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Don't forget, also, that an individual who is AB has the A and B antigens on each and every erythrocyte.  The antigens themselves are NOT direct gene product, as they are, in effect, immunodominant sugar residues, and only proteins can be direct gene products.  In the case of the A and B genes, the direct gene products are, respectively, N-acetyl-D-galactosaminyl transferase and D-galacosyl transferase, both of which transfer their respective sugars from a UDP donor molecule.  However, these two enzymes are competitive.  As a result, sometimes the A transferase "wins the battle" between the two, and the A antigen ends up being expressed more strongly on the red cells than the B antigen, and sometimes the B transferase "wins the battle" between the two, and the B antigen ends up being expressed more strongly on the red cells than the A antigen.

In the case of the latter, an individual who is genetically A2B can, phenotypically, appear to be A3B, and so genotyping the individual may not help.  All that having been said, the fact that (roughly speaking) 50% of the antigens expressed on each red cell will be a normal B antigen, it is not surprising that there would be no"mixed-field" reaction with anti-A,B.

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