antigens, with the D antigen being the most significant due to its ability to provoke a strong immune response.
These two systems are important for blood transfusions because incompatible blood types can trigger severe immune reactions. For example, if a person with type B blood receives type A blood, their anti-A antibodies will attack the transfused red blood cells, leading to complications like agglutination and hemolysis. Matching both ABO and Rh types is a standard practice to ensure patient safety during transfusions.
The Diversity of Other Blood Groups
Beyond the well-known ABO and Rh systems, the International Society of Blood Transfusion (ISBT) recognizes 48 human blood group systems as of May 2025, containing 366 distinct red cell antigens. These “minor” blood groups include systems such as Kell, Duffy, Kidd, MNS, Lewis, and P.
For instance, the Kell blood group includes the K antigen, found on the red cells of about 9% of the Caucasian population. Antibodies to these antigens can develop, particularly in individuals who have received multiple blood transfusions or during pregnancy.
Identifying these diverse blood groups and the antibodies against them is important in specific medical scenarios. For patients requiring repeated transfusions, checking for antibodies to these minor blood groups helps prevent delayed transfusion reactions. In some cases, rare blood types within these systems may be needed for patients with specific antibodies, making the identification of these antigens a factor in finding compatible blood.
How Blood Groups Are Inherited
A person’s blood group is determined by the genes inherited from their biological parents. These genes provide instructions for producing the specific antigens found on the surface of red blood cells. The ABO blood group, for instance, is governed by the ABO gene located on chromosome 9, which has three main forms, called alleles: A, B, and O.
Alleles A and B are codominant, meaning that if both are inherited, both A and B antigens will be expressed, resulting in AB blood type. The O allele is recessive, so it only determines the blood type if two O alleles are inherited, leading to type O blood. For example, a child inheriting an A allele from one parent and an O allele from the other will have type A blood because A is dominant over O.
Similarly, the Rh factor is also genetically determined. The genes responsible for Rh blood type are located on chromosome 1. The Rh-positive allele follows an autosomal dominant pattern. This means that inheriting just one Rh-positive allele from either parent will result in Rh-positive blood. To be Rh-negative, an individual must inherit two Rh-negative alleles, one from each parent.
Blood Group Antigens in Health and Medicine
Blood group antigens are crucial for safe blood transfusions. Matching blood types between donor and recipient is necessary to prevent adverse immune reactions, where the recipient’s antibodies could attack the donor’s red blood cells. These reactions can range from mild to severe, including acute or delayed hemolytic reactions, which involve the destruction of red blood cells. Routine blood typing and cross-matching procedures identify compatible blood before transfusion.
Blood group antigens are also important in preventing Hemolytic Disease of the Newborn (HDN), particularly concerning Rh incompatibility. This condition can occur when an Rh-negative mother is exposed to Rh-positive red blood cells from her fetus, typically during pregnancy or childbirth. The mother’s immune system may produce antibodies against the RhD antigen, and these antibodies can cross the placenta in subsequent pregnancies, attacking the Rh-positive fetal red blood cells.
The introduction of Rh immunoglobulin (RhIg) has significantly reduced the incidence of severe HDN. RhIg is an injection given to Rh-negative mothers during pregnancy and after delivery, preventing the mother’s immune system from forming antibodies against the RhD antigen. Beyond transfusions and HDN, blood group antigens also have other biological functions, such as contributing to cell membrane structure and participating in transport across cell surfaces. Some antigens may even influence susceptibility or resistance to certain infectious diseases, indicating their broader biological significance.