Fc receptors on human neutrophils: electron microscopic study of natural surface distribution

The membrane receptor for the Fc portions of IgG (FcR) was examined on the cell surface of human neutrophils using electron microscopic markers of soluble immune complexes composed of ferritin (Fer) and rabbit 7S anti-Fer prepared in forty-fold and 120-fold antigen excess than needed at equivalence. By using negative staining coupled with electron microscopy, most of the immune complexes in forty-fold antigen excess were seen to be composed of one anti-Fer antibody and one or two Fer particles, suggesting that most of the indicator molecules are 'monovalent ligands' in terms of Fc pieces available per single immune complex molecule. FcR on neutrophils labelled with both indicators at 0 degrees in the presence of sodium azide were clustered as discontinuous patches of varying length over the cell surface. The pre-incubation of neutrophils at 37 degrees for 30 min prior to labelling did not alter the grouped distribution of FcR. No diffuse Fer labelling was observed. The clustering of FcR remained the same even after cross-linking the soluble complexes with F(ab')2 anti-Fer into multivalent ligands at 0 degrees. We favour the clustering of FcR as the natural surface representation on human neutrophils rather than an initial redistribution induced by the ligands. The findings are discussed with relation to the natural distribution of other surface antigens.