<P> The other main function is that of defense in the immune system . Lymph is very similar to blood plasma: it contains lymphocytes . It also contains waste products and cellular debris together with bacteria and proteins . Associated organs composed of lymphoid tissue are the sites of lymphocyte production . Lymphocytes are concentrated in the lymph nodes . The spleen and the thymus are also lymphoid organs of the immune system . The tonsils are lymphoid organs that are also associated with the digestive system . Lymphoid tissues contain lymphocytes, and also contain other types of cells for support . The system also includes all the structures dedicated to the circulation and production of lymphocytes (the primary cellular component of lymph), which also includes the bone marrow, and the lymphoid tissue associated with the digestive system . </P> <P> The blood does not come into direct contact with the parenchymal cells and tissues in the body (except in case of an injury causing rupture of one or more blood vessels), but constituents of the blood first exit the microvascular exchange blood vessels to become interstitial fluid, which comes into contact with the parenchymal cells of the body . Lymph is the fluid that is formed when interstitial fluid enters the initial lymphatic vessels of the lymphatic system . The lymph is then moved along the lymphatic vessel network by either intrinsic contractions of the lymphatic passages or by extrinsic compression of the lymphatic vessels via external tissue forces (e.g., the contractions of skeletal muscles), or by lymph hearts in some animals . The organization of lymph nodes and drainage follows the organization of the body into external and internal regions; therefore, the lymphatic drainage of the head, limbs, and body cavity walls follows an external route, and the lymphatic drainage of the thorax, abdomen, and pelvic cavities follows an internal route . Eventually, the lymph vessels empty into the lymphatic ducts, which drain into one of the two subclavian veins, near their junction with the internal jugular veins . </P> <P> The lymphatic system consists of lymphatic organs, a conducting network of lymphatic vessels, and the circulating lymph . </P> <P> The primary or central lymphoid organs generate lymphocytes from immature progenitor cells . </P>

What are the main organs in the lymphatic system