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The organs and tissues involved in immune system activity are spread throughout the body. Some are solely dedicated to protecting the body from foreign organisms and toxins. Others fulfill multiple functions, with their involvement in immune system activity only part of their overall role in the body.

Most major organs and tissues involved in immune system response may be considered part of the lymphatic system. This system consists of organs and tissues that are responsible for draining fluid (lymph) from spaces in tissues and returning it to the bloodstream. Lymph is composed of plasma, white blood cells and other substances. It continuously flows out of the thin walls of the capillaries (tiny blood vessels that carry oxygen and nutrients to individual body cells) into surrounding body tissues where its proteins, minerals and nutrients provide nourishment.

Although most lymph is reabsorbed into the capillaries, some remains in the spaces surrounding the cells. The lymph vessels are a network of tubes that branch off into tissues throughout the body. They collect excess lymph and also remove and transport damaged cells, bacteria, and foreign material that may have entered body tissue.

A major component of the immune system, the lymphatic system produces and stores some of the cells needed to fight infections and disease. The system includes the lymph nodes (also known as lymph glands), 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 system cells and filtering foreign materials from lymph. When the lymph nodes recognize antigens in the lymph fluid, they respond by enlarging and producing additional cells to help combat infection. Thus, enlarged lymph nodes, or swollen glands, are a common result of immune system activity – and a symptom of many different diseases.

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 produces cells to fight infection, stores healthy blood cells and filters out damaged or worn-out blood cells, platelets and cellular waste.
  
  
• Thymus. An organ located in the front of the chest at the base of the neck that is most active before birth and in infancy. Its main function is the creation of certain types of white blood cells (T cells).
  
  
• Bone marrow. The soft, inner section of most human bones are composed of blood-forming cells, fat cells and tissues that support the growth of blood cells, including the white blood cells that respond to antigens within the body.
  
  
• Adenoid and tonsils. These masses of lymphoid tissue are located at the back of the throat. They store white blood cells that produce antibodies against foreign materials that are breathed in or swallowed. The tonsils and adenoid 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., in cases of recurring tonsillitis).