Resting T lymphocytes. Coloured scanning electron micrograph (SEM) of resting T lymphocytes from a human blood sample. T lymphocytes, or T cells, are a type of white blood cell and components of the body's immune system. They mature in the thymus. T lymphocytes recognise a specific site on the surface of pathogens or foreign objects (antigens), bind to it, and become activated to produce antibodies or cells to eliminate that antigen. Specimen courtesy of Professor Greg Towers, University College London, UK. Magnification: x7000 when printed at 10 centimetres wide.