Illustration of a blood vessel with red blood cells (erythrocytes) trapped in a fibrin mesh (white) forming a clot. The production of fibrin is triggered by cells called platelets, activated when a blood vessel is damaged. The fibrin binds the various blood cells together, forming a solid structure called a blood clot. A blood clot is a normal response, preventing an excessive loss of blood. However, inappropriate clotting is a major cause of heart attacks and strokes.