The lymphatic system in vertebrates is a network of conduits that carry a clear fluid called lymph. It also includes the lymphoid tissue that the lymph travels through. Lymphoid tissue is found in many organs, particularly the lymph nodes, and in the lymphoid follicles associated with the digestive system such as the tonsils. The system also includes all the structures dedicated to the circulation and production of lymphocytes, which includes the spleen, thymus, bone marrow and the lymphoid tissue associated with the digestive system. The lymphatic system as we know it today, was first described independently by Olaus Rudbeck and Thomas Bartholin.
The dissolved constituents of the blood do not directly come in contact with the cells and tissues in the body, but first enter the interstitial fluid, and then the cells of the body. Lymph is the fluid that is formed when interstitial fluid enters the conduits of the lymphatic system. The lymph is not pumped through the body like blood, it is moved mostly by the contractions of skeletal muscles.
The lymphatic system has three interrelated functions. It is responsible for the removal of interstitial fluid from tissues. It absorbs and transports fatty acids and fats as chyle to the circulatory system. The last function of the lymphatic system is the transport of antigen presenting cells (APCs), such as dendritic cells, to the lymph nodes where an immune response is stimulated.
The study of lymphatic drainage of various organs is important in diagnosis and treatment of cancer. The lymphatic system, because of its physical proximity to many tissues of the body, is responsible for carrying cancerous cells between the various parts of the body in a process called metastasis. The intervening lymph nodes can trap the cancer cells. If they are not successful in destroying the cancer cells the nodes may become sites of secondary tumors.
Diseases and other problems of the lymphatic system can cause swelling and other symptoms. Problems with the system can impair the body's ability to fight infections.

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