Glycoproteins, consisting of oligosaccharides covalently bound to proteins, are the most abundant membrane carbohydrates and have oligosaccharide components facing the outside world. Some of them play key roles in the recognition of foreign substances or cells. Glycolipids, consisting of a carbohydrate and a lipid, help cells recognize other cells. Membrane carbohydrates, which typically are branched, are highly diverse. A glycoprotein consists of a protein with an attached oligosaccharide.
Most glycoproteins have an extracellular exposure, location, or function.
The blood group antigens are a set of oligosaccharides attached to the surface of erythrocytes. The structural differences between the various oligosaccharides are quite small, yet this variation is critical if you are receiving or donating blood.