Blood groups are a biological classification system based on inherited variations found on the surface of red blood cells. These variations are determined by the presence or absence of specific proteins or carbohydrate molecules, which act as antigens. These surface markers are recognized by the immune system and determine which blood types are compatible for donation and receipt. Classification is important for safety in medical procedures, particularly blood transfusions.
The Major Classification Systems
The ABO system is the most significant classification, dividing blood into four main types: A, B, AB, and O. This grouping is based on whether red blood cells possess the A antigen, the B antigen, both, or neither. Plasma, the liquid component of blood, contains corresponding antibodies that develop early in life against any antigens the person’s own red blood cells lack.
A person with Type A blood has A antigens on their red cells and anti-B antibodies in their plasma. Type B blood has B antigens and anti-A antibodies. Type AB individuals possess both A and B antigens but neither anti-A nor anti-B antibodies. Conversely, Type O blood has neither A nor B antigens but carries both anti-A and anti-B antibodies in the plasma.
The Rhesus (Rh) factor is the second most important system, determined by the presence or absence of the RhD protein (D antigen) on the red cell surface. If the D antigen is present, the blood is Rh-positive (\(Rh+\)); if absent, it is Rh-negative (\(Rh-\)). Unlike ABO antibodies, anti-Rh antibodies are acquired only after an Rh-negative person is exposed to Rh-positive blood, usually through transfusion or pregnancy. Combining the two systems results in the eight common blood types.
The Role of Genetics in Blood Type
Blood type is determined by genes inherited from both parents, following Mendelian inheritance patterns. The ABO gene, located on chromosome 9, has three alleles: A, B, and O. The A and B alleles are co-dominant; inheriting both results in Type AB blood, expressing both antigens.
The O allele is recessive, so it is only expressed if a person inherits an O allele from both parents, resulting in Type O blood. For example, a person with the genotype AO will have Type A blood because the A allele is dominant over the recessive O allele. The combination of these three alleles dictates one of the four ABO blood types.
Rh factor inheritance is controlled by genes, primarily located on chromosome 1, and follows a dominant-recessive pattern. The Rh-positive allele is dominant, meaning inheriting just one copy results in Rh-positive blood. A person is only Rh-negative if they inherit the recessive Rh-negative allele from both parents. Since the ABO and Rh genes are inherited independently, the two systems combine to determine the complete blood type.
Compatibility and Clinical Significance
The clinical significance of blood groups lies in ensuring compatibility for transfusions to prevent a life-threatening immune reaction. When an incompatible blood type is transfused, the recipient’s pre-existing antibodies recognize the donor’s red cell antigens as foreign. This triggers a reaction where the antibodies bind to the foreign red blood cells, causing them to clump together, a process known as agglutination. This immune response, called a hemolytic transfusion reaction, can lead to kidney failure and be fatal.
Compatibility rules establish O-negative (\(O-\)) blood as the universal donor for red blood cells because it lacks A, B, and RhD antigens. This means it will not be attacked by any recipient’s antibodies. Conversely, AB-positive (\(AB+\)) blood is considered the universal recipient because AB+ individuals possess all three major antigens and have no corresponding antibodies to attack transfused red cells. While these types are used in emergencies, the safest practice is always to match the donor and recipient blood types exactly.
Blood type also holds importance during pregnancy if an Rh-negative mother is carrying an Rh-positive fetus. During delivery, or sometimes earlier, a small amount of the baby’s Rh-positive blood can enter the mother’s bloodstream. The mother’s immune system recognizes the RhD antigen as foreign and begins producing anti-Rh antibodies, a process called Rh sensitization.
These maternal anti-Rh antibodies can cross the placenta in subsequent pregnancies and attack the red blood cells of a future Rh-positive fetus, leading to Hemolytic Disease of the Newborn (HDN). To prevent sensitization, Rh-negative mothers are routinely given an injection of Rh immune globulin (RhoGAM). This injection destroys any fetal Rh-positive cells in the mother’s circulation before her own immune system can develop a lasting immune memory. The injection is typically administered between 26 and 28 weeks of pregnancy and again shortly after the delivery of an Rh-positive baby.