Study of details of thrombus formation at the single-cell level. Irradiation of hematoporphyrin with a 488-nm laser beam results in production of reactive oxygen species (ROS), mainly O₂⁻. Free radicals activate all cells present in the irradiated field—white blood cells, red blood cells, and platelets, but maily endothelial cells. Activated endothelial cells release von Willebrand factor (VWF), regulated through inflammatory cytokine signaling. Discoid platelets adhere to the activated endothelium through GP1bα-VWF interaction and form a platelet-rich, occlusive thrombus. Single-cell dynamics and thrombus formation details are visualized during laser excitation and ROS production. Professional illustration by Paulette Dennis.