Activated granulocyte. Coloured scanning electron micrograph (SEM) of an activated granulocyte, or polymorphonucleocyte (PMN), white blood cell. Granulocytes are part of the immune system and are characterized by the presence of granules in their cytoplasm. They are released from the bone marrow in response to foreign bodies and infection. Magnification x5000 when printed 10cm wide.