ABO Antigens
The H antigen is an essential precursor to the ABO blood group antigens. The H locus, which is located on chromosome 19, contains 3 exons that span more than 5 kb of genomic DNA; it encodes a fucosyltransferase that produces the H antigen on RBCs. The H antigen is a carbohydrate sequence with carbohydrates linked mainly to protein (with a minor fraction attached to ceramide moiety). It consists of a chain of β-D-galactose, β-D-N-acetylglucosamine, β-D-galactose, and 2-linked, α-L-fucose, the chain being attached to the protein or ceramide.
The ABO locus, which is located on chromosome 9, contains 7 exons that span more than 18 kb of genomic DNA. Exon 7 is the largest and contains most of the coding sequence. The ABO locus has three main alleleic forms: A, B, and O. The A allele encodes a glycosyltransferase that bonds α-N-acetylgalactosamine to the D-galactose end of the H antigen, producing the A antigen. The B allele encodes a glycosyltransferase that bonds α-D-galactose to the D-galactose end of the H antigen, creating the B antigen.
In the case of the O allele, when compared to the A allele, exon 6 lacks one nucleotide (guanine), which results in a loss of enzymatic activity. This difference, which occurs at position 261, causes a frameshift that results in the premature termination of the translation and, thus, degradation of the mRNA. This results in the H antigen remaining unchanged in case of O groups.
The majority of the ABO antigens are expressed on the ends of long polylactosamine chains attached mainly to band 3 protein, the anion exchange protein of the RBC membrane, and a minority of the epitopes are expressed on neutral glycosphingolipid.
Read more about this topic: ABO Blood Group System