We have raised an antibody to the IgG Fc receptor (FcR) of human mononuclear phagocytes by immunizing a goat with FcR purified by ligand affinity from a human monocyte line (U937). This antiserum, which inhibited the binding of IgG ligand to the receptors on U937, precipitated from detergent lysates of surface-radioiodinated U937 cells a 72 Kd sialoglycoprotein (p72) identified as the FcR by several previously published criteria. Two other bands seen in autoradiograms of SDS gels were precipitated by this antiserum: a 40 to 43 Kd band that co-purified with p72 and a 170 Kd protein that was not present in the immunogen. Fractionation of the IgG of this antiserum into two subclasses yielded one subclass (IgG1) in which anti-p72 activity was considerably enriched relative to antibody activities against other molecules. This antiserum precipitated p72 not only from U937 but from HL60 cells and from human peripheral blood monocytes as well, indicating common antigens on the p72 molecules from these three cells. However, p72 was not recovered from lysates of surface-iodinated human polymorphonuclear leukocytes or murine macrophage lines. Anti-p72 activity was not completely removed by adsorption with intact U937, suggesting that the antiserum recognizes portions of p72 that are not exteriorly disposed, perhaps noncarbohydrate portions of the molecule. We expect this antiserum to be useful for a number of studies of receptor structure and function.

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