Anti-β2GP1 autoantibodies amplify thrombus formation in venules. Effect of purified anti-β2GP1 autoantibodies on thrombus size and fibrin generation in venules. Anti-β2GP1 autoantibodies were infused into wild-type mice 5 minutes before laser-induced venule wall injury. Platelet and fibrin imaging was performed using anti-CD42 antibody labeled with Dylight 649 (0.1 μg/g mouse) and anti-fibrin antibody labeled with Alexa 488 (0.5 μg/g mouse). Platelet thrombus size and fibrin generation at the site of laser-induced injury were determined by calculating median fluorescence values at 649 nm and 488 nm over 3 minutes, respectively. After initial laser injury of the venule wall, a thrombus composed of platelets (A) and fibrin (B) was generated; 15 minutes after infusion of 10 μg of anti-β2GP1 autoantibodies, anti-β2GP1 autoantibody–induced changes in platelet thrombus size and fibrin generation were observed. The kinetics of the fluorescence signals associated with platelets and fibrin over 180 seconds after vessel injury are shown before (black) and after (gray) infusion of anti-β2GP1 autoantibodies.