The native Celts tended to have more type O blood, while the other populations tended to have more type A.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.Some evolutionary biologists theorize 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. Origin theories
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.
His third group (C) indicated absence of both A and B antigens, but contains anti-A and anti-B. The following year, his students Adriano Sturli and Alfred von Decastello discovered the fourth type (but not naming it, and simply referred to it as "no particular type"). In 1910, Ludwik Hirszfeld and Emil Freiherr von Dungern introduced the term O (null) for the group Landsteiner designated as C, and AB for the type discovered by Sturli and von Decastello. They were also the first to explain the genetic inheritance of the blood groups. Classification systems
Czech serologist Jan Janský independently introduced blood type classification in 1907 in a local journal. He used the Roman numerical I, II, III, and IV (corresponding to modern O, A, B, and AB). Unknown to Janský, an American physician William L. Moss devised a slightly different classification using the same numerical; his I, II, III, and IV corresponding to modern AB, A, B, and O.These two systems created confusion and potential danger in medical practice. Mosss system was adopted in Britain, France, and US, while Janskýs was preferred in most European countries and some parts of US. To resolve the chaos, the American Association of Immunologists, the Society of American Bacteriologists, and the Association of Pathologists and Bacteriologists made a joint recommendation in 1921 that the Jansky classification be adopted based on priority. But it was not followed particularly where Mosss system had been used.In 1927, Landsteiner had moved to the Rockefeller Institute for Medical Research in New York.