The lymphatic system is a network of vessels within the body that collects fluid from the spaces between tissue cells (interstitial fluid), which has filtered out through the blood capillaries. The lymphatic system then returns that fluid, including escaped blood proteins, to the bloodstream. The lymphatic system also removes particles such as bacteria and molecular proteins from the tissues, carries fat from the intestine to the blood, and produces lymphocytes, a type of white blood cell that is an important part of the immune system. The clear, watery fluid carried by the lymph vessels is called lymph; the lymph in the vessels periodically flows through lymph nodes, or glands, which filter foreign matter out of the lymph with phagocyte cells and add lymphocytes. The lymph vessels originate as very thin-walled capillaries, which merge into larger lymph vessels, which in turn drain in the shoulder region into...