Lymphatic System Functions and Formation

Functions of the Lymphatic System

• Returns leaked fluids from the vascular system to ensure sufficient blood volume, estimated to be about 3 liters.
• Transports dietary fats from the intestine to the bloodstream via lacteals; fatty lymph is called chyle.
• Provides the body protection against cancer and infection.

Lymph Formation and Movement

• Interstitial fluid that enters lymphatic capillaries is called lymph.
• Lymph moves through the following structures: lymph capillaries -> lymph vessels -> lymph trunks -> lymph ducts.

Lymphatic Capillaries

• Lymphatic capillaries are present in all tissues except bone and teeth.
• These capillaries are very permeable due to the fact that the endothelial cells forming the wall of the lymphatic capillaries are not tightly joined.
• Adjacent endothelial cells overlap to form easily openable mini-valves.
• Collagen filaments anchor endothelial cells to surrounding structures; interstitial fluid volume increase opens mini-valves, allowing fluid into the lymphatic capillaries but not out.
• Lymphatic capillaries are blind-ended tubes where adjacent endothelial cells overlap, forming flaplike minivalves.

Lymphatic System and Immune Response

• Proteins in interstitial space easily enter lymphatic capillaries.
• Lymphatic capillaries enlarge during inflammation, facilitating the uptake of cell debris, pathogens, and cancer cells.
• These particles are then transported via lymph to lymph nodes, where immune system cells remove them.

Lymphatic Vessels

• Lymphatics in the skin travel with superficial veins.
• Lymphatics in the trunk and digestive viscera travel with deep arteries.
• There exist five lymphatic trunks: two subclavian, two bronchomediastinal, two lumbar, two jugular, and one intestinal trunk.
• The right lymphatic duct drains lymph from the right side of the head and the right arm.
• The left lymphatic duct (thoracic duct) receives lymph from the rest of the body.
• The thoracic duct originates at the cisterna chyli.
• Each duct empties lymph into the venous circulation at the junction of the internal jugular and subclavian veins on its respective side of the body (right side of the heart).
• Lymphatics function as low-pressure conduits.
• The milking action of active skeletal muscles, pressure changes in the thorax during breathing, and valves aid lymph movement.

Lymphoid Cells and Tissue Types

• Five types of lymphoid cells: Lymphocytes (B and T cells), dendritic cells (APCs), macrophages, and reticular cells (produce reticular fiber stroma).
• There are two types of Lymphoid tissue: Diffuse lymphoid tissue and Lymphoid follicles (nodules).
• Diffuse lymphoid tissue consists of a loose arrangement of lymphoid cells and reticular fibers and is found in almost every organ (except the thymus), with large collections in mucous membranes of the digestive tract.
• Lymphoid follicles (nodules) are solid, spherical bodies consisting of tightly packed lymphoid cells and reticular fibers.
• Follicles have light staining germinal centers with rapidly dividing B lymphocytes, which can be found as part of large organs such as lymph nodes and aggregates in the small intestine (Peyer's patches) and appendix.

Lymphoid Organs

• Primary lymphoid organs are where lymphocytes mature, examples being the red bone marrow (where B cells mature) and thymus (where T cells mature).
• Secondary lymphoid organs are where mature lymphocytes encounter antigens.
• Secondary lymphoid organs include the spleen, lymph nodes, tonsils (collection of mucosa associate lymphoid tissues), and Peyer's patches (aggregated lymphoid nodules).

Lymph Nodes

• Lymph nodes are the most important secondary lymphoid organ, clustering along lymphatic vessels throughout the body.
• There are hundreds of lymph nodes embedded in connective tissue, particularly in the inguinal, axillary, and cervical regions.
• Lymph nodes are surrounded by a capsule of dense connective tissue, and the capsule's extensions (trabeculae) divide the node into compartments.
• The capsule, trabeculae, reticular fibers, and fibroblasts form the supporting network (stroma) of the lymph node.
• The functioning part (parenchyma) consists of the cortex and medulla.
• The cortex contains a germinal center with proliferating B cells, while the outer edge of the follicle contains T cells and dendritic cells.
• Medullary cords in the medulla contain B cells, plasma cells, macrophages, and T cells.
• The two main functions of lymph nodes: cleansing the lymph and activating the immune system.
• Buboes are infected pus-filled lymph nodes.
• Lymphadenopathy is characterized by swollen, inflamed lymph nodes.
• Cancer-infiltrated lymph nodes are swollen, but not painful.

Spleen

• The spleen is the largest lymphoid organ.
• It serves as a site for B and T cell activation and proliferation, immune surveillance, and blood cleansing, removing debris, foreign matter, aged, and defective blood cells/platelets via macrophages.
• Spleen functions include recycling red blood cell breakdown products, storing blood platelets and monocytes, and serving as a site for erythrocyte production in the fetus.
• Spleen components: fibrous capsule, trabeculae, red pulp, and white pulp.
• White pulp is involved in immune function and contains lymphocytes.
• Red pulp contains erythrocytes and macrophages responsible for destroying worn-out red blood cells and pathogens.

MALT (Mucosa Associated Lymphoid Tissue)

• MALT includes the tonsils, Peyer's patches, and appendix.
• Tonsils form a ring of lymphoid tissue that gathers and removes pathogens entering the pharynx via food and inhaled air.
• The lymphoid tissue of tonsils contains germinal centers and scattered lymphocytes.
• Tonsils are not encapsulated; the epithelium forms tonsillar crypts, allowing bacteria trapped in the mucosal epithelium to access the lymphoid tissue.
• Peyer's patches are aggregated lymphoid nodules found in the distal part of the intestine.
• The appendix is an aggregation of lymphoid nodules.

Thymus

• The Thymus is the primary lymphoid organ and is prominent in newborns.
• The thymus increases in size during the first year and remains highly active, but stops growing during adolescence and atrophies.
• It continues to produce immunocompetent cells and is broken into lobules containing an outer cortex and inner medulla containing rapidly dividing lymphocytes (the bulk of thymic cells) and scattered macrophages.
• The medulla contains fewer lymphocytes and thymic corpuscles, and the thymic corpuscles are where regulatory T cells develop, which help prevent autoimmunity.

Differences between the Thymus and other Lymphoid Organs:

• Lacks follicles due to the absence of B cells.
• Does not directly fight antigens but functions strictly in T lymphocyte maturation. It contains a blood-thymus barrier that isolates immature T lymphocytes from antigens to prevent premature activation.
• Its stroma is made up of epithelial cells, not reticular fibers, providing the environment for T lymphocytes to become immunocompetent.

Lymphoid-related Pathologies

• Lymphadenopathy: enlarged, sometimes tender lymph nodes in response to infection, commonly referred to as swollen glands.
• Lymphoma: cancers of the lymphoid organs, particularly the lymph nodes, broadly categorized into Hodgkin's and non-Hodgkin's lymphoma.
• Hodgkin's lymphoma: characterized by painless, nontender enlargement of lymph nodes, commonly in the neck, chest, and axilla; has a high cure rate (90%-95%) if diagnosed early.
• Burkitt lymphoma: a form of B cell non-Hodgkin lymphoma.