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Illustration of virus particles (round) surrounded by antibody, or immunoglobulin, molecules. The y-shaped antibody molecules have two arms that can bind to specific antigens, for instance 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/KEITH CHAMBERS)