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QUESTION: In your book you write that blood type O may have been the more common blood type in early human history. In genetics, most early forms are eventually displaced by later forms (survival of the fittest). If this is so, why did type O not just disappear?
ANSWER: There are probably several good possible explanations for the continuance of the type O blood group, the first being the shear amount of O gene still found in the gene pool, and the fact that, to a certain degree, it is still self-replicating. Another example would be eye color: If brown is dominant to blue, theoretically blue eye color frequency should diminish, however it hasn't.
A recent article may explain from a more practical persepctive why the gene for type O blood persists. It appears that individuals with a rare disorder called Leukocyte Adhesion Deficiency type II (characterized by recurrent infections, persistent high white blood count, and severe mental and growth retardation) are deficient in the ability of their white blood cells to 'traffic' to sites of inflammation. LAD II patients exhibit a deficiency in the expression of cell surface structures that include the blood type O (actually H) antigen.
Thus it may be that the manufacture of H antigen (the only antigen type O's manufacture, but also made to varying degrees by all the other blood types as well) may be to help the cells of our immune system get to 'where the action is.'
If so, the maintenance of genes necessary to manufacture H antigen is vitally important to our continued existence and may explain why human breast milk is so rich in fucose (the H antigen). It helps the infant's developing immune system fight off infection efficiently until it matures sufficiently to do so by itself.
Becker DJ, Lowe JB. Leukocyte adhesion deficiency type II. Biochim Biophys Acta 1999 Oct 8;1455(2-3):193-204