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.