The lymphatic system is an important part of the immune system and can be described as the drainage system of the human body. The lymph drainage is a one-way flow, not a circulation.
The lymphatic system consists of all cells, tissues, and organs that contain aggregates of lymphocytes, including the lymph organs, lymph nodes, lymph ducts, and lymph vessels.
The lymphatic system functions as a filtration system for the body fluid -- acts as an active supporter of the blood and circulatory system. However, one of the main functions of this system is defending the human body against microorganisms and other harmful foreign substances or antigens. When microorganisms and other foreign invaders enter into the body, the components of lymphatic system filters these harmful invaders and add antibodies. The lymph node filters these foreign invaders from lymph, the spleen from blood, and the lymphoid aggregates from tissue fluid.
The lymphatic system maintains fluid balance in the body and also absorbs fatty substances from the small intestine. Water, protein and other constituents of blood continuously exit the blood vessels and enter into the spaces between tissues. At that point, these constituents of blood are called as interstitial fluid or lymph. The lymph vessels collect the interstitial fluid (lymph) from the spaces between tissues and through the lymphatic system, the interstitial fluid passes through lymph nodes where it is filtered, and after that, the interstitial fluid returns to the blood again.