Little is known of any relationship that might exist between immunoglobulins on the one hand and mucous membranes and exocrine secretions on the other. Exocrine secretions varywidelyin complexitydependingon thesource. Even uncomplicated seromucous secretions arise from different cell types and glands, and contain, in addition to the major mucus glycoprotein fractions and IgA (immunoglobulin A), a large number of other constituents including most of the serum proteins. The most important constituent of mucus is a high-molecular-weight glycoprotein, and this is responsible for the physicochemical properties of the material. It has a characteristic amino acid and carbohydrate composition. Thus it contains relatively large amounts of proline and the hydroxy amino acids, serine and threonine, but is deficient in aromatic and sulphur-containing amino acids. The native glycoprotein has a molecular weight of several million, and more than one-half of this is contributed by carbohydrate. As the oligosaccharide units have an average size of only eight to ten monosaccharide residues, this means that the glycoprotein contains many thousands of oligosaccharide units closely packed along the polypeptide chain. Such an arrangement renders the material resistant to denaturation aild to proteolysis, ideal properties to fulfil the role of niucosal protection and lubrication, particularly in the gastrointestinal tract.

The oligosaccharide units are 0-glycosidically linked from N-acetylgalactosamine to the hydroxyl groups of either serine or threonine. In addition to the linkage sugar, the average oligosaccharide unit contains one or two residues of fucose, three or four of galactose, three or four of N-acetylglucosamine and none or one residue of N-acetylneuraminic acid.