Valency of antibodies

Most antibodies in the serum of hyperimmune animals, i.e. of the sort used in the original precipitation studies which measured their valency properties, are known to consist predominantly of immunoglobulin G (IgG) which, as described above, exhibits at equivalence a valency approaching two consistent with its determined structure. While all antibodies have a basic unit structure similar to IgG, some other classes of antibody exist in polymeric form and in consequence may express a greater valency.

IgA can exist in polymers of various sizes, the most common being found in secretions as a dimer with the two units joined end-to-end by a J chain. Such molecules have a structural valency of four. This is of little significance in considering antibodies in serum as most IgA in serum of normal individuals is monomeric. However, the polymeric secretory IgA active at mucosal surfaces may owe its protective properties in part to the distribution of binding sites and polyvalent nature.

Serum IgM antibodies are constructed on a penta-meric form with ten combining sites. In some circumstances, especially in primary responses, IgM may be a significant component of specific serum antibody and its multivalency offers certain distinct advantages in efficient bacterial and viral agglutination and complement fixation. In practice steric hindrance between binding sites and the local availability of antigen epitopes for all binding sites may reduce the effective valency of IgM to less than that predicted by structure. In reactions of pure IgM antibody with soluble antigen a valency of five is given as the normal property of this molecule.

The special properties of IgE antibodies depend upon it first attaching through receptors to the surface of mast cells and basophils. In this position they are known to bind antigen (allergen) to initiate the release of mediators of immediate hypersensitivity. Evidence suggests that the trigger mechanism for mediator release depends upon the cross-linking of IgE molecules on the cell membrane by allergen, and the divalent property of IgE would be an essential feature of this mechanism.

Recent studies suggest that antigen valency may also be a critical factor in mast cell degranulation. In an experimental system with rat leukemic mast cells, a conjugate of dinitrophenyl (DNP) hapten with a carrier molecule, with high DNP valency (DNP32-HSA) gave a much greater degranulation response by anti-DNP IgE-coated mast cells than the same allergen design with lower DNP valency.

See also: Affinity; Agglutination; Allergens; Antigen-binding site; Antigens; Antiserum; Epitopes; Hapten; IgA; IgE; IgG; IgM; Immunogen; Immunoglobulin structure; Joining J chain; Precipitation reaction.

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