Interaction of carbohydrate antigens with antibodies

In the 1960s Kabat showed that polyclonal anti-dextran antibodies bound maximally to about five sequentially linked sugar residues of the antigen. Modern work on monoclonal antibodies and sharply defined carbohydrate immunodeterminants have since shown that the determinant in an antigenic polysaccharide can be 2-4 sugars long. Affinity constants for the entire immunodeterminant can range from 10' to 106m '. These values suffice for the initiation of an immune response. Antibodies elicited by polysaccharides can be directed against either the interior segments or the terminal segment of the antigenic chain, and the antibody can bind the polysaccharide chain on its surface, or inside cavities. In the case of a monoclonal antibody to a(l—»6)-dextran it was shown that one group of monoclonal antibodies could bind the dextran all along its chain. Another set of monoclonal antibodies could bind only to a stretch of residues of the dextran that had to include the single chain-end glucose residue of the dextran. The latter was held inside a pronounced cavity, while its three sequential sugars were held near the surface of the immunoglobulin. It was found that an antibody combining area was thus made up of a set of subsites, and these could be mapped. Each one could accommodate a single sugar residue of the polysaccharide. All together these subsites can bind the entire immunodeterminant of four sugar residues. Generally, the major subsite shows an affinity of 102-10' m ' for its single carbohydrate residue, while the other subsites often possess far less affinity for their particular sugar residues. In the case of a comprehensively studied monoclonal antibody to a homopoly-saccharide (galactan), it was found that the immunoglobulin could also maximally bind four sugar residues. Here the monoclonal immunoglobulin could access and bind these repetitively occurring four galactosyl residues in the interior of the polysaccharide chain. All four subsites of the immunoglobulin were on the protein surface, and the subsite with the major affinity was located within the sequence of four. In yet another study, a monoclonal anti-S/jig-ella dysenteriae type 1 antibody could also bind interiorly located repeating determinants of the bacterium's O-specific heteropolysaccharide.

See also: ABO blood group system; Affinity; Antibody-antigen intermolecular forces; Antigen-binding site; Antigens; Bacterial cell walls; Lectins; Tumor antigens; Vaccines.

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