Rarest ABO Type – Bombay Phenotype

October 26, 2011 at 12:00 pm
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By Billie Rubin, Hemoglobin’s Catabolic Cousin, reporting from the labs of Stanford Blood Center

Just in case the excitement of the ABO blood group system was beginning to subside, there is one other very, very rare ABO type first described in India. It is known as the “Bombay phenotype” and on the surface it looks like a type O. It is found in one out of 10,000 people in India, and one in a million in Europe.

The building blocks from which the A & B antigens are made is known as the H antigen. A and B genes transform most H’s (not all) into As and/or Bs, respectively. Type O has the largest amount of “untransformed” H antigens.

Most people have HH or Hh genes but Bombay phenotypes don’t. They are recessive hh. With no building blocks for A or B antigens, they look like a type O but they can make antibodies to the foreign H antigen – regular type Os don’t. They are incompatible with almost everybody! Only another Bombay phenotype can be transfused to them, usually from rare frozen inventories.

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