A type of white blood cell (leucocyte) that has a large nucleus and little cytoplasm. Lymphocytes are formed in the lymph nodes and provide about a quarter of all leucocytes. They are important in the body’s defence and are responsible for immune reactions as the presence of antigens stimulates them to produce antibodies. There are two principal populations of lymphocytes: B lymphocytes (see b cell), which produce circulating antibodies and are responsible for humoral immunity; and T lymphocytes (see t cell), which are responsible for cell-mediated immunity.