English: (A) The lymphatic system includes the primary and secondary lymphoid organs and a series of lymphatic vessels, providing a one-way drainage route from all tissues back ultimately to the blood circulation via the great veins in the neck. In the primary lymphoid organs (bone marrow and thymus) immune cell production and maturation takes place, whereas secondary lymphoid organs (lymph nodes, spleen, and mucosa associated lymphoid organs such as Peyer’s patch, tonsils and adenoids) are the sites for lymphocyte activation. The initial dermal lymphatic capillaries absorb interstitial material and fluid to make lymph which drains into lymphatic collectors. Lymph is pumped from the gut and lower half of the body to the cisterna chyli, a sac-like structure situated below the diaphragm, and then on to the thoracic duct. (B) The thoracic duct is responsible for the lymph drainage coming from most of the body with the exception of the right sides of the head and neck, the right side of the thorax and the right upper limb that drain primarily into the right lymphatic duct. Both ducts drain into the great veins of the neck. (C) The intricate dermal lymphatic capillary network drains downstream into the lymphatic collector vessels on route to the lymph nodes. (D) Oak leaf-shaped initial lymphatic capillary cells are connected via discontinuous junctions or buttons allowing the fluid to enter the system passively; the lymphatic collector endothelial cells, on the other hand, present with continuous junctions or zippers. Collectors differ from initial lymphatics by possessing intraluminal valves, smooth muscle cells (SMC) and a continuous basement membrane. Contractions of the lymphangions, the vessel segment between two valves, generate the pressure gradient ensuring the unidirectional flow of lymph. Image in (A) modified from OpenStax College under a CC BY 3.0 license. (C) modified from OpenLearn Create under a CC BY-NC-SA 4.0 license.