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 ). Figure 1 An overview of the human immune system. T‐lymphocytes and B‐lymphocytes originate in the bone marrow and then are processed in different body organs before proceeding to the lymph nodes, where they provide the underpinnings of the two types of immune responses. The transformation of stem cells into B-lymphocytes and T-lymphocytes begins about the fifth month after fertilization, and a full set is complete a few months after birth. These cells then migrate to the lymphoid organs in the lymph nodes, spleen, tonsils, adenoids, and other organs of the lymphatic system. To initiate the immune response, microorganisms are phagocytized and their antigens are processed in phagocytic cells such as macrophages. The antigenic determinants are displayed on the surface of the phagocytic cells and presented to the appropriate B-lymphocytes and T-lymphocytes to provoke an immune response. Clonal selection. The clonal selection theory helps explain how lymphocytes recognize antigenic determinants and respond. According to this theory, small populations (clones) of lymphocytes bear receptors on their cell membranes. Production of these receptors is genetically determined. On B-lymphocytes, the receptors consist of antibody molecules, while on T-lymphocytes, they are clusters of amino acids. When lymphocytes encounter an antigenic determinant on the surface of a macrophage, their receptors match with the antigenic determinant and a stimulation follows. A match is also made between a set of molecules called major histocompatibility (MHC) molecules and their receptors. The clonal selection theory suggests that B-lymphocytes and T-lymphocytes exist for all antigenic determinants even before contact with an antigen is made. The theory also says that antigenic determinants stimulate the lymphocytes to endow their progeny with identical specificity. The B-lymphocytes and T-lymphocytes that might potentially react with the body's own cells are deleted or in some way inactivated to ensure that an immune response to the host organism does not develop. Two general types of immunity exist for specific resistance to disease. They are antibody-mediated (humoral) immunity, centered in B-lymphocytes, and cell-mediated immunity, centered in T-lymphocytes.