Lymphocytes, a type of white blood cell, are the smallest white blood cells, produced by the lymphoid organs, and are important cells for the immune function of the body. Lymphocytes can be divided into T lymphocytes, B lymphocytes and natural killer cells (NK cells.) Both T cells and B cells initially originate from hematopoietic tissues, T cells circulate with the blood to the thymus and mature beneath the thymus, B cells mature in the bone marrow, T cells are involved in cellular immunity, and B cells produce antibodies. Natural killer cells, or NK cells, are not dependent on antigen stimulation and exert a spontaneous cytotoxic effect with the ability to kill target cells.