Cells of the immune system are associated with the lymphatic system of the body and its specialized cells. Lymphocytes of the lymphatic system are derived from stem cells of the bone marrow. These undifferentiated precursor cells proliferate throughout life and replenish the mature cells of the immune system.
B-lymphocytes and T-lymphocytes. There are two major pathways for the differentiation of stem cells into immune cells. Certain of the stem cells produce B-lymphocytes (B-cells) while other stem cells form T-lymphocytes (T-cells). B-lymphocytes are so named because in birds, they are formed in the bursa of Fabricius. The equivalent site in humans has not been identified but is believed to be the bone marrow. T-lymphocytes undergo their conversion in the thymus gland, an organ in the neck tissues near the trachea and thyroid gland (Figure 1 ).