Lymphatic System

Abstract

The lymphatic system consists of a network of vessels lined with endothelial cells and interconnected lymph nodes, distributed throughout most of the body. It functions as a tissue drainage system, controlling the volume of blood and extracellular fluid and preventing tissue oedema. The lymphatic system collects tissue fluids and cells from most organs and tissues, transports it as lymph via a combination of intrinsic and extrinsic lymphatic pumps, ultimately returning these cells to blood. Dendritic cells patrol the tissues detecting danger signals and bringing foreign antigens back to the lymph nodes via afferent lymphatics where adaptive immune responses are induced. Thus, the lymphatic system integrates the immune system and widely dispersed organs and tissues, allowing for their protection from infections. It also plays a role in the turnover of macromolecules, which make up the extracellular matrix, fat absorption from the gut and immune function.

Key Concepts

  • The lymphatic system comprises a network of vessels lined by endothelial cells.
  • Lymphatic vessels drain nearly all tissues and organs in the body and interconnect lymph nodes.
  • Afferent lymphatics transport lymph from the tissues to lymph nodes, whereas efferent lymphatics transport lymph away from lymph nodes.
  • Lymphatics are essential for returning proteins, fluids and cells to blood, absorbing fat from the gut and are involved in the homeostasis of interstitial macromolecules.
  • The lymphatic system is a key regulator of extracellular fluid volume and is involved in preventing oedema.
  • The lymphatics play a central role in integrating the immune system, carrying lymphocytes, professional antigen‐presenting cells and antigens from the tissues to the lymph nodes.
  • Lymphatic endothelial cells function as professional antigen‐presenting cells, communicate directly with lymphocytes and antigen‐presenting cells to control cell traffic.

Keywords: lymph; tissue fluid homeostasis; lymphocytes; tissue oedema; lymph nodes; endothelial cells

Figure 1. The lymphatic system drains most tissues and organs in the body, and interconnects the lymph nodes. Reprinted by permission from Macmillan Publishers Ltd: Brown P (2005) Lymphatic system unlocking the drains. Nature 436: 456–458.
Figure 2. Scheme showing the principal pathways of lymph flow in mammals. PLN, peripheral lymph node and MLN, mesenteric lymph node.
Figure 3. Afferent lymphatics consist of initial lymphatics which drain into collecting lymphatics. These collecting lymphatics are characterized by a smooth muscle wall and bicuspid valves. In skin, pre‐collector ducts, which lack the muscle layer, lie between the lymphatic capillaries and collecting ducts. Black arrows indicate direction of lymph flow.
Figure 4. Scheme of a peripheral lymph node showing the pathways by which cells enter and leave the node. Afferent lymph delivers approximately 10% of the lymphocytes entering a lymph node with the remaining 90% entering from blood via specialized post‐capillary venules, or high endothelial venules (HEV), within the node. In most mammals, lymphocytes leave the lymph node via efferent lymphatics. Arterial blood is shown in red and venous blood is shown in blue. Arrows indicate the direction of cell traffic.
Figure 5. Scheme showing pathways of lymphocyte recirculation through blood and lymph. The journey from blood through lymph nodes and back into lymph takes approximately 1 day. Tissue‐specific streams direct gut‐homing cells to the intestines and skin‐homing cells to skin. Lymphocytes are added to the pool from their sites of production in thymus and bone marrow. PLN, peripheral lymph node and MLN, mesenteric lymph node.
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Washington, Elizabeth A, Mahakapuge, Thilini AN, Scheerlinck, Jean‐Pierre Y, and Kimpton, Wayne G(May 2015) Lymphatic System. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000523.pub3]