Biomolecules:
Carbohydrates

Variations on a Theme

The ABO Blood Type System

One large role carbohydrates play in cells is in cell-cell recognition. Because of all their possible structural variation discussed in the previous page, carbohydrates allow specific recognition between cells using relatively small molecules.

One well-known example of cell-cell recognition mediated by carbohydrates is the ABO blood type system. Anybody who has donated or received blood probably knows the following facts about blood compatibility. People with blood type O can receive only O type blood but can donate to any blood type. People with blood type A can receive only A and O type blood but can donate to A and AB recipients. Similarly, people with blood type B can receive only B and O type blood but can donate to B and AB recipients. People with blood type AB can receive any type of blood but can only donate to other AB types. All people synthesize a precursor carbohydrate, called the H antigen, which is attached to lipids or proteins on the outer surface of red blood cells. Specific enzymes synthesized by the ABO genes attach additional monosaccharides to the H antigen, and the completed carbohydrate determines that person's blood type.

In the radio boxes at left, click on a blood type to see the structure of the ABO blood type antigens.

	A
	B
	AB
	O
The A gene codes for a protein that adds N-acetyl galactosamine (yellow) in an α1-->3 glycosidic bond to the galactose residue at the end of the H antigen. The B gene codes for a protein that adds galactose (green) in an α1-->3 glycosidic bond to the galactose residue at the end of the H antigen. Since both A and B genes are present in AB individuals, some H antigens get an N-acetyl galactosamine residue (yellow) while others get a galactose residue (green). O individuals have neither the A nor the B gene, so no additional carbohydrate is added to the H antigen.