Small lymphocytes are the cells responsible for specific immunity (Figure4.2). They make up approximately 20% of the blood leukocytes. There are twopopulations of small lymphocytes that mature at different sites in the body and have distinct functions (Figure 4.14).
The precursors of small lymphocytes originate in the bone marrow by division of lymphoid stem cells. Some small lymphocytes remain in the bone marrow where they mature into B lymphocytes. When maturation is complete, B lymphocytes have antibodies on their surface that are receptors for an individual epitope. Thus a single B lymphocyte is specific for an epitope and is capable of clonal division and of making antibody to it when stimulated appropriately by the immunogen. Thus they are responsible for humoral immunity. The second population of small lymphocytes, known as T lymphocytes, leave the bone marrow when immature and complete theirmaturation in the fetal thymus, a bilobed organ situated just above the heart. During their development in the thymus, T lymphocytes first acquire specificity for an epitope, by producing a cell surface receptor. They then mature into one of two T cell subsets. Cells of the first subset are known as cytotoxicprecursors or TC cells. When appropriately stimulated, TCcells develop into cytotoxic T lymphocytes (CTL) that are capable of killing virus-infected cells.Cells of the second subset of T cells are the helper T lymphocytes or THcells. When stimulated by an immunogen, TH cells develop into cytokine-secreting TH cells that produce an array of cytokines that control the activities of both specific and nonspecific cells of the immune system. Thus TH cells have a central role in the regulation of all immune responses.
When mature, both B and T lymphocytes are released into the circulation. However, small lymphocytes are not confined to the blood and many move into the lymphoid tissues: the spleen, lymph nodes, tonsils and the mucosa-associated lymphoid tissues found in the respiratory, gastrointestinal and urogenital tracts. Small lymphocytes constantly move between the blood and the lymphoid systems, a phenomenon known as recirculation. The route of this recirculatory process is shown in Figure 4.15. Lymph is the fluid that drains from the tissues into small lymphatic vessels. These merge with larger lymphatic vessels, the largest of which, the thoracic duct, delivers the lymph to the blood at its junction with the left subclavian vein. En route to the thoracic duct, lymph is filtered through many lymph nodes.Small lymphocytes circulating in the blood are able to move between the
endothelial cells lining the blood vessels that supply the lymph nodes. These blood vessels have a specialized endothelium which aids this process. By crossing the blood vessel wall, the small lymphocytes enter the lymph node and from this they enter the lymphatics and, eventually, return to the blood.