The ABO system was discovered as in 1900 and is considered the first and clinically most important system. The ABO gene and its 7 coding exons give rise to one of two principally different glycosyltransferases. The A glycosyltransferase (GTA) catalyzes the addition of a donor substrate, UDP-N-acetylgalactosamine, to an acceptor substrate known as the H antigen. The B glycosyltransferase (GTB) differs by only four amino-acid substitutions from GTA and performs the same enzymatic reaction but uses UDP-galactose as donor substrate. In this way, genetic polymorphism gives rise to two related antigens in this system. Any polymorphism or mutation that changes the activity or specificity of the encoded enzyme may therefore alter the ABO phenotype. Alterations that completely abolish enzymic activity give rise to the blood group O phenotype, in which the H antigen remains unconverted and no A or B antigen can be detected. If the genetic alteration decreases the activity of the enzyme, or alters its subcellular location and thereby decreases conversion of H to A or B, a weak A or B subgroup phenotype can result. Furthermore, certain polymorphisms result in promiscuous enzymes that can synthesize both A and B antigen, thereby resulting in the so-called cisAB or B(A) phenotypes. The A phenotype is divided into A1 and A2. The former is more prevalent in all populations and has approximately 5 times more A epitopes per red cell. The GTA1 is also better than GTA2 at synthesizing certain forms of A, .e.g. A type 3 and 4.
In addition to the A and B antigens, two other antigens are included in the ABO system, namely A,B and A1. The former is a joint epitope on A or B antigen and is therefore present in both A, B and AB phenotypes. The exact biochemical nature of the A1 antigen has been more controversial but has been proposed to represent A type 4.