Lymphocytes, a variety of white blood cells, are involved in the immune response. They are of two kinds: B lymphocytes (production of antibodies) and T lymphocytes (cell-mediated immunity). These are small mobile cells found in the blood, bone marrow and lymphoid tissues (spleen, lymph nodes) that play a central role in the immune system. There are several families of lymphocytes, one of which is responsible for the synthesis of antibodies, but all of them are part of the leukocytes (white blood cells) family.