Cytophilic activity of IgG2 from sera of unimmunized guinea-pigs.

The interaction between 7S IgG2 from unimmunized guinea-pigs and macrophage-rich peritoneal exudate cells from oil-stimulated guinea-pigs was examined by I-labelled IgG2 uptake. The cell uptake of I-labelled IgG2 reaches a maximum after 90 minutes at 20°; increased levels of labelling (1 to 23 atoms of iodine per immunoglobulin molecule) lead to slightly enhanced uptake, but the effect does not exceed experimental variability. Peripheral white blood cells bind relatively low levels of IgG2; the uptake by peritoneal cells is attributable primarily to a subpopulation of glass-adherent cells, presumably macrophages. Binding of IgG2 to macrophages is reversible even with a mild washing procedure. Repeated washing of cells did not affect the association constant, but the apparent number of receptor sites per cell was progressively reduced. After correction for the washing procedure employed, the number of IgG2 receptor sites was estimated as 2.5±0.4 x 10 per cell; the association constant (Ka) was 1.46±0.45 x 10 L/M at 20°.