The lymphatic system is a network of vessels, fluid, and lymphoid tissues that maintains fluid balance, transports certain absorbed nutrients, and supports the immune system. Its circulating fluid, lymph, forms when fluid surrounding body cells enters lymphatic vessels. The system includes lymph nodes, the spleen, thymus, bone marrow, and lymphoid tissue associated with mucosal surfaces. Although closely integrated with immunity, it also performs essential transport functions distinct from immune defense. (cancer.gov)
Fluid balance and lymph formation
Fluid continuously moves from blood capillaries into the spaces between tissue cells, where it becomes interstitial fluid. The lymphatic system collects fluid and dissolved substances from these spaces and returns them to venous blood. This drainage contributes to homeostasis by maintaining tissue fluid levels and blood volume. It also returns proteins that have escaped from blood vessels and would otherwise accumulate outside the circulation. (training.seer.cancer.gov)
Lymph resembles blood plasma, but its composition varies with the tissue being drained. It can contain proteins, immune cells, cellular debris, and absorbed fats. “Interstitial fluid” and “lymph” therefore describe fluid in different anatomical compartments rather than two entirely separate substances: tissue fluid becomes lymph when it enters a lymphatic vessel. (training.seer.cancer.gov)
Vessels and drainage pathways
The smallest lymphatic vessels are blind-ended capillaries within tissues. Their thin walls contain overlapping endothelial cells that allow interstitial fluid to enter. These capillaries connect to larger collecting vessels, which carry lymph through regional lymph nodes and eventually into larger trunks and ducts. Unlike the blood circulation, this network principally transports fluid away from tissues rather than forming a closed circuit through them. (training.seer.cancer.gov)
Lymph movement does not depend on a central pump equivalent to the heart. Skeletal muscle contractions, breathing-related pressure changes, and contractions of smooth muscle in collecting vessel walls help propel it. One-way valves limit backward flow. These mechanisms allow lymph to move from peripheral tissues toward the veins despite relatively low pressures within the vessels. (training.seer.cancer.gov)
The main drainage pathways are asymmetric. The right lymphatic duct receives lymph from the right upper limb and the right sides of the head and chest. The thoracic duct drains most of the remaining body. Lymph enters the venous circulation near the junctions of the internal jugular and subclavian veins in the neck. (training.seer.cancer.gov)
Lymphoid organs
Lymphoid organs are commonly divided into primary and secondary organs. The primary organs are bone marrow and the thymus, where lymphocytes develop and mature. In humans, B cells mature principally in bone marrow, whereas T cells arise from marrow-derived precursors and mature in the thymus. Secondary organs provide sites where mature lymphocytes encounter foreign material and participate in immune responses. (openstax.org)
Lymph nodes are small, usually bean-shaped organs positioned along lymphatic vessels. Prominent groups occur in the neck, armpits, and groin, with additional nodes deeper in the chest and abdomen. Lymph enters through afferent vessels, passes through internal channels called sinuses, and leaves through efferent vessels. Nodes contain immune cells that examine material arriving from their drainage territories. (training.seer.cancer.gov)
The spleen, located in the upper left abdomen, is the largest lymphoid organ. Unlike lymph nodes, it filters blood rather than incoming lymph. Its white pulp supports immune responses, while its red pulp removes aging or damaged red blood cells. Other lymphoid tissues include the tonsils and aggregates in the intestine, such as Peyer’s patches, which are positioned near potential routes of microbial entry. (training.seer.cancer.gov)
Immune surveillance
Lymphatic drainage brings foreign substances and immune cells from tissues into lymph nodes. Dendritic cells and macrophages capture material and participate in its processing. Within lymphoid organs, encounters between lymphocytes and antigens help initiate adaptive immune responses. Thus, lymph nodes are both filtering structures and organized sites of immune-cell interaction. (openstax.org)
Activated B cells can differentiate into plasma cells, which secrete antibodies. T cells perform other functions, including coordinating immune activity and destroying infected cells. Lymphocytes circulate through blood and lymph and reside in lymphoid organs; their movement connects local tissue events with immune responses elsewhere in the body. (openstax.org)
Absorption of dietary fats
Specialized lymphatic capillaries called lacteals lie within the intestinal villi. During digestion and absorption, they take up dietary fats and fat-soluble vitamins, transporting these substances toward the venous circulation. Fat-rich intestinal lymph has a milky appearance and is called chyle. This pathway differs from the direct entry of many other absorbed nutrients into intestinal blood capillaries. (training.seer.cancer.gov)
Clinical significance
Impaired lymphatic drainage can cause lymphedema, an accumulation of fluid in tissues that produces swelling. Cancer can obstruct lymph flow, while lymph-node removal and radiation treatment can damage drainage pathways. Cancer-related lymphedema can involve an arm, leg, or other region served by the affected vessels and nodes. (cancer.gov)
Lymphoma comprises cancers involving lymphocytes and may affect lymph nodes, other lymphoid organs, or sites outside them. Other cancers can spread through lymphatic vessels to regional nodes. A sentinel lymph-node biopsy examines the first node or nodes likely to receive drainage from a tumor; its findings can contribute to determining whether cancer has spread and to cancer staging. (training.seer.cancer.gov)