Sera from patients with chronic ITP with anti-GPIb-IX antibodies induce MSD unfolding and shear-dependent platelet signaling. (A) Enzyme-linked immunosorbent assay absorbance values for representative patient samples binding to GPIb-IX. Bars represent the values for plasma binding to purified GPIb-IX plus bovine serum albumin minus the values for bovine serum albumin binding alone. (B) Graphs of percentage of positive events for P-selectin (top) and phosphatidylserine (bottom) exposure in healthy donor platelets reconstituted in plasma from healthy donors, patient 02, or patient 11 under sheared (30 dyn/cm²) or static conditions (n = 4-5). (C) Graphs of fold increase (sheared/static) in 5G6 and WM23 binding to healthy donor platelets reconstituted in plasma from healthy donors, patient 02, or patient 11. Bars represent the mean of duplicate (n = 2) values for each condition. Variance in WM23 fold change was 12% to 15% of total. Variance in 5G6 fold change was 5% to 17% of total. ****P ≤ .0001.