White blood cells and a platelet. Coloured scanning electron micrograph (SEM) of white blood cells and a single platelet (blue). 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 other cells and each other, forming a clot. White blood cells (red lymphocytes and purple neutrophils) are involved in defending the body against both infectious disease and foreign materials. Magnification: x3000 when printed at 10 centimetres wide.