Blood clot. Coloured scanning electron micrograph (SEM) of red blood cells (erythrocytes) trapped in a fibrin mesh (blue). This image shows the internal surface of a PICC line with rouleaux formation. A PICC (peripherally inserted central catheter) line is used to give chemotherapy or other treatments such as liquid nutrition. 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. Rouleaux are stacks or aggregations of red blood cells that form due to the unique disc shape of the cells. Rouleaux form when the plasma protein concentration is high. Magnification x 8000 at 10cm wide