Blood Group ABO/Rh
The blood group of each individual is determined by the surface proteins expressed on the membrane of the red blood cells. Of the multitude of systems described, the most relevant, especially for human-to-human compatibility, are the ABO and Rh systems.
Blood group is defined by the proteins or antigens expressed on the surface of erythrocytes or red blood cells. There are several known classificatory systems, but the most relevant for the categorization of the blood group in the population are the ABO system and the Rh system.
In the case of the ABO system the antigens anchored to the surface of red blood cells are sugars that are produced by a series of reactions in which enzymes catalyze the transfer of sugar units. A person's DNA determines the type of enzymes he has and, therefore, the type of sugar antigens that end up on his red blood cells. An individual can express on the surface of the erythrocyte antigen A, B, both or neither, giving rise to the 4 possible blood groups of this system, A, B, AB or O respectively. The A allele encodes a glycosyltransferase that produces the antigen with the sugar N-acetylgalactosamine, the B allele to generate the sugar D-galactose, and the O allele for an enzyme with no function hence no sugar is expressed on the surface of the erythrocyte. Blood group O is the most common blood group worldwide, followed by blood group A. Group B is less common, and group AB is the least common.
In contrast, Rh blood group antigens are proteins. A person's DNA contains the information to produce the protein antigens. The RhD gene codes for the D antigen, which is a large membrane protein. Some people have a version of the gene that does not express the D antigen on the cell surface and, therefore, the RhD protein is absent on their red blood cells. Hence, the two existing blood group types for the Rh system, Rh+ and Rh- respectively.
The anchoring of antigens on the surface of red blood cells that determines the blood group depends on the genetics of each individual. Based on several studies, 4 markers associated with antigen production have been identified. These polymorphisms are found in the ABO and RHD genes. The ABO gene codes for a differential glycosyltransferase enzyme that catalyzes the addition of the corresponding monosaccharide on the so-called H antigen, a molecule present on the surface of red blood cells from which the A and B antigens are formed. The A antigen is produced by an enzyme that adds a molecule of N-acetyl galactosamine to the H antigen, while the B antigen is produced by an enzyme that adds a D-galactose to the H antigen. On the other hand, the ability to produce the Rh antigen is determined by the genotype in the RHD gene. This gene is also linked to the RHCE gene, so that their genotypes are inherited together and both can be used to determine the Rh group.
IMPORTANT: With this analysis we make a prediction of a person's blood type thanks to the genetic variants described for each blood group. The reliability of this technique is 90%, so there may be cases (10%) in which genetics do not match the blood group. Therefore, to know exactly your blood type, you must perform a blood biochemical analysis.
13.5 million variants
Dean L. Blood Groups and Red Cell Antigens. National Center for Biotechnology Information [May 2022].