The lymphatic system consists of organs, nodes and vessels that are responsible for draining a fluid called lymph and returning it to the bloodstream. Lymph is composed of plasma, white blood cells and other substances.

Lymph fluid continually flows out of the thin walls of the capillaries into surrounding body tissues, where its proteins, minerals and nutrients provide nourishment to the tissues.  

Although most of the lymph is reabsorbed into the capillaries, some of the lymph remains in the spaces surrounding the cells. The lymph vessels are a network of tubes or canals that branch off into tissues throughout the body. They collect the excess lymph from the tissues and carry it back to the blood. Without this process, the excess lymph would remain in the body tissues and cause swelling. Lymph vessels also remove and transport damaged cells, bacteria, cancer cells and other foreign material that may have entered the tissue fluids.

A major component of the immune system, the lymphatic system defends the body from foreign material by producing and storing some of the cells needed to fight infections and disease. The system includes lymph nodes, small, bean-shaped organs that are clustered in various areas of the body, including the neck, armpit, chest and groin. The lymph nodes are located along the lymph vessels and their functions include producing immune cells (e.g., lymphocytes, monocytes and plasma cells) and filtering foreign materials from lymph.  When the lymph nodes recognize antigens or other foreign material in the lymph fluid, they respond by enlarging and producing additional lymphocytes and plasma cells to help combat infection.

In addition to the lymph fluid, lymph vessels and lymph nodes, the lymphatic system also involves a number of organs and tissue masses, including:

• Spleen.  An organ located under the rib cage on the left side of the abdomen. It is involved in the destruction of worn-out or damaged red blood cells and platelets as well as production of certain white blood cells. It produces lymphocytes and other cells to fight infection, stores healthy blood cells and filters out damaged or worn-out blood cells, platelets and cell waste.
  
  
• Thymus. An organ located in the front of the chest at the base of the neck that is most active before birth. During fetal development and childhood, its main function is developing T lymphocytes (T cells), a type of white blood cell. The T cells mature in the thymus and then travel to the lymph nodes. The activity and size of the thymus diminish after birth. It continues to decline with age as it is gradually replaced by fat tissue. In an older patient, what appears to be a yellowish-colored thymus may reveal only small remnants of thymus tissue surrounded by fat.
  
  
• Bone marrow. The soft, inner section of bone. It is composed of blood-forming cells, fat cells and tissues that support the growth of blood cells. There are two types of bone marrow, yellow and red. Found in the cavities of large bones, yellow marrow consists mainly of fat cells and a few primitive blood cells. Red marrow is tissue in which blood cells are produced. Blood cells produced in the bone marrow include:
  
  
• White blood cells (to fight infections)
  
  
• Red blood cells (carry oxygen to tissues in the body)
  
  
• Platelets (help develop blood clots and control bleeding)
  
  
• Adenoids and tonsils. These are masses of lymphoid tissue located at the back of the throat. Their role in the lymphatic system includes producing antibodies against antigens or other foreign materials that are breathed in or swallowed. Tonsils and adenoids often become swollen when they are fighting off an infection in the throat. Physicians may recommend that they  be surgically removed if they become chronically enlarged (e.g., after several attacks of tonsillitis).