Structure and Components of the Lymphatic System
- The lymphatic system consists of lymphatic vessels, lymphoid organs, lymphoid tissues, and circulating lymph.
- Primary lymphoid organs include the thymus and bone marrow, where lymphocytes are generated from immature progenitor cells.
- Secondary lymphoid organs, such as lymph nodes and the spleen, maintain mature naive lymphocytes and initiate an adaptive immune response.
- Lymphatic vessels carry lymph throughout the body, passing through lymph nodes that filter out unwanted materials.
- Lymph ducts, including the right lymphatic duct and thoracic duct, empty into subclavian veins to return to the blood circulation.

Primary and Secondary Lymphoid Organs
- The bone marrow is responsible for the creation of T cell precursors and the production and maturation of B cells.
- B cells immediately join the circulatory system, while T cells travel to the thymus for further development and maturation.
- The thymus increases in size in response to postnatal antigen stimulation and is most active during the neonatal and pre-adolescent periods.
- The thymus is where T lymphocytes mature and become immunocompetent.
- Lack of the thymus results in severe immunodeficiency and high susceptibility to infection.
- Secondary lymphoid organs, including lymph nodes and the spleen, maintain mature naive lymphocytes and initiate an adaptive immune response.
- Lymphocyte activation by antigens occurs in secondary lymphoid organs.
- Activation leads to clonal expansion and affinity maturation.
- Mature lymphocytes recirculate between the blood and secondary lymphoid organs until they encounter their specific antigen.
- The spleen produces immune cells, removes particulate matter and aged blood cells, and synthesizes antibodies in its white pulp.

Functions of the Spleen
- The spleen produces immune cells to fight antigens and removes particulate matter and aged blood cells.
- The spleen synthesizes antibodies in its white pulp and removes antibody-coated bacteria and blood cells.
- The red pulp of the spleen gets rid of aged red blood cells and pathogens through macrophages.
- The spleen is a center of activity for the mononuclear phagocyte system and plays a role in recycling erythrocyte components.
- The absence of the spleen can lead to a predisposition to certain infections.

Lymphatic Vessels and Other Lymphoid Tissue
- Lymphatic vessels conduct lymph between different parts of the body.
- Lymph capillaries absorb interstitial fluid from the tissues.
- Larger collecting vessels include the right lymphatic duct and the thoracic duct.
- Lymph vessels propel absorbed fluid forward into the collecting ducts.
- Intraluminal valves ensure unidirectional flow of lymph without reflux.
- Lymphoid tissue is concerned with immune functions in defending the body against infections and tumors.
- Lymphoid tissue consists of connective tissue with various types of leukocytes, mostly lymphocytes.
- Lymphoid follicles are regions densely packed with lymphocytes.
- Lymphoid tissue can be well-organised as lymph nodes or loosely organised as mucosa-associated lymphoid tissue (MALT).
- The central nervous system also has lymphatic vessels.

Function and Clinical Significance of the Lymphatic System
- The lymphatic system removes interstitial fluid from tissues.
- It absorbs and transports fatty acids and fats from the digestive system.
- It transports white blood cells to and from the lymph nodes.
- The lymph transports antigen-presenting cells to the lymph nodes for immune response stimulation.
- Long-chain fatty acids are absorbed to the lymph and transported via the thoracic duct.
- The lymphatic system is the primary site for cells relating to the adaptive immune system.
- Cells in the lymphatic system react to antigens presented or found by other cells.
- An immunological cascade begins when an antigen is recognised, involving the activation and recruitment of more cells.
- The study of lymphatic drainage is important in the diagnosis, prognosis, and treatment of cancer.
- The lymphatic system carries cancerous cells between different parts of the body in a process called metastasis.
- Enlarged lymph nodes can trap cancer cells and may become sites of secondary tumors.
- Lymphadenopathy refers to one or more enlarged lymph nodes, which can be reactive or caused by infections, connective tissue diseases, or cancers.
- Lymphedema is the swelling caused by the accumulation of lymph and can occur due to damage or malformations in the lymphatic system.

Lymphatic system (Wikipedia)

"Lymphatic drainage" redirects here. For the massage technique, see Manual lymphatic drainage.

The lymphatic system, or lymphoid system, is an organ system in vertebrates that is part of the immune system, and complementary to the circulatory system. It consists of a large network of lymphatic vessels, lymph nodes, lymphoid organs, lymphoid tissues and lymph. Lymph is a clear fluid carried by the lymphatic vessels back to the heart for re-circulation. The Latin word for lymph, lympha, refers to the deity of fresh water, "Lympha".

Lymphatic system
Human lymphatic system
Details
Identifiers
Latin	systema lymphoideum
MeSH	D008208
TA98	A13.0.00.000
TA2	5149
FMA	7162 74594, 7162
Anatomical terminology [edit on Wikidata]

Unlike the circulatory system that is a closed system, the lymphatic system is open. The human circulatory system processes an average of 20 litres of blood per day through capillary filtration, which removes plasma from the blood. Roughly 17 litres of the filtered blood is reabsorbed directly into the blood vessels, while the remaining three litres are left in the interstitial fluid. One of the main functions of the lymphatic system is to provide an accessory return route to the blood for the surplus three litres.

The other main function is that of immune defense. Lymph is very similar to blood plasma, in that it contains waste products and cellular debris, together with bacteria and proteins. The cells of the lymph are mostly lymphocytes. Associated lymphoid organs are composed of lymphoid tissue, and are the sites either of lymphocyte production or of lymphocyte activation. These include the lymph nodes (where the highest lymphocyte concentration is found), the spleen, the thymus, and the tonsils. Lymphocytes are initially generated in the bone marrow. The lymphoid organs also contain other types of cells such as stromal cells for support. Lymphoid tissue is also associated with mucosas such as mucosa-associated lymphoid tissue (MALT).

Fluid from circulating blood leaks into the tissues of the body by capillary action, carrying nutrients to the cells. The fluid bathes the tissues as interstitial fluid, collecting waste products, bacteria, and damaged cells, and then drains as lymph into the lymphatic capillaries and lymphatic vessels. These vessels carry the lymph throughout the body, passing through numerous lymph nodes which filter out unwanted materials such as bacteria and damaged cells. Lymph then passes into much larger lymph vessels known as lymph ducts. The right lymphatic duct drains the right side of the region and the much larger left lymphatic duct, known as the thoracic duct, drains the left side of the body. The ducts empty into the subclavian veins to return to the blood circulation. Lymph is moved through the system by muscle contractions. In some vertebrates, a lymph heart is present that pumps the lymph to the veins.

The lymphatic system was first described in the 17th century independently by Olaus Rudbeck and Thomas Bartholin.