Activated platelets, coloured scanning electron micrograph (SEM). Platelets are blood cell fragments that play an essential role in blood clotting and wound repair, and can also activate certain immune responses. They are formed in the red bone marrow, lungs, and spleen by fragmentation of very large cells known as megakaryocytes. 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 (now known as activated platelets) change their shape, 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: x2,000 when