Effect of exogenous IL1ra on phagocytosis of anti-D–coated RBCs after 2 hours of incubation in monocytes and granulocytes. The indicated final concentrations of IL1ra were co-incubated with anti-D–opsonized RBCs and WBCs at an RBC/WBC ratio of 500:1 for 2 hours, and phagocytosis was measured by flow cytometry. ○ indicates monocytes; and •, granulocytes. The data are expressed as a percentage of control of phagocytosis and were calculated by the formula: (percentage of 37°C-positive cells + IL1ra/percentage of 37°C-positive control cells – IL1ra) × 100. The results are expressed as the means ± SDs of 3 independent experiments. Significance values are indicated (*P < .002; **P < .001).