The Lymphatic System: Structure and Function

• The lymphatic system returns fluids that have leaked from blood vessels back into the bloodstream.
• The lymphatic system consists of a network of lymphatic vessels (lymphatics), lymph fluid, and lymph nodes (which cleanse the lymph).
• Lymphoid organs and tissues house phagocytic cells and lymphocytes, providing the structural basis of the immune system.
• The structures include the spleen, thymus, tonsils, lymph nodes, and other lymphoid tissues.

Distribution and Structure of Lymphatic Vessels

• Lymphatic vessels transport lymph in a one-way system, ensuring it flows only toward the heart.
• Lymph vessels (lymphatics) encompass lymphatic capillaries and larger lymphatic vessels.
• Lymphatic capillaries are blind-ended vessels that weave between tissue cells and blood capillaries.
• They are absent from bones, teeth, bone marrow, and the central nervous system, where CSF is used for drainage.
• Lymphatic capillaries are similar to blood capillaries, but more permeable, allowing them to take up larger molecules and particles, such as proteins, cell debris, pathogens, and cancer cells.
• They also act as a route for pathogens or cancer cells to travel throughout the body.
• Increased permeability results from overlapping endothelial cells, forming one-way minivalves.
• These minivalves are anchored by collagen filaments to the matrix; increases in ECF volume further open them.
• Decreases in ECF cause minivalves to close.
• Lacteals are specialized lymph capillaries present in the intestinal mucosa that absorb digested fat and deliver fatty lymph (chyle) to the blood.
• Lymph capillaries drain into larger collecting lymphatic vessels, which consist of collecting vessels, trunks, and ducts.
• The structures and tunics of Lymphatic vessels are similar to veins, but have thinner walls, more internal valves, and anastomose more frequently.
• Lymphatic trunks are formed by the union of the largest collecting vessels and drain large areas of the body.
• They are named for the regions they drain, including paired lumbar, bronchomediastinal, subclavian, jugular trunks, and a single intestinal trunk.
• Lymph is delivered from trunks into the right lymphatic duct, draining the right upper arm and right side of the head and thorax.
• Lymph is delivered the thoracic duct, draining the rest of the body.
• Each duct empties lymph into venous circulation at the junction of the internal jugular and subclavian veins on its side of the body.

Lymph Transport

• The lymph system is a low-pressure system, similar to the venous system.
• Lymph is propelled by the milking action of skeletal muscle, pressure changes in the thorax during breathing, valves to prevent backflow, pulsations of nearby arteries, and contractions of smooth muscle in the walls of lymphatics.
• Physical activity increases lymph flow, while immobilization keeps needed inflammatory material in the area for faster healing.

Homeostatic Imbalance

• Lymphedema is severe localized edema caused by anything that prevents normal lymph return to the blood, such as tumors blocking lymphatics or removal of lymphatics during cancer surgery.
• Lymphedema may improve if some lymphatic pathways remain and enlarge.

Lymphoid Cells

• Lymphoid cells encompass immune system cells found in lymphoid tissue and supporting cells that form lymphoid tissue structures.
• Lymphocytes, cells of the adaptive immune system, mature into T cells (T lymphocytes) and B cells (B lymphocytes).
• T and B cells protect against antigens, anything the body perceives as foreign, such as bacteria, toxins, viruses, mismatched RBCs, and cancer cells.
• T cells manage immune responses, and some attack and destroy infected cells.
• B cells produce plasma cells, which secrete antibodies that mark antigens for destruction by phagocytosis.
• Macrophages phagocytize foreign substances and help activate T cells.
• Dendritic cells capture antigens, deliver them to lymph nodes, and activate T cells.
• Reticular cells produce reticular fibers called stroma in lymphoid organs, providing a network-like support.

Lymphoid Tissue

• Main functions include housings and providing proliferation sites for lymphocytes.
• It offers surveillance vantage points for lymphocytes and macrophages as they filter through lymph.
• Lymphoid tissue is largely composed of reticular connective tissue, where macrophages live on reticular fibers, offering spaces for lymphocytes returning from patrolling the body.
• The two main types of lymphoid tissue are diffuse lymphoid tissue and lymphoid follicles (nodules).
• Diffuse lymphoid tissue has a loose arrangement of lymphoid cells and reticular fibers, found in virtually every body organ, with larger collections in the lamina propria of mucous membranes.
• Lymphoid follicles (nodules) are solid, spherical bodies consisting of tightly packed lymphoid cells and reticular fibers, containing germinal centers of proliferating B cells, and may form part of larger lymphoid organs (nodes), with isolated aggregations in Peyer's patches and the appendix.

