![]() ![]() ![]() to produce immune cells to fight antigens.However, the thymus is not where the infection is fought, as the T cells have yet to become immunocompetent. Therefore, one of the most important roles of the thymus is the induction of central tolerance. In addition, thymic stromal cells allow for the selection of a functional and self-tolerant T cell repertoire. The thymus provides an inductive environment for the development of T cells from hematopoietic progenitor cells. Research on bony fish showed a buildup of T cells in the thymus and spleen of lymphoid tissues in salmon and showed that there are not many T cells in non-lymphoid tissues. T cells mature from thymocytes, proliferate, and undergo a selection process in the thymic cortex before entering the medulla to interact with epithelial cells. In most species, the thymus consists of lobules divided by septa which are made up of epithelium which is often considered an epithelial organ. The loss or lack of the thymus results in severe immunodeficiency and subsequent high susceptibility to infection. The thymus is where the T lymphocytes mature and become immunocompetent. However, residual T cell lymphopoiesis continues throughout adult life, providing some immune response. At puberty, by the early teens, the thymus begins to atrophy and regress, with adipose tissue mostly replacing the thymic stroma. The thymus is located between the inferior neck and the superior thorax. It is most active during the neonatal and pre-adolescent periods. The thymus increases in size from birth in response to postnatal antigen stimulation. The thymus and the bone marrow constitute the primary lymphoid organs involved in the production and early clonal selection of lymphocyte tissues. The primary (or central) lymphoid organs generate lymphocytes from immature progenitor cells. The lymphatic system consists of a conducting network of lymphatic vessels, lymphoid organs, lymphoid tissues, and the circulating lymph. Structure Anatomy of the lymphatic system showing primary and secondary lymphoid organs The lymphatic system was first described in the 17th century independently by Olaus Rudbeck and Thomas Bartholin. In some vertebrates, a lymph heart is present that pumps the lymph to the veins. Lymph is moved through the system by muscle contractions. The ducts empty into the subclavian veins to return to the blood circulation. ![]() The right lymphatic duct drains the right side of the region and the much larger left lymphatic duct, known as the thoracic duct, drains the left side of the body. ![]() Lymph then passes into much larger lymph vessels known as lymph ducts. These vessels carry the lymph throughout the body, passing through numerous lymph nodes which filter out unwanted materials such as bacteria and damaged cells. The fluid bathes the tissues as interstitial fluid, collecting waste products, bacteria, and damaged cells, and then drains as lymph into the lymphatic capillaries and lymphatic vessels. įluid from circulating blood leaks into the tissues of the body by capillary action, carrying nutrients to the cells. Lymphoid tissue is also associated with mucosas such as mucosa-associated lymphoid tissue (MALT). The lymphoid organs also contain other types of cells such as stromal cells for support. Lymphocytes are initially generated in the bone marrow. These include the lymph nodes (where the highest lymphocyte concentration is found), the spleen, the thymus, and the tonsils. Associated lymphoid organs are composed of lymphoid tissue, and are the sites either of lymphocyte production or of lymphocyte activation. The cells of the lymph are mostly lymphocytes. Lymph is very similar to blood plasma, in that it contains waste products and cellular debris, together with bacteria and proteins. The other main function is that of immune defense. One of the main functions of the lymphatic system is to provide an accessory return route to the blood for the surplus three litres. Roughly 17 litres of the filtered blood is reabsorbed directly into the blood vessels, while the remaining three litres are left in the interstitial fluid. The human circulatory system processes an average of 20 litres of blood per day through capillary filtration, which removes plasma from the blood. Unlike the circulatory system that is a closed system, the lymphatic system is open. (The Latin word for lymph, lympha, refers to the deity of fresh water, " Lympha"). Lymph is a clear fluid carried by the lymphatic vessels back to the heart for re-circulation. It consists of a large network of lymphatic vessels, lymph nodes, lymphoid organs, lymphoid tissues and lymph. The lymphatic system, or lymphoid system, is an organ system in vertebrates that is part of the immune system, and complementary to the circulatory system. ![]()
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