Cells recognize one another because of the saccharides attached to cell surfaces.
They are present usually as oligosaccharides associated through covalent links to lipids and/or proteins forming Glycoconjugates. The lipid or protein part is integrated into the cell membrane structure, with the saccharide part towards the external membrane surface.
Membrane carbohydrates (2-10% of the membranes) are on the extracellular surface bounded to lipids or proteins of the membrane, forming glycoconjugates that serve as docking sites in cell recognition, adhesion and receptor action. These sugars include mainly glucose, galactose, mannose, fucose, N-acetyl galactosamine and N-acetyl glucosamine.
The different kinds of Glycoconjugates include:
Proteoglycans: In the Proteoglycans, the Glucosaminoglycan moety forms the greater fraction of the molecule (tipically a proteoglycan consists of 95 % of carbohydrates) and is the main site of biological activity, providing multiple binding sites. They are found mainly in the extracellular matrix. They are major components of connective tissue.
Glycoproteins: Membrane bound glycoproteins participate in a wide range of cellular phenomena, including cell recognition, cell surface antigenicity, etc. In the glycoproteins, the majority of the molecule consist of proteins; they have one or more oligosaccharides attached to a protein, and they usually are branched and do not have serial repeats, so they are rich in information, forming highly specific sites for recognition and high affinity binding by other proteins
Glycolipids: are membrane lipids in which the hydrophilic head groups are oligosaccharides.
As in glycoproteins, glycolipids act as specific sites for recognition by carbohydrate binding proteins. The four types of human RBC have different oligosaccharides (antigens) in their cell membranes. Blood groups depends on the gangliosides (a kind of sphingolipid) in the surface of the RBC .