A short sequence of sugars (oligosaccharide) found on many cells in the human body, but especially attached to red blood cell membranes and floating free in plasma and secretions. The H antigen must be made before either A or B antigen (the two main antigens in the ABO blood group) can be made (and when either of those antigens is made, the chain ceases to have H activity, resulting in a reciprocal relationship between the amount of H and the amount of A and/or B on a red cell). H is made when a fucose sugar residue is added to a pre-existing oligosaccharide through the action of either the H or Se gene product. Rarely, a person lacks both of the above-mentioned genes. This situation, known as the “Bombay Phenotype,” results in production of a very strong anti-H as well as anti-A and anti-B, making all blood except that from another person with Bombay incompatible.
I recorded a video in 2014 on the H antigen and the Bombay phenotype called, “What the H?” so check it out for more information.
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