The lymphatic system is involved in the immune response by circulating lymph, a fluid containing cells that fight infection.
The human body fights disease via the immune system. The immune system relies heavily on the lymphatic system, which is made up of organs and cells that store and circulate fluids involved in immune response. The primary function of the lymphatic system is the collection and transport of lymph, a colorless fluid that contains white blood cells, which help fight infection. Lymph travels through lymphatic vessels. A lymphatic vessel, similar to a vein or artery, is one of many such vessels distributed throughout the body that carry lymph. The process of delivering lymph to infected sites closely involves draining interstitial fluid from the spaces between cells. Interstitial fluid, which is the liquid outside of the body's cells, can contain oxygen, sugars, amino acids, and proteins that cells require in order to carry out the processes of life. The lymphatic system replaces interstitial fluid with lymph when needed, but it also carries interstitial fluid to the blood vessels, where it is returned to the blood. Lymph always travels towards the heart and white blood cells produced by and collected in the lymphatic system are distributed through the body by the circulatory system. The lymphatic system essentially controls an ebb and flow of both lymph and interstitial fluid to maintain a balance of fluid in the body. Without the lymphatic system, this fluid would build up, compromising homeostasis, which is the way the body balances and regulates such internal functions.
In addition, the lymphatic system involves the digestive system because fats absorbed from the small intestine are collected as lymph in a mechanism very similar to the collection of interstitial fluid. Fats in the small intestine are absorbed into the lymphatic system and are transported into the circulatory system. Some fats pass directly through the gastrointestinal tract, but most are absorbed through the lymphatic system instead. This method of absorption protects the liver from damage, because the liver does not process fats as well as it does other nutrients.