Human red blood cells trapped in a fibrin blood clot, coloured scanning electron micrograph (SEM). Human red blood cells trapped in a fibrin blood clot. Platelets (not seen) are cell fragments in the blood that play an essential role in blood clotting and wound repair, and can also activate certain immune responses. Platelets in the blood are small oval disks and are termed non-activated platelets or thrombocytes. They are the body's first line of defence against excessive blood loss. When an injury such as a cut is sustained, platelets change their shape (now known as activated platelets), become sticky and build up on a vessel wall to form a plug. They are also involved in the secretion of a chemical platelet factor which helps produce threads of fibrin. Bloods cells collect in the entangled fibrin and platelet mass forming a blood clot at the site. Magnification: x2, 400 when shortest axis printed at 25 millimetres.