Unlike blood, the body’s lymph flows through an open-ended network
Alongside the blood vessels runs a second network of vessels, nodes and organs that carries lymph, the fluid that seeps out of blood into the tissues. Described independently by Olaus Rudbeck and Thomas Bartholin in the 17th century, it drains that fluid back and hosts much of the immune system.
The name comes from the Latin lympha, linked to a deity of fresh water. Blood runs in a closed circuit, but the lymphatic system is open-ended. Fluid that leaks from capillaries into the tissues delivers nutrients and picks up waste, bacteria and damaged cells, then drains into lymphatic vessels as lymph, a liquid much like blood plasma whose cells are mostly lymphocytes.
On its way back, lymph passes through lymph nodes that strain out bacteria and debris. The human body has nearly 800 of these bean-shaped nodes, about 300 in the head and neck, and their lymphocyte-packed follicles swell when they meet a foreign antigen. Lymph then collects in two ducts: the right lymphatic duct serves the upper right of the body, and the far larger thoracic duct serves the rest, both emptying into the subclavian veins.
Lymphocytes are born in the primary lymphoid organs, the bone marrow and thymus, plus the fetal liver and yolk sac; birds add the bursa of Fabricius. B cells leave the red marrow ready to hunt pathogens, while T cell precursors travel to the thymus to mature and learn to tolerate the body’s own tissue. The thymus, between the lower neck and upper chest, is busiest in infancy and childhood and shrinks from adolescence, largely replaced by fat.
Secondary organs such as the nodes, spleen, tonsils and Peyer’s patches are where mature lymphocytes meet antigens and launch adaptive responses. The spleen makes antibodies in its white pulp and clears old red cells in its red pulp. Until the fifth month of pregnancy it also makes red blood cells, and in sudden blood loss it contracts to release its store.
Source: Lymphatic system
