The lymphatic system is the body's network of organs, ducts, and tissues that filter harmful substances out of the fluid that surrounds body tissues. Lymphatic organs include the bone marrow, thymus, spleen, appendix, tonsils, adenoids, lymph nodes, and Peyer's patches (in the small intestine). The thymus and bone marrow are called primary lymphatic organs, because lymphocytes are produced in them. The other lymphatic organs are called secondary lymphatic organs.

Lymphocytes are a type of white blood cell (WBC), which is highly concentrated in lymphatic fluid. This clear fluid, also called lymph, travels through the lymphatic vessels, which connect the lymphatic organs. The terminal lymphatic vessels feed into the thoracic duct that returns body fluids to the heart prior to blood reoxygenation. The reincorporated fluid originates in the bloodstream, bathes organs and tissues, and is returned to the bloodstream after passing through lymphatic filters that function as...