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Dendritic cell. Coloured scanning electron micrograph (SEM) of a protective cell of the human immune system known as a dendritic cell. The long projections seen on the cell's surface are "feet" to help it move. These cells process foreign antigens which then act like an alarm signal, alerting other immune cells of the body to the infection. Dendritic cells found in the upper layer of the skin (the epidermis) are known as histiocytes or Langerhans cells. In the central nervous system they are known as microglia, and in the liver as Kupffer cells. Magnification:x 3000 when printed at 10cm wide (KEYSTONE/SCIENCE PHOTO LIBRARY/STEVE GSCHMEISSNER/SCIENCE PHOTO LIBRARY)