Lymphoid Organs

• These are grouped into two functional categories: primary and secondary.
• Primary lymphoid organs, the red bone marrow and thymus, are where T and B cells mature.
• T and B cells originate in bone marrow, but only B cells mature there, while T cells mature in the thymus.
• Secondary lymphoid organs are where mature lymphocytes first encounter their antigen and become activated, including nodes, the spleen, MALT (mucosa-associated lymphoid tissue), and diffuse lymphoid tissues.

Lymph Nodes

• Lymph nodes are the principal secondary lymphoid organs, with hundreds found throughout the body.
• Most lymph nodes are embedded deep in connective tissue in clusters along lymphatic vessels.
• Some are nearer the body surface in the inguinal, axillary, and cervical regions, where collecting vessels converge into trunks.
• The two main functions are cleansing the lymph by acting as "filters," where macrophages remove and destroy microorganisms and debris, preventing unwanted substances from being delivered to the blood.
• They also activate the immune system by offering a place for lymphocytes to become activated and mount an attack against antigens.
• Lymph nodes vary in shape and size, but are mostly bean-shaped and small (less than 2.5 cm).
• They are surrounded by an external fibrous capsule.
• Capsule fibers extend inward as trabeculae, dividing the node into compartments.
• Two histologically distinct regions are the cortex and medulla.
• The superficial cortex contains follicles with germinal centers heavy with dividing B cells, while the deep cortex houses T cells in transit.
• Abundant dendritic cells are closely associated with both T and B cells, playing a role in activating both lymphocytes.
• Medullary cords extend inward from the cortex and contain B cells, T cells, and plasma cells.
• Lymph sinuses are found throughout the node, consisting of large lymphatic capillaries spanned by crisscrossing reticular fibers, where macrophages reside, checking for and phagocytizing foreign matter.
• Lymph enters the convex side of the node via afferent lymphatic vessels, travels through the subcapsular sinus and smaller sinuses throughout the cortex and medulla, then exits the concave side at the hilum via efferent lymphatic vessels.
• Fewer efferent vessels cause flow to stagnate, allowing lymphocytes and macrophages time to function as lymph travels through several nodes.

Spleen

• The spleen is a blood-rich organ about the size of a fist, located in the left side of the abdominal cavity.
• It's the largest lymphoid organ, served by the splenic artery and vein, which enter and exit at the hilum.
• Functions include lymphocyte proliferation, immune surveillance and response, and cleansing blood of aged blood cells and platelets as macrophages remove debris.
• The spleen stores breakdown products of RBCs (e.g., iron), blood platelets, and monocytes.
• It may also be a site of fetal erythrocyte production.
• The spleen is encased by a fibrous capsule and has trabeculae, histologically consisting of white pulp (where immune function occurs) and red pulp (where old blood cells and bloodborne pathogens are destroyed).
• White pulp contains mostly lymphocytes on reticular fibers, found around central arteries as islands of white in a sea of red pulp.
• Red pulp is rich in RBCs and macrophages that engulf them.

Mucosa-Associated Lymphoid Tissue (MALT)

• This lymphoid tissue in mucous membranes throughout the body protects from pathogens trying to enter the body.
• It is found in the mucosa of the respiratory tract, genitourinary organs, and digestive tract.
• The largest collections of MALT are found in tonsils, Peyer's patches, and the appendix.

Tonsils

• These are the simplest lymphoid organs, forming a ring of lymphatic tissue around the pharynx.
• Names are according to location, including palatine tonsils (at the posterior end of the oral cavity, largest, and most often infected), lingual tonsil (lumpy collection of follicles at the base of the tongue), pharyngeal tonsil (also called adenoids, in the posterior wall of the nasopharynx), and tubal tonsils (surrounding openings of the auditory tubes into the pharynx).
• The tonsils function to gather and remove pathogens in food or air, and contain follicles with germinal centers and scattered lymphocytes, are not fully encapsulated.
• Overlying epithelium invaginates, forming tonsillar crypts, where bacteria or particulate matter is trapped.

Peyer's Patches

• These are clusters of lymphoid follicles in the wall of the distal portion of the small intestine, structurally similar to tonsils.
• They aid in destroying bacteria and generating "memory" lymphocytes.
• They also prevent breaching of the intestinal wall.

Appendix

• The appendix is an offshoot of the first part of the large intestine, containing numerous lymphoid follicles.
• Its location aids in destroying bacteria and preventing breaches of the intestinal wall, also generating "memory" lymphocytes.

Thymus

• The thymus is a bilobed lymphoid organ found in the inferior neck.
• It extends into the mediastinum and partially overlies the heart.
• The thymus functions as a lymphoid organ where T cells mature.
• Most active and largest in size during childhood, it stops growing during adolescence and gradually atrophies, though still produces immunocompetent cells, but more slowly.