Representative flow cytometric histogram fluorescence of THP-1 phagocytosis of W6/32 platelets. (A) Control THP-1 phagocytosis of W6/32-opsonized platelets at 0°C (······) and 37°C (—) and (B) THP-1 phagocytosis of W6/32-opsonized platelets at 37°C in the presence of nonopsonized red blood cells (RBC; 10:1 RBC:THP-1 ratio, —), Rh immune globulin-opsonized RBC (1:1 RBC:THP-1 ratio, ——), or Rh immune globulin-opsonized RBC (10:1 RBC:THP-1 ratio, ······). The median values of the THP-1 fluorescence for W6/32-opsonized platelets at 0°C was 4.2 (± 1.2) and at 37°C was 79.6 (± 28.4, N = 91). These values were used to calculate the percentage inhibition of (C) W6/32-opsonized platelet phagocytosis by nonopsonized RBC (□), RBC opsonized with Rh immune globulin (●), or RBC bound with Rh immune globulin F(ab′)₂ fragments (○); and (D) IgG autoantibody-opsonized platelet phagocytosis by nonopsonized RBC (□), RBC opsonized with Rh immune globulin (●), or RBC bound with Rh immune globulin F(ab′)₂ fragments (●). The data in panels C and D are presented as the mean percent inhibition (± SD) at the indicated RBC:THP-1 ratios (N = 20 for panel C and N = 8 for panel D) and were calculated by the formula: (1 − phagocytic index [PI]))×(added RBC)÷PI (platelets alone) × 100. Statistical significance (P values) by an unpaired t test is shown: all solid symbols were compared with the corresponding nonopsonized RBC inhibition, ★; P < .001.