The lymphatic system is an extensive and interconnected system of vessels, spaces, and nodes in the body which circulates lymph, a fluid made from protein and other fluids. Lymph is initially part of the blood, but as blood flows through the body, lymph slowly leaks out, taking hazardous substances with it and filtering these substances through the lymph nodes, which trap these substances. Over time, the lymph slowly works its way around the body, eventually re-entering the heart so that it can be circulated all over again.
Many people are familiar with the circulatory system, which moves blood around the body, but they may not be as familiar with the lymphatic system, because it isn't quite as flashy. However, this system is still extremely important, as lymph performs a number of vital functions in the body, especially in the context of the immune system. You have probably seen your own lymph; if a cut has ever oozed a clear fluid, that was lymph.
Lymph can be found all over the body, filling the spaces between muscles, organs, and other structures in the body, and circulating through a series of valves which allow the lymph to flow only in one direction. Unlike the blood, lymph does not have a central pump, relying instead on muscle contractions and other natural functions of the body to force lymph through the lymphatic system, where it will eventually drain into the thoracic duct, which brings lymph back to the heart.
Lymph generates several types of cells which are important to the immune system, and it also traps substances which the body identifies as dangerous. It also plays a role in the transport of fatty acids and other substances, helping the digestive tract process food more efficiently. As lymph circulates, passing through the lymph nodes, it becomes more turbid, abandoning potentially hazardous materials and seeking others to quarantine to keep the body healthy.
Damage or interruption to this system can result in a range of medical conditions. You may be familiar with lymphedema, a condition in which lymph pools in the limbs because it cannot circulate. Lymphoma is also related to the lymphatic system, as the name implies, and elephantiasis is also linked with disruption to the lymphatic system. You may also notice that your lymph nodes become swollen when you are sick, because your body is isolating the agent responsible to help you get better.