RUSSELL KIGHTLEY / SCIENCE PHOTO LIBRARY RUSSELL KIGHTLEY / SCIENCE PHOTO LIBRARY
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T cell dependent B cell activation, computer artwork. Invading pathogens (round, small) are phagocytosed (engulfed) by macrophages, a type of white blood cell. Fragments of protein (antigen) from the pathogen are displayed on the macrophage's surface. The antigen is recognised by helper T lymphocytes (centre right), which secrete chemicals to activate other immune cells, including B lymphocytes (centre left). The B lymphocytes differentiate into plasma cells (large cells bottom right), which produce large numbers of antibodies (y-shaped) that recognise the antigen. The antibodies either neutralise the pathogen or flag it for destruction by other cells.
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