Activated T lymphocytes and red blood cells (RBCs). Coloured scanning electron micrograph (SEM) of activated T lymphocytes and RBCs 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. Morphologically the cells become more adherent and flattened with many more processes when activated. Red blood cells are biconcave, disc-shaped cells that transport oxygen from the lungs to body cells. They circulate in the blood and also remove carbon dioxide to the lungs for exhalation. Their red colour is due to the oxygen-carrying protein haemoglobin. Magnification: x5000 when printed at 10 centimetres wide.