Anatomy & Physiology

Anatomy & Physiology Study Guide lymphatic duct. Lymph drains into the thoracic duct from the left side of the body superior to the diaphragm and region inferior to the diaphragm. The smaller right lymphatic duct collects lymph from the right side of the body superior to the diaphragm. When drainage is blocked, and lymph cannot flow from a limb, lymphedema results. 21.3 Lymphocytes Twenty to thirty percent of leukocyte population is accounted for by Lymphocytes. Three classes of lymphocytes circulate in blood: T (thymus-dependent) cells, B (bone marrow-derived) cells, and NK (natural killer) cells. Approximately 80 percent of circulating lymphocytes are classified as T cells. The primary types of T cells are the following: • Cytotoxic T cells: The body uses these cells to attack cells (usually via direct contact) in the body infected by a virus. Therefore, they are the principal cells used for the production of cell- mediated immunity or cellular immunity. • Helper T cells: These stimulate the activation and function of both T cells and B cells. • Suppressor T cells: These obstruct the activation and function of both T cells and B cells. Ten to fifteen percent of the circulating lymphocytes are accounted for by B cells. These cells can be stimulated to differentiate into plasma cells, which are responsible for the production and secretion of antibodies, which are soluble proteins also known as immunoglobulins. Antibodies bind to specific chemical targets called antigens. The remaining 5–10 percent of circulating lymphocytes are large granular lymphocytes known as NK cells. These lymphocytes attack infected cells, foreign cells, and cancer cells that appear in normal tissues. Their continuous “policing” of peripheral tissues has been called immunological surveillance . 21.4 Lymphocyte Production Lymphocyte production, or lymphopoiesis , involves the bone marrow, thymus, and peripheral lymphoid tissues. The bone marrow produces two distinct populations of lymphoid stem cells. One group of lymphoid stem cells remains in the bone marrow. Divisions of these cells produce immature NK cells and B cells. B cell development involves intimate contact with large stromal cells in the bone marrow. Stromal cells produce an immune system hormone, or cytokine, called interleukin-7 , that promotes the differentiation of B cells. The second group of lymphoid stem cells migrates to the thymus. Under the influence of thymic hormones, the lymphoid stem cells repeatedly divide, producing the various kinds of T cells. 21.5 Lymphoid Tissues Lymphoid tissues are connective tissues dominated by lymphocytes. In a lymphoid nodule or lymphatic nodule, the lymphocytes are densely packed in an area of areolar tissue. In many regions, lymphoid nodules form large clusters. Lymphoid nodules are found in the connective tissue deep to Achieve Page 281 of 368 ©2018