Blood clot. Coloured scanning electron micrograph (SEM) of red blood cells (erythrocytes, rounded objects) trapped in a fibrin mesh (strands). 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 (thrombosis, forming a thrombus) is a major cause of heart attacks, embolisms, and strokes.