The lymphatic system plays a particularly important role in the elimination of toxic waste and bacteria from the tissues, and is a vital part of the body's immune system. It also carries digested fats away from the intestines. The lymphatic system is made up of lymph fluid, lymphatic vessels, bone marrow [the production site of lymphocytes], and other structures and organs that contain lymph, such as lymph nodes, the spleen and tonsils. The clear fluid in tissue spaces, called interstitial fluid, collects accumulated waste and carries it into the lymphatic vessels where it becomes lymph. Lymphatic vessels are similar to the veins of the cardiovascular system, but the vessel walls are much thinner and have more valves. Unlike the bloodstream, the lymphatic vessels travel in one direction only, from the tissues to the bloodstream. In a continual process, this fluid collects in the tissues from the arterial system. Once it is swept up into the lymphatic system, the fluid is cleansed and filtered by the lymph nodes before it re-enters the bloodstream. Unlike the circulatory system, the lymph vessels depend largely on muscles, not the heart, to pump fluid through its vessels. A sluggish lymphatic system is often due to lack of exercise, but it can also be the result of excess waste or fluid in the body. When an infection is being fougth in nearby tissues, lymph glands become swollen with the additional burden. If the lymphatic system is not capable of halting the progress of infection, infectious material will be allowed to spill into the bloodstream. The tonsils and spleen are well endowed with lymphatic tissue. The tonsils, a collection of lymph nodes at the back of the throat, are defence stations designed to fight off infectious agents that are inhaled or ingested before they can do greater damage or reach farther into the body. The spleen, located in the upper left of the abdomen, does not actually filter lymph because it does not have lymphatic vessels. Its contribution to immunity is the manufacturing of lymphocytes, and the destruction of damaged or worn-out red blood cells.