EVIDENCE OF A 2 STAGE NATURE OF PRECIPITIN REACTIONS

Precipitin reactions are traditionally explained as macromolecular interactions giving rise to very large molecular lattices or frameworks that due to their very large size are insoluble. We have questioned this concept experimentally by comparing the reactions of native and succinylated IgG (20% modified lysine residues). By use of a variety of different techniques we have found that (1) succinylation of IgG does not change the principal ability of IgG to react with its antigen, (2) succinylation of IgG gives rise to altered precipitin behaviour in the biologically important mild antigen excess region, (3) difference turbidity measurements of precipitin reactions in antibody excess show highly significant differences, (4) zonal centrifugations of selected immune complex mixtures show that succinylated complexes are more water soluble than complexes formed of native IgG and (5) theoretical calculations show that very large complexes (as typical lattice or framework aggregates) are thermodynamic unlikely. It is a unique feature of typical precipitin reactions that no free antigen is present before the equivalence zone. To explain this it is necessary to introduce two different molecular mechanisms or stages. In the first (and immune specific) stage of the reaction, a series of compositionally different complexes is formed. In a second stage some of these complexes create a hydrophobic phase by non-immune-specific interactions. The remaining smaller complexes will be distributed between the two phases. According to this hypothesis the distinguishing absence of free or complexed antigen in solution before the equivalence zone can be explained. © 1979.