Glucosyltransferase determines blood type Remember when we solved the blood type mystery (in Chapter 3)? When we were trying to figure out what determines blood type, we discovered that it was the work of an enzyme. I remember! The ABO blood group system is classified according to the differences between the "sugar chains" on the surface of red blood cells. A sugar chain is a collection of monosaccharides connected together! And the differences between those chains are the monosaccharides at the tips, right?