Function 
Glycosyltransferase (GT) are enzymes which catalyze the transfer of monosaccharide to alcohol, carbohydrate, protein or inorganic phosphate[1]. 
Galactosyltransferase (AGT) is a GT which catalyzes  the transfer of galactose during the synthesis of polysaccharides[2].
Fucosylglycoprotein 3-AGT (GTB) or histo-blood group ABO system transferase is the basis of the blood ABO system.  It involves 3 carbohydrate antigens: A, B and H. 
Flavonoid GT glycosylates flavonoids (plant metabolites with two phenyl rings and one heterocyclic ring)[3].  For details on flavonoid GT see Vitis vinifera Flavonoid 3-O-Glucosyltransferase (Vv3GT). 
  Relevance 
Individuals with blood type A, B and AB has glycosyltransferase activitiy which converts antigen H to A or B.  Individuals with blood type O lack this activity.  For details see Human ABO(H) Blood Group Glycosyltransferases.
  Disease 
Mutations in mannose GT can cause muscular dystrophy and neuronal migration disorder[4].
  Structural highlights 
The  of cyclodextrin GT contains [5]. Water molecules shown as red spheres.