Tn antigen refers to the "oligosaccharide" (in fact monosaccharide) structure N-acetylgalactosamine (GalNAc) linked to serine or threonine by a glycosidic bond, i.e. as an O-glycan. It is not usually found on healthy cell surfaces, but may be found on cancer cells. Addition of single monosaccharide residues creates disaccharide antigens: the Thomsen-Friedenreich antigen (TF antigen or T antigen) is formed by substitution with galactose (Gal(b1-3)GalNAc); the sialyl Tn antigen (STn antigen) is formed by substitution with sialic acid (Neu5Ac(a2-6)GalNAc. Further substitution gives the normal core structures and O-glycans found on healthy cell surfaces.
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Tn antigen Wikipedia(Text) CC BY-SA