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The cytophilic activity of human immunoglobulins.


 The ability of human myeloma immunoglobulins of different subclasses to bind to the surface of guinea pig macrophages was investigated. Using 125I-labeled proteins, it was found that those belonging to the subclass IgG1 generally bound to a greater extent than did those in other subclasses, and that the cell-associated radioactivity remained largely on the cell surface during the short periods of observation employed. The Fc fragment of one such IgG1 protein was shown to contain virtually all of the cytophilic property of the parent molecule. Pre-incubation of guinea pig macrophages with IgG1 proteins, but not with representatives of other groups, resulted in a pronounced inhibition of the binding of cytophilic antibody, suggesting that the inhibiting proteins occupied the same site on the macrophage membrane involved in cytophilic binding.