The lymphatic system is responsible for a variety of functions, including filtration and certain immunological processes. Ranging over the entire body, lymph vessels are as extensive as blood vessels and are in fact considered part of the circulatory system together with the cardiovascular structures, although sometimes “circulatory” is used to designate only the latter. When oxygen-rich blood passes into the (small "hair-like" vessels) for deployment in a given organ or body tissue, some of the fluid portion of the blood escapes from the vessels and washes over the body tissue; most of this fluid is reabsorbed, but the rest drains into the open lymph vessels. This transparent, yellowish fluid is known as “lymph” (for the etymology of this word, see p. 319). The lymphatic system does not have a “heart” structure to pump it, but rather moves through the vessels by contractions brought on by movement of the musculoskeletal system, and valves in the vessels block the lymph from flowing backwards. All lymph below the (Grk. "barrier") and from the left side of the upper body drains into the (Grk. "having to do with the chest") duct, while lymph from the right side of the upper body drains into the right lymphatic duct; both ducts eventually return the lymph to the blood vessels through the subclavian veins, which are located "underneath the ."

Lymph vessels provide a special service in the small intestine. The intestinal (Lat. "hairs") absorb digested fats, creating a milky fluid that is absorbed by the lacteals (a special kind of lymph vessel), which then transport this fluid, known as “,” to the thoracic duct, from which it enters the bloodstream, just as lymph from other parts of the body does.

At various points along the lymph vessels are lymph (Lat. “knot”), accumulations of lymphoid tissue that help in filtration and immunological processes. These are surrounded by a capsule, which has several internal extensions known as “” (pl. of the Lat. dim. trabs, “beam, timber”). Lymph is "carried to" the node by several vessels and then follows the channels made by the trabeculae and a fibrous network, where lymphocytes (a type of white blood cell) can identify and destroy foreign particles and cancer cells, a process known as “” (lit. “cell eating”). The lymph is then "carried out" through the end of the node via an vessel. Because of their design, lymph nodes are clinically useful for the detection of cancer, since cancer cells are often lodged in the nodes themselves (the detection of breast cancer relies particularly on the axillary nodes found in the shoulder area, named after the Latin axilla, “”).

Although the 1st-century AD doctor Rufus of Ephesus claimed that the spleen had no function whatsoever, and although the well-respected St. Isidore (c. AD 560-636) thought that it got its name from the fact that it just took up empty space (as if it were an abbreviation of the -declension Latin noun supplementum), the spleen is critical to lymphatic functions. It is structured much like a lymph node, except that it filters blood instead of lymph; this is done through the “red pulp,” while the “white pulp” contains lymphocytes that help to fight off infection.

Several other structures also contribute to the lymphatic system. The tonsils, so called in antiquity (Lat. tonsillae), are two almond-shaped masses at the back of the throat that contribute to the immune system (note that “tonsil” usually designates a “palatine tonsil,” though the term may be used more broadly to refer to other lymphoid tissues). The “-shaped” adenoids, also known as the “pharyngeal tonsils,” serve the same function. The thymus was so called as early as Galen and literally means “warty excrescence” in Greek. The exact function of the thymus is not fully understood, although it is the locus of lymphocyte production and thereby aids the body’s fight against infection; it is normally more active in children than adults.