Click here . Natural killer cells resemble macrophages, large granular lymphocytes . They kill many virusinfected cells and cancer cells , particularly in connection with T lymphocytes. Lymphocytes are also produced in the bone marrow. Lymphocytes are not motile like granulocytes or macrophages, and they enter the circulation through lymphatic channels that drain into the venous system of the blood.
Some lymphocytes leave and reenter the circulation, surviving for many years.
They are found in large numbers in the lymph nodes, tonsils, thymus, spleen and lymphoid tissue of the gastrointestinal tract. When stimulated by antigen for example on a microbe, the lymphocyte divides several times into daughter cells and eventually generates a clone of identical lymphocytes. Some of these cells remain in the circulation, others patrol the tissues of the body. The larger number of lymphocytes capable of reacting to the same microbe is responsible for the immunological memory that is manifested, if the organism encounters the same microbe later in time. T lymphocytes participate in what is called cell-mediated immune responses , for example against bacteria and parasite.