Platelet particle formation, size, and annexin binding induced by anti–GPIIIa49-66 Ab or Ca²⁺ ionophore. (A) Studies with anti–GPIIIa49-66 Ab. Gel-filtered platelets were labeled with either CD61-FITC (left panels) or annexin-FITC (right panels) incubated with buffer (A,E), control IgG (B,F), or anti–GPIIIa49-66 (C,G) for 4 hours at 37°C and analyzed by flow cytometry (representative of 3 experiments). (B) Studies with Ca²⁺ ionophore. Similar studies were performed with Ca²⁺ ionophore as agonist (D,H). The numbers in the quadrants refer to the percentage of cells in the quadrant as determined by fluorescent labeling.