Chapter 20: The Lymphatic System and Lymphoid Organs and Tissues
Overview of the Lymphatic System
Definition and Function: The lymphatic system is a specialized set of organs and tissues that returns fluids leaked from blood vessels back into the blood circulation. It provides the structural basis of the immune system by housing phagocytic cells and lymphocytes.
The Three Essential Parts: * Lymphatic Vessels (Lymphatics): An elaborate network of drainage vessels. * Lymph: The fluid contained within these vessels. * Lymph Nodes: Specialized organs that cleanse and filter the lymph as it passes through the system.
Lymphoid Organs and Tissues: These structures include the spleen, thymus, tonsils, lymph nodes, and several other lymphoid tissues (e.g., Peyer’s patches and the appendix).
Lymphatic Vessels (Lymphatics)
Drainage and Directionality: The lymphatic system returns interstitial fluid and leaked plasma proteins back to the blood. It is a one-way system that ensures lymph flows only toward the heart.
Lymphatic Capillaries: * Structure: These are blind-ended vessels that weave between tissue cells and blood capillaries in loose connective tissues. * Location Constraints: They are conspicuously absent from bones, teeth, and bone marrow. * Permeability Characteristics: They are similar to blood capillaries but are much more permeable, allowing them to take up larger molecules and particles such as proteins, cell debris, pathogens, and cancer cells. This permeability can unfortunately act as a route for pathogens or cancer cells to travel throughout the body. * Structural Specializations for Permeability: * Minivalves: Endothelial cells overlap loosely to form one-way minivalves. * Collagen Filaments: These anchor the minivalves to the matrix. An increase in extracellular fluid (ECF) volume opens the minivalves further, while a decrease in ECF causes them to close.
Lacteals: Specialized lymphatic capillaries located in the intestinal mucosa. They absorb digested fat and deliver a fatty lymph called chyle to the blood.
Larger Lymphatic Vessels: * Hierarchy: Lymph capillaries drain into increasingly larger vessels known as collecting lymphatic vessels, which then lead to trunks and finally ducts. * Anatomy vs. Veins: They have tunics similar to veins but possess thinner walls, more internal valves, and anastomose (connect) more frequently.
Lymphatic Ducts: * Right Lymphatic Duct: Drains lymph from the right upper arm and the right side of the head and thorax. * Thoracic Duct: Drains lymph from the rest of the body. In approximately half of the population, it begins as an enlarged sac called the cisterna chyli. * Venous Entry: Both ducts empty lymph into the venous circulation at the junction of the internal jugular and subclavian veins on their respective sides of the body.
Lymph Transport and Associated Clinical Conditions
Transport Mechanisms: The lymphatic system is a low-pressure system, similar to the venous system. It lacks a pump and relies on: * Milking action of skeletal muscles. * Pressure changes in the thorax during breathing. * Valves to prevent backflow. * Pulsations of nearby arteries. * Contractions of smooth muscle in the walls of the lymphatics.
Activity Levels: Physical activity increases the flow of lymph. Conversely, immobilization of an injured area keeps inflammatory material in the area to facilitate faster healing.
Clinical - Homeostatic Imbalance: * Lymphangitis: A condition where lymphatic vessels appear as painful red lines under the skin. This is caused by inflammation of larger lymphatic vessels that contain vaso vasora (vessels of vessels), which become congested with blood. * Lymphedema: Severe localized edema caused by anything that prevents the normal return of lymph to the blood (e.g., tumors blocking vessels or the surgical removal of lymphatics during cancer treatment).
Lymphoid Cells and Tissues
Lymphoid Cells Categories: * Immune System Cells: Found in lymphoid tissue. * Lymphocytes: The main cells of the adaptive immune system. They mature into T cells ( lymphocytes), which manage the immune response and destroy infected cells, or B cells ( lymphocytes), which produce plasma cells that secrete antibodies. Antibodies mark antigens (anything foreign, such as bacteria, toxins, viruses) for destruction. * Macrophages: Phagocytize foreign substances and help activate cells. * Dendritic Cells: Capture antigens and deliver them to lymph nodes; also aid in cell activation. * Supporting Cells: * Reticular Cells: Produce reticular fibers called stroma. The stroma is a network-like scaffolding that supports immune cells within lymphoid organs.
Lymphoid Tissue Characteristics: * Function: Houses and provides proliferation sites for lymphocytes; offers surveillance points for lymphocytes and macrophages. * Composition: Largely composed of reticular connective tissue (a type of loose connective tissue). Macrophages live on the fibers, while lymphocytes occupy the spaces between them.
Types of Lymphoid Tissue: * Diffuse Lymphoid Tissue: A loose arrangement of lymphoid cells and fibers found in virtually every body organ, specifically in the lamina propria of mucous membranes. * Lymphoid Follicles (Nodules): Solid, spherical bodies of tightly packed cells and fibers. They contain germinal centers where cells proliferate and may exist as isolated aggregations (e.g., Peyer’s patches and the appendix).
Functional Categories of Organs: * Primary Lymphoid Organs: Where and cells mature. Both originate in the red bone marrow; cells mature in the marrow, while cells mature in the thymus. * Secondary Lymphoid Organs: Where mature lymphocytes first encounter antigens and become activated. These include the lymph nodes, spleen, MALT (mucosa-associated lymphoid tissue), and diffuse lymphoid tissues.
The Lymph Nodes
Distribution: Hundreds of nodes are found along lymphatic vessels. Major clusters are located in the inguinal, axillary, and cervical regions where collecting vessels converge into trunks.
Primary Functions: 1. Cleansing the Lymph: Act as filters where macrophages destroy microorganisms and debris. 2. Immune System Activation: Provide sites for lymphocytes to monitor for antigens and mount attacks.
Internal Structure: * Size: Generally bean-shaped and small ( or approximately ). * Capsule and Trabeculae: Surrounded by an external fibrous capsule with strands called trabeculae extending inward to divide the node into compartments. * Cortex: The superficial area contains follicles with germinal centers heavy with dividing cells. The deep cortex houses cells in transit. Dendritic cells are also abundant here. * Medulla: Contains medullary cords (inward extensions of the cortex containing , , and plasma cells) and lymph sinuses (large capillaries spanned by reticular fibers).
Circulation Path: Lymph enters the convex side via afferent lymphatic vessels, flows through the subcapsular sinus into smaller sinuses in the cortex and medulla, and exits the concave side at the hilum via efferent lymphatic vessels. Because there are fewer efferent than afferent vessels, flow stagnates slightly, allowing time for immune cells to function.
Clinical Considerations: * Buboes: Inflamed, swollen, and tender lymph nodes (often called swollen glands) that occur when nodes are overwhelmed. This was a hallmark of the bubonic plague. * Cancer: Lymph nodes can be secondary cancer sites. Cancer-infiltrated nodes are usually swollen but not painful, unlike infected nodes.
The Spleen
Overview: The largest lymphoid organ, about the size of a fist, located in the left side of the abdominal cavity below the stomach. It is served by the splenic artery and vein, which enter/exit at the hilum.
Primary Functions: * Site for lymphocyte proliferation and immune surveillance. * Cleanses the blood of aged blood cells and platelets. * Stores breakdown products of red blood cells (e.g., iron) for reuse. * Stores blood platelets and monocytes. * May serve as a site for fetal erythrocyte production.
Histology Components: * White Pulp: Where immune functions occur; consists mostly of lymphocytes on reticular fibers clustered around central arteries. * Red Pulp: Where old blood cells and pathogens are destroyed; rich in red blood cells and macrophages. It contains splenic cords (reticular tissue) and splenic sinusoids (venous sinuses).
Clinical - Homeostatic Imbalance: The spleen has a thin capsule and can rupture from trauma. While splenectomies (surgical removal) were once common, it is now known the spleen can often repair itself. If removed, the liver and