Human red blood cells and activated platelets trapped in a fibrin blood clot, composite coloured scanning electron micrograph (SEM). Platelets in the blood are small oval disks and are termed nonactivated platelets or thrombocytes. Platelets serve as the body's first line of defence to prevent excessive blood loss. When an injury such as a cut is sustained, platelets (now known as activated platelets) change their shape, become sticky and build up on a vessel wall to form a plug. Bloods cells collect in the entangled fibrin and platelet mass forming a blood clot at the site. Platelets are also involved in the secretion of a chemical platelet factor which helps produce threads of fibrin. Platelets can also activate certain immune responses. Magnification: x1, 375 when shortest axis printed at 25 millimetres.