Enzymes that catalyze the transfer of glycosyl group to an acceptor.
Most often another carbohydrate molecule acts as an acceptor, but inorganic phosphate can also act as an acceptor, such as in the case of phosphorylases. Some of the enzymes in this group also catalyze hydrolysis, which can be regarded as transfer of a glycosyl group from the donor to water.
Subclasses include the hexosyltransferases, pentosyltransferases, sialyltransferases, and those transferring other glycosyl groups.
The ABH antigens are not primary gene products but instead they are the enzymatic reaction products of glycosyltransferases.
The ABO system occurs as a result of polymorphism of complex carbohydrate structrures of glycoproteins and glycolipids expressed at the surface of erythocytes or other cells, or present in secretions, as glycan units of mucin glycoproteins. 
|Allele ||Protein ||% in USA among caucasians | blacks | orientals|
|A1||A1 transferase ||22 | 12 | 18|
|A2||A2 transferase ||7 | 6 | rare|
|B ||B transferase ||6 | 12 | 17|
Immuno-dominant structures of A and B antigens, GalNAc alpha1->3 (Fuc alpha1->2) Gal- and Gal alpha1->3 (Fuc alpha1->2) Gal-, respectively, are synthesized by a series of reactions; the A and B transferases encoded by the functional alleles (A and B alleles) of a single gene at the ABO locus, catalyze the last step of the synthesis, while the transferase coded by the O allele is non-functional; therefore, the acceptor substrate, (H antigen: Fuc alpha1->2 Gal-) remains without a further modification and the A and B determinants are absent.