On the composition of insoluble antigen-antibody complexes

A relationship is established between the behavior of precipitin curves in the far antibody and antigen excess regions and the composition of certain soluble and insoluble antigen-antibody complexes. The primary assumption made is that antigen-antibody reactions proceed according to the principle of mass action. It is found that important features of the behavior of precipitin curves can be examined by plotting them logarithmically, yielding graphs which appear to have definite limiting slopes as the abscissa approaches ±∞. The values of these two slopes are related to the composition of certain of the smaller insoluble antigen-antibody complexes present in the reacting system. When applied to experimental data on antigen-rabbit antibody systems the method developed indicates that surprisingly small complexes must be insoluble: complexes as small as those composed of three antigen molecules and two antibody molecules. Contrasted to this, it is found that in the case of flocculating horse antibodies reacting with antigen insoluble complexes must be composed of at least some dozens of antigen and antibody molecules.