Normal human sera contain a factor required for phagocyte responses toward IgG-opsonized platelets. (A) Respiratory burst activity of PMN toward platelets in NHS or anti-HPA-1a antibody-containing FNAIT sera or saline solutions, analyzed by the conversion of dihydrorhodamine-1,2,3 to fluorescent rhodamine-1,2,3, measured by fluorescence-activated cell sorter. (B-C) PMN respiratory burst activity toward IgG-opsonized platelets was only observed in NHS. (B) Platelets were opsonized with the human monoclonal anti-HPA-1a IgG1 antibody; B2G1, subsequently washed, was resuspended in either PBS, HEPES, or NHS together with PMN to initiate phagocyte responses. (C) Platelets were preincubated with either NHS (unopsonized) or opsonized with FNAIT serum, washed with PBS, and resuspended in the indicated media before addition to PMN, and then the respiratory burst was measured after 45 minutes at 37°C. Data are representative of 3 independent experiments, showing mean ± standard deviation. Statistical comparisons were performed as follows: (A) 1-way analysis of variance (ANOVA) with Tukey’s posttest; (B-C) 2-way ANOVA with Bonferroni posttest. ***P ≤ .001.