In the context of blood type, the O antigen refers to the H…
In the context of blood type, the O antigen refers to the H antigen that remains unmodified because the individual inherited two non-functional O alleles of the ABO gene. These O alleles result in an inactive glycosyltransferase enzyme, so no sugar is added to the H antigen. Thus, the term O antigen is functionally the same as the H antigen – just not further modified. The H antigen is a core sugar structure on the surface of red blood cells (RBCs). It is produced when the FUT1 gene encodes a fucosyltransferase enzyme that adds fucose to a precursor glycan. This H antigen acts as the scaffold for further modification by the ABO enzymes: The A allele encodes a glycosyltransferase that adds N-acetylgalactosamine (GalNAc) to the H antigen. The B allele encodes a glycosyltransferase that adds galactose. The O allele produces an inactive enzyme, leaving the H antigen unchanged (O antigen). In rare individuals with the Bombay phenotype, a mutation in the FUT1 gene prevents the synthesis of the H antigen altogether. As a result, they cannot express A, B, or O antigens, even if they carry functional A or B alleles, because there is no H antigen to modify. People with the AB blood type inherit one A allele and one B allele, and both functional enzymes are expressed. Therefore, they modify the H antigen in two different ways, leading to co-expression of both A and B antigens on the surface of their red blood cells. Because of this, AB individuals do not produce anti-A or anti-B antibodies and are considered universal recipients for red blood cell transfusions. Here is a summary table for five individuals: Individual ABO Genotype FUT1 Status RBC Surface Antigen(s) Ana AA Functional A antigen Bruno BO Functional B antigen Caio OO Functional H antigen (O antigen) Daniela AB Functional A and B antigens Eduarda AO Non-functional None (Bombay phenotype) Daniela has the AB blood type. Which of the following best explains the enzymatic mechanism responsible for the presence of both A and B antigens on her red blood cells?
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