Oligosaccharides play a role in cell recognition/identity. Oligosaccharides form the blood group antigens. In some cells, these antigens are attached as O-linked glycans to membrane proteins. Alternatively, the oligosaccharide may be linked to a lipid molecule to form a glycolipid. These oligosaccharides determine the blood group types in humans (Figure 9.29). There are actually 14 genetically characterized blood group systems with more than 100 different blood group antigens.

Molecules of the blood group antigens represent only a special case of a much more general phenomenon - cell marking by oligosaccharides (Figure 9.30a). In multicellular organisms, different kinds of cells must be marked on their surfaces so that they can interact properly with other cells and molecules. The surface of many cells are nearly covered with polysaccharides, which are attached to either proteins or lipids in the cell membrane. Some animal cells have an extremely thick coating of polysaccharides called a glycocalyx.

For oligosaccharides or polysaccharides to serve as recognition signals, there must be proteins that bind to them specifically. One such class is the immunoglobulins. Another very diverse group of saccharide-binding proteins is the lectins. In plants, lectins appear to play defensive roles and aid in adhering nitrogen-fixing bacteria to roots. In animals, lectins seem to be involved in interactions between cells and proteins of the intercellular matrix, such as collagen, and help to maintain tissue and organ structure.

The cell surfaces of many cancer cells are abnormal, which may account for the loss in tissue specificity that such cells commonly exhibit.

Properties of oligosaccharides that aid in their role as cellular markers:

They can present a wide variety of structures in relatively short chains. The multiple possible monomers, linkages, and branching patterns allow a vast, but specific vocabulary.

They are very potent antigens (antibodies can be elicited swiftly against them)