Analytic study of lattice-precipitation based on a plausible model of irreversible reactions leads to results not at all in agreement with the existing experimental data. The idea of competition between irreversible adsorption of antibody and simultaneous precipitation of the resulting compounds is inadequate to explain immune precipitation. The failure arises directly from the fact that aggregation proves to be much too slow a process to compete effectively with the more rapid progress of the initial reactions.

Since the initial reaction of antibody with antigen is probably complete at a time when aggregation is just beginning, it seems likely that antibody is actually removed from its compounds during the subsequent precipitation. This obviously calls for reversible initial reactions, and a process of selective precipitation similar to that previously described.

Certain alternative explanations are discussed in the light of this study, and its possible bearing on some unexplained features of the immune reactions is considered.

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