A Disquisition on the Energetics of Immunoglobulin Binding to Receptors in Vivo and in Vitro

Abstract

The binding constant of Fc-moieties of IgC and their receptors (R), derived via the law of mass action, yields values that are of the order of 10⁶ to 10⁸ L/M. In circulating blood, phagocytic R must be bouna 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 blooa scream. 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 those 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 K_a as well as of the number of R per cell.