magnitude of this response and terminating it over time. Certain T lymphocytes called suppressor T cells diminish or suppress responses of the other lymphocytes to antigen. Characterization of these cells has proved to be difficult, but recent studies have convincingly shown that these cells belong to the population of CD4 T lymphocytes that coexpress the CD25 and FOXP3 marker proteins. CD4CD25FOXP3 suppressor T cells originate in the thymus and account for about 5% of the total population of T cells. They secrete cytokines such as IL-10 and transforming growth factor  (TGF-), the latter a potent suppressor of proliferation of specific classes of T and B effector cells.
Suppressor T cells diminish or suppress antibody formation by B cells, as well as lower the ability of CTLs to provide a cell-mediated immune response. They play important roles in delayed hypersensitivity reactions (allergic reactions) by downregulating responses to antigen that enter the body through the skin or mucosa. They are also important in the prevention of graft rejection. Suppressor T lymphocytes may also function in the regulation of erythroid cell maturation in bone marrow.
Activated T lymphocytes synthesize a variety of cytokines.
Cytokines are soluble polypeptide substances, synthesized mainly by activated T lymphocytes, which affect the function of immune system effector cells (T and B cells), monocytes, macrophages, and other APCs. In general, cytokines and growth factors are similar in nature; the distinction between them is related to their effects on their target cell populations. Cytokines are defined as substances that are involved in immune defense mechanisms and act on lymphocytes, whereas growth factors act on other somatic cells. Included among these substances are chemotactic and mitogenic agents, migration inhibitory factors, interferon, and interleukins. Cytokines serve as chemical messengers between cells of the immune system and act locally on the same cell that secreted them (autocrine control) or on neighboring cells (paracrine control). In a way similar to hormones, they may also communicate the state of the immune system to cells in other systems (e.g., central nervous system, endocrine system, and hemopoietic system). Cytokines function through specific receptors. Therefore, cells regulated by cytokines possess cytokine receptors.
Interleukins are synthesized mainly by helper CD4 T lymphocytes and to a lesser extent by monocytes, macrophages, and endothelial cells. Interleukins promote growth and differentiation of T cells, B cells, and hematopoietic cells. Currently, more than 29 interleukins have been identified. Interleukin 2 was the first cytokine to be discovered and characterized. Mutations in the genes encoding several cytokine receptors have been identified in several  immunodeficiency disorders, bacterial sepsis, certain lymphoid cancers, and diseases of autoimmunity. For instance, individuals with a mutation in the IL-12 receptor gene cannot mount an effective immune response against mycobacterial (fungal) infections. Cytokines have been used with promising results to prevent transplant rejection, reverse cellular deficiencies after chemotherapy and radiation therapy, and treat certain cancers. The major functions of selected interleukins are summarized in Table 14.3.
APCs interact with helper CD4 T lymphocytes to facilitate immune responses.