T-lymphocyte white blood cells, coloured scanning electron micrograph (SEM). T-lymphocytes (or T-cells, round) are a type of white blood cell. They are an important part of the body's immune system. They track down and destroy foreign bodies and infected cells. They also help to mediate the production of antibodies to invading organisms, such as bacteria and viruses. T- lymphocytes originate in the bone marrow, but mature in the thymus gland.