2.4.5. The lymphatic system

The lymphatic system is part of the circulatory system, comprising a network of conduits called lymphatic vessels that carry a clear fluid called lymph (from Latinlympha “water goddess”) directionally towards the heart.

Roughly 17 liters of the filtered plasma actually get reabsorbed directly into the blood vessels, while the remaining 3 liters are left behind in the interstitial fluid. The primary function of the lymph system is to provide an accessory route for these excess 3 liters per day to get returned to the blood. Lymph is essentially recycled blood plasma.

Lymphatic organs play an important part in the immune system, having a considerable overlap with the lymphoid system. Lymphoid tissue is found in many organs, particularly the lymph nodes, and in the lymphoid follicles associated with the digestive system such as the tonsils. Lymphoid tissues contain lymphocytes, but they also contain other types of cells for support. The system also includes all the structures dedicated to the circulation and production of lymphocytes (the primary cellular component of lymph), which includes the spleen, thymus, bone marrow, and the lymphoid tissue associated with the digestive system.

Functions

The lymphatic system has multiple interrelated functions:

• It is responsible for the removal of interstitial fluid from tissues
• It absorbs and transports fatty acids and fats as chyle from the digestive system
• It transports white blood cells to and from the lymph nodes into the bones
• The lymph transports antigen-presenting cells (APCs), such as dendritic cells, to the lymph nodes where an immune response is stimulated.

Organization

The lymphatic system can be broadly divided into the conducting system and the lymphoid tissue.

• The conducting system carries the lymph and consists of tubular vessels that include the lymph capillaries, the lymph vessels, and the right and left thoracic ducts.
• The lymphoid tissue is primarily involved in immune responses and consists of lymphocytes and other white blood cells enmeshed in connective tissue through which the lymph passes. Regions of the lymphoid tissue that are densely packed with lymphocytes are known as lymphoid follicles. Lymphoid tissue can either be structurally well organized as lymph nodes or may consist of loosely organized lymphoid follicles known as the mucosa-associated lymphoid tissue (MALT).

Lymphoid tissue associated with the lymphatic system is concerned with immune functions in defending the body against theinfections and spread of tumors. It consists of connective tissue with various types of white blood cells enmeshed in it, most numerous being the lymphocytes.

The lymphoid tissue may be primary, secondary, or tertiary depending upon the stage of lymphocyte development and maturation it is involved in. (The tertiary lymphoid tissue typically contains far fewer lymphocytes, and assumes an immune role only when challenged with antigens that result in inflammation. It achieves this by importing the lymphocytes from blood and lymph.)

Primary lymphoid organs

The central or primary lymphoid organs generate lymphocytes from immature progenitor cells.

The thymus and the bone marrow constitute the primary lymphoid tissues involved in the production and early selection of lymphocytes.

Secondary lymphoid organs

Secondary or peripheral lymphoid organs maintain mature naive lymphocytes and initiate an adaptive immune response. The peripheral lymphoid organs are the sites of lymphocyte activation by antigen. Activation leads to clonal expansion and affinity maturation. Mature lymphocytes recirculate between the blood and the peripheral lymphoid organs until they encounter their specific antigen.

Secondary lymphoid tissue provides the environment for the foreign or altered native molecules (antigens) to interact with the lymphocytes. It is exemplified by the lymph nodes, and the lymphoid follicles in tonsils, Peyer’s patches, spleen, adenoids, skin, etc. that are associated with the mucosa-associated lymphoid tissue (MALT).

Lymph nodes

A lymph node is an organized collection of lymphoid tissue, through which the lymph passes on its way to returning to the blood. Lymph nodes are located at intervals along the lymphatic system. Several afferent lymph vessels bring in lymph, which percolates through the substance of the lymph node, and is drained out by an efferent lymph vessel.

The substance of a lymph node consists of lymphoid follicles in the outer portion called the “cortex,” which contains the lymphoid follicles, and an inner portion called “medulla,” which is surrounded by the cortex on all sides except for a portion known as the “hilum.” The hilum presents as a depression on the surface of the lymph node, which makes the otherwise spherical or ovoid lymph node bean-shaped. The efferent lymph vessel directly emerges from the lymph node here. The arteries and veins supplying the lymph node with blood enter and exit through the hilum.

Lymph follicles are a dense collection of lymphocytes, the number, size and configuration of which change in accordance with the functional state of the lymph node. For example, the follicles expand significantly upon encountering a foreign antigen. The selection of B cells occurs in the germinal center of the lymph nodes.

Lymph nodes are particularly numerous in the mediastinum in the chest, neck, pelvis, axilla (armpit), inguinal (groin) region, and in association with the blood vessels of the intestines.

Lymphatics

Tubular vessels transport back lymph to the blood ultimately replacing the volume lost from the blood during the formation of the interstitial fluid. These channels are the lymphatic channels or simply called lymphatics.