Virtually all immunoglobulins contain significant amounts of carbohydrate in CH2 region. There is however, a considerable quantitative and qualitative variation, even among molecules from the same species and of the same class.
The carbohydrate component of the immunoglobulin is an oligosaccharide with variable mono-saccharide units. It may include simple mannose rich units as in IgM or complex N- acetyl lactosamine rich units as in IgG. In IgG, the carbohydrate portion is about 2.5% of the total molecule and it is equally divided between the two gamma heavy chains. It is linked through amino acids, aspartic acid and threonine residues in the polypeptide chain.
Mostly there will be no glycosylation in Fab fragments; therefore glycosylation generally has no effect on antigen binding property of antibody. It, however, does influence effector’s function controlled by Fc fragment.
The biological role of the carbohydrate in immunoglobulins is yet unknown. It is postulated that it may play a role in the passage of immunoglobulins through biological membranes during their secretion from the cell or their passage through the placental barrier, and also in certain other biological functions of the immunoglobulins.