Circulating blood neutrophils constitutively possess IgGs bound to their FcγRs. A: For the measurement of IgGs present on neutrophils in whole blood, erythrocytes were lysed by dilution of 900 μl of whole blood in 11 ml of an NH4Cl buffer containing 138.3 mM NH4Cl, 21.5mM Tris, pH 7.4 as previously described . After the blood became translucent, leukocytes were recovered by a 5 minute centrifugation at 800 g. The amount of IgGs bound on neutrophils was measured by flow cytometry using FITC-labeled anti-human IgG antibodies. The same measurement was performed on isolated neutrophils incubated with or without 10% autologous PPP for 20 minutes. B: Neutrophils were pre-incubated with blocking FcγRs antibodies before incubation with PPP and analysis as in A. Results are expressed as mean +/- SEM of five independent experiments. *** P < 0.0005 (one-sample t test).