Neutrophil receptors for IgG and complement: their roles in the attachment and ingestion phases of phagocytosis.

The functional roles of IgG and C3b in phagocytosis by human peripheral neutrophils were investigated. Phagocytosis of Staphylococcus aureus in the presence of human serum was severely depressed by heat inactivation of serum at 56 degrees C for 30 min. Experiments with varying particle: leukocyte ratios in the presence of complement-inactivated sera showed that particle-bound C3b can mediate a 10-fold enhancement of the overall phagocytic rate. When sheep erythrocytes were sensitized with either IgG or IgM, only the former were bound to and readily internalized by neutrophils. Erythrocytes sensitized with both IgM and C3b were bound but not internalized. Furthermore, the presence of Fc fragments during incubation of S. aureus or latex beads with neutrophils in the presence of IgG or fresh serum affected a total inhibition of internalization but did not significantly alter adherence. Quantitative data regarding IgG sensitization indicated that bound C3b results in at least a 3-fold decrease in the amount of sensitizing IgG required for 50% maximal phagocytic response by neutrophils. On the basis of the above results, it is argued that particle-bound C3b functions primarily in the adherence phase and that bound IgG serves as a trigger for the internalization phase of phagocytosis.