A and B antigens were originally identified on red blood cells. However, they were also later identified on other types of cells and in secretion. For example, endothelial cells that form the linings of capillaries express these antigens depending on blood type. Therefore, the ABO blood group is important not only for blood transfusion, but also for cell/tissue/organ transplantation. Also, blood, hairs, and seminal fluid are important pieces of evidence at crime scenes. Therefore, ABO blood typing has played an important role in excluding suspects in forensic investigations. Additionally, A and B antigens are not solely restricted to humans. Both the same and similar antigens have been found in other species of organisms. For example, chimpanzees express A and O blood groups, whereas gorillas express B blood groups. In addition to primates, many mammals and vertebrates, plants, and certain microorganisms have been shown to express the same or similar antigens. The evolution of the ABO system is, therefore, of scientific interest.
The expression of A and B antigens is not always constant. It fluctuates during development, differentiation, and even carcinogenesis of cells. The elucidation of how the expression of these antigens is controlled was an important issue to research. Additionally, why "naturally occurring" antibodies against A and B antigens are present in the plasma of individuals who do not express those antigens is an important question to be answered.
A and B antigens are not protein antigens but rather oligosaccharide antigens with the chemical structures of GalNAc α1-3 (Fuc α1-2) Gal- and Gal α1-3 (Fuc α1-2) Gal-, respectively. The immunodominant sugars are GalNAc (N-acetyl-D-galactosamine) and D-galactose for A and B antigens, respectively. The difference between these two sugars is that GalNAc has -NHCOCH3 at the C2 position whereas galactose has a smaller –OH at this position.