Blood clot. Coloured Scanning Electron Micrograph (SEM) of a blood clot, also known as a thrombus. Red blood cells (erythrocytes) are seen trapped in a web of white fibrin threads made of an insoluble protein. Small platelets (green) have also become enmeshed in the clot. When activated, platelets become spiky (as here), clump together and release chemical factors into the blood. These factors cause fibrin threads to form. A white blood cell of the immune system is also trapped (yellow). A clot forms when a blood vessel is injured or diseased. A clot that forms inside a blood vessel may lead to a heart attack if it blocks a heart artery. Magnification: x4,200 at 6x7cm size. x2,100 at 35mm size.