Interaction of IgG and its fragments with red cells

The red cell uptake of ¹³¹I following incubation of red cells with ¹³¹I] IgG prepared from normal donors was shown to be IgG and not trace contaminants such as transferrin, lipids or iodide. The criteria used were immunodiffusion, DEAE chromatography, gel filtration and exchange with unlabelled lipoproteins and plasma. The uptake of ¹³¹I]IgG was pH and ionic strength dependent and was influenced by the proportion of cells to IgG present during the reaction. With constant cell concentration the uptake of ¹³¹I]IgG increased progressively as more IgG was added to the cells and approached an asymptotic value suggesting that there was saturation of red cell binding sites. When the IgG concentration was kept constant the uptake of IgG was inversely proportional to the red cell concentration. No difference in the molar binding of IgG, Fab or Fc was found indicating that the non-antibody binding of IgG does not preferentially involve any part of the IgG molecule. The molar quantities of carefully prepared ¹³¹I]IgG bound to red cells were similar to those obtained with ¹³¹I]BSA. The non-antibody red cell binding of IgG was contrasted with the antibody type of IgG binding.