ABO Blood Groups
ABO Blood groups represent the first described human molecular polymorphism. The ABO gene encodes variants of a protein (a glycosyltransferase) that produces the short sugar chains on glycoproteins and glycolipids that define the A, B, or O antigens. These antigens are found on red blood cells, plasma glycoproteins, and other cell types in various tissues. Individual humans can have one of four blood types based on the two alleles inherited from both parents at the ABO locus: blood type A, B, AB, or O. We still lack a definitive explanation of why humans individually differ in the molecular composition of their blood and combine the absence of particular ABO antigens with circulating antibodies against the missing molecule(s). Several lines of evidence indicate that this system evolved to protect populations from parasites and pathogens that use these ABO sugar chains for invasion of the human host. Furthermore, infections by enveloped viruses can be considered “nanotransplantations”: the viruses are covered with the ABO antigens acquired from the previous host and can be targeted by circulating anti-ABO antibodies when infecting new individuals of different blood types. The same antibodies underlie crucial importance of ABO blood typing for transfusion and transplantation medicine. Recent comparative genome studies have revealed that this polymorphic system is ancient and shared between humans and non-human primates, this despite the fact that none of the great ape species carries all four ABO blood types. Historically, ABO allele frequencies across human populations have been used for pseudoscientific claims of superiority of certain regional populations and similar pseudoscience perpetuates claims for ABO blood group-based diets and/or personality types.