Illustration of an activated platelet, or thrombocyte. Platelets are fragments of white blood cells that, under normal circumstances, are small and biconcave in form. However, if there is a break in the surface of a blood vessel the platelets come into contact with molecules they are not used to and become activated. They become amorphous in form, with long projections (pseudopodia) that help them adhere to the vein and each other, forming a clot over the site of the break.