Physiology and immunology of the thymus gland

The thymus is a giand located in the upper anteriorportion of the chest cavity just behind the sternurn,Under the evolutionary pressure exerted by the emergence of adaptive immunityAnd its inherent risk to from receptorsThat recognize self molecules, this gland appeared about 500 milT cells in order to prevent autoimmunity and orchestrate selftolerance.The thymus has thus become a crucial lymphoid organ in which cells arriving from the bone marrow undergo a finely tuned process of selection based on the specificity of T-cell receptors (TCRs) and differentiate into mature T-cells.The development of thymocitesn involves a stringent selection in which only 1-3% of these cells succeed in survival and can leave the gland to colonize the periphery and give origin to effective immune cells. Duting maturation in thymus, T cells are first positively selected for uselfulness and then negatively selected against autoreactivity. These intrathymis events are governed by sequential interactions of thymocites with different stromal cell types during the migration through the thymus essentially from the external to the internal part of the thymic lobuli. The mature T cells called naïve T cells leave the organ and contribute to the peripheral T cell poll. The complex process of intrathymic maturation of T-lymphocites involvesvarious thymic specific factors and several other molecules. Indeed T cell maturation requires either direct cell-to-cell or paracrine interactions that occur via cytokines or thymic hormones produced by the cells of the thymic microenvironment. For a long toime the functions of the thymus have remained obscure. The first demonstration of its crucial role in the ontogeny and development of the immune system was provided in 1961 when it was shown that mice thymectomized immediately after birth had poorly developed lymphoid tisses impaired immune responses and susceptibility to infections. Althought cells present in the thymus were believed to be immunoincompetent a few years later it was shown that they could proliferate after an antigenic challenge and produce cells unable to synthetize antibodies. Such cells were capable of enabling other lymphocyte to differentiate to antibody-forming cells. This was the first demonstration in mammalians of the existence of two major subsets of lymphocytes now known as T- and B-cells. It required a re-evaluation of many immunological phenomena such as tolerance memory and autoimmunity and it was followed by a huge number of studies elucidating many of the mysteries of the immune system.