The lymphatic system helps maintain fluid balance and defend the body against disease. It contains lymphatic vessels, lymph nodes, and organs that collect excess fluid and produce lymphocytes. Diseases of the lymphatic system include elephantiasis and mononucleosis. Elephantiasis causes enlarged tissues due to blocked lymph vessels and edema. Mononucleosis is caused by the Epstein-Barr virus and causes fever, sore throat, and fatigue. The immune system helps the body develop immunity against viruses, fungi, and other pathogens through cells, tissues, and organs. Severe combined immunodeficiency syndrome is a congenital disorder where people are born with little to no immune system, leaving them susceptible to infections.