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White blood cells known as lymphocytes arise from by mitosis of stem cells in the bone marrow. Some lymphocytes migrate to the thymus and become T cells that circulate in the blood and are associated with the lymph nodes and spleen.
The stem cells give rise to two families of white blood cells: the myeloid cells (named after bone marrow) and the lymphoid cells, which take their name from the lymphatic system. Both families of leukocyte normally leave the bone marrow and enter other tissues through the high endothelial venules, or HEV.