Antibodyantigen Intermolecular Forces

Carel J van Oss, Department of Microbiology, School of Medicine, State University of New York at Buffalo, Buffalo, New York, USA

The different types of bonds occurring in the specific interaction between antigen (Ag) and antibody (Ab) molecules are all of the weak physical variety. The three different classes of bonding forces in question are (not necessarily in the order of their relative importance): 1) van der Waals or electrodynamic forces; 2) hydrogen bonding or polar forces; 3) electrostatic forces. Covalent bonds are not involved in Ag-Ab interactions. These physical, noncovalent forces also provide the bonds between other biological entities, such as various ligand and cell surface receptors, carbohydrates and lectins, and enzymes and their substrates. (In enzyme-substrate reactions however, covalent bonds can also occur, in addition to the three types of physical forces mentioned above.)

Human Ab molecules (immunoglobulins) have from two (IgG, IgA, IgD, IgE), to ten identical valencies (IgM), while secretory IgA (S-IgA) has four valencies. Each paratope on an immunoglobulin molecule represents one valency. Ag molecule (or particles) can have from a few to a great many valencies (each epitope represents one valency). The multiple valencies of such Ag molecules may be all different, or all identical, or some epitopes on a given Ag may be different and others the same. Ag and Ab molecules of a given specificity can combine into Ag-Ab complexes in virtually every conceivable proportion: the Ag-Ab reaction is essentially nonstoi-chiometric. Thus, the valency of an Ag molecule can only be determined under conditions of excess Ag. In both cases the resulting Ag-Ab complexes will tend to be soluble; insoluble complexes only occur at intermediate Ag-Ab ratios, i.e. close to the optimal, or equivalence ratio.

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