General Features of Antigen-Antibody Interactions

Antigens and their corresponding antibodies precipitate by cross-linking to form an insoluble network. Polysaccharides have multiple, repetitive immunodeterminants and virtually none have demonstrable tertiary structure in solution (except, perhaps, under viscous stress). The number of these immunodeterminant groupings on each macromolecule is large. In the case of dextran, for instance, there are several thousand of them (if the dextran has a molecular weight of several million), even if the determinant involves the hep-tasaccharide. There is, thus, ample opportunity to form a precipitating, crosslinked complex with divalent (or polyvalent) antibody molecules. 

If some of the antibodies in the antibody population of a serum against antigen A also precipitate with another antigen B, the process is called a cross-reaction between A and B. Cross-reactions are caused by antigenic determinants that A and B have in common. 

In contrast to polysaccharides, protein antigens do have tertiary structure in solution this leaves only a limited number of amino acid sequences exposed at the surface of the molecule. Of these determinants, few, if any, will be repetitive. It would therefore be ex- 

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