<P> The two common O alleles, O01 and O02, share their first 261 nucleotides with the group A allele A01 . However, unlike the group A allele, a guanosine base is subsequently deleted . A premature stop codon results from this frame - shift mutation . This variant is found worldwide, and likely predates human migration from Africa . The O01 allele is considered to predate the O02 allele . </P> <P> Some evolutionary biologists theorize that the I allele evolved first, followed by O (by the deletion of a single nucleotide, shifting the reading frame) and then I . This chronology accounts for the percentage of people worldwide with each blood type . It is consistent with the accepted patterns of early population movements and varying prevalent blood types in different parts of the world: for instance, B is very common in populations of Asian descent, but rare in ones of Western European descent . Another theory states that there are four main lineages of the ABO gene and that mutations creating type O have occurred at least three times in humans . From oldest to youngest, these lineages comprise the following alleles: A101 / A201 / O09, B101, O02 and O01 . The continued presence of the O alleles is hypothesized to be the result of balancing selection . Both theories contradict the previously held theory that type O blood evolved first . </P> <P> It is possible that food and environmental antigens (bacterial, viral, or plant antigens) have epitopes similar enough to A and B glycoprotein antigens . The antibodies created against these environmental antigens in the first years of life can cross-react with ABO - incompatible red blood cells that it comes in contact with during blood transfusion later in life . Anti-A antibodies are hypothesized to originate from immune response towards influenza virus, whose epitopes are similar enough to the α - D-N - galactosamine on the A glycoprotein to be able to elicit a cross-reaction . Anti-B antibodies are hypothesized to originate from antibodies produced against Gram - negative bacteria, such as E. coli, cross-reacting with the α - D - galactose on the B glycoprotein . </P> <P> However, it is more likely that the force driving evolution of allele diversity is simply negative frequency - dependent selection; cells with rare variants of membrane antigens are more easily distinguished by the immune system from pathogens carrying antigens from other hosts . Thus, individuals possessing rare types are better equipped to detect pathogens . The high within - population diversity observed in human populations would, then, be a consequence of natural selection on individuals . </P>

In immunological terms the proteins that make up the abo blood types are considered to be