Differences between the Fc receptors of two lymphocyte subpopulations of human peripheral blood.

The Fc receptors of human peripheral blood lymphocytes bearing stable membrane Ig (B cells) and those bearing cytophilic membrane Ig (UL cells) were evaluated for binding avidity and interaction with human 'Ia-like' alloantigens. Titration experiments showed that binding of soluble antigen-antibody complexes to UL cells was readily detected at low concentations (5-10 microgram/ml), whereas high concentrations (400-800 microgram/ml) were necessary to detect binding to most B lymphocytes. Binding at all concentrations was dependent on an intact Fc portion of the antibody molecule within the antigen-antibody complex. F(ab')2 fragments of antibodies against human 'Ia-like' antigens inhibited binding of complexed Ig to B cells but not UL cells. These differences are compatible with the possibility that the Fc receptors of the two cell populations are distinct molecular entities or, alternatively, are the same molecules and differ in quantity, distribution, or mobility on the surface of the two cell types.