A disquisition on the energetics of immunoglobulin binding to receptors in vivo and in vitro.

The binding constant of Fc-moieties of IgG and their receptors (R), derived via the law of mass action, yields values that are of the order of 10(6) to 10(8) L/M. In circulating blood, phagocytic R must be bound rather strongly to IgG, which is normally present in high concentrations, so that it is unlikely that Fc-R mediated interactions between rather sparse sensitized particles and phagocytes take place to any significant degree in the blood stream. However, in the spleen, where Fc-R mediated interactions do play a more important role, the situation is different, due to: a) an increased cell concentration; b) a decreased relative IgG concentration; c) a locally very high macrophage concentration, with large numbers of R per cell. It can be shown that these changed conditions in the spleen cause a shift in the equilibrium of the Fc-R interaction in favor of sensitized particle Fc-R binding, with diminished involvement of freely circulating IgG. The law of mass action can also be used to predict the degree of washing of phagocytic cells needed to remove bound immunoglobulin. Conversely, measurement of the concentrations of free and bound immunoglobulin at different dilutions allows the determination of Ka as well as of the number of R per cell.