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Antibody response, conceptual image. Computer artwork representing immunoglobulin (Ig) molecules responding to a flu virus. Each Y-shaped molecule has two arms (top) that can bind to specific antigens, for instance bacterial or viral proteins. In doing this they mark the antigen for destruction by phagocytes, white blood cells that ingest and destroy foreign bodies. Antibodies can also kill some pathogens directly, and can neutralise toxins. (KEYSTONE/SCIENCE PHOTO LIBRARY/RAMON ANDRADE 3DCIENCIA)