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 ABO/Rh
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.
Dean L. Blood Groups and Red Cell Antigens. National Center for Biotechnology Information [May 2022].
ABO blood group system. Encyclopedia Britannica, [May 2022]