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Figure 1.
The immunodominant A and B trisaccharide epitopes are formed
from the common H disaccharide substrate by  1,3-N-acetylgalactosaminyltransferase
(GTA), defined by the blood group A gene, and -galactosyltransferase
(GTB), defined by the blood group B gene, respectively.
Conversely, the strategy used for enzymatic conversion of blood
group A and B antigens to H involves exoglycosidases that
specifically hydrolyze the 1,3GalNAc
( -N-acetylgalactosidase,
A-zyme) or the 1,3galactose
( -galactosidase,
B-zyme) to form the common H structure found on O RBCs. Black
arrows indicate the different C-2 N-acetyl group of GalNAc and
OH group of Gal in the immunodominant A and B epitopes,
respectively. The immunodominant epitopes are positioned at the
termini of oligosaccharide chains on glycolipids and
glycoproteins as indicated by R. Increased complexity in ABH
oligosaccharide structures are provided by the oligosaccharide
carrier chain (R). On human RBCs most structures are based on
type 2 polylactosamine chains with repeating Gal  1-4GlcNAc
disaccharide units (both glycolipids and N-linked
glycoproteins). A minor amount of type 1 chain ABH structures
with Gal 1-3GlcNAc
are found as glycolipids adsorbed from plasma^3.
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