IgG‐binding sites on macrophage cell membrane. I. Identification of two distinct Fc receptors on mouse peritoneal macrophages

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Abstract
Evidence was presented concerning the existence on mouse peritoneal macrophages of 2 separate and distinct Fc receptors, 1 for cytophilic monomeric Ig[immunoglobulin]G (mIgG) and the other for polymeric IgG. The latter Fc receptor recognizes heat-aggregated IgG and antigen-complexed IgG. The major findings of our studies were: the different susceptibility of the 2 Fc receptor types by pronase, trypsin or phospholipase C; the independent modulation of these 2 binding sites on the cell membrane; the inability of mIgG to inhibit the binding of particulate antigen-complex IgG ligand; the ability of mIgG molecules which are devoid of the cytophilic property to attach to the macrophage surface upon their polymerization induced by heating or antigen. The results were discussed in terms of cytophilic and opsonic Fc receptor types, which may provide different functional abilities for normal macrophages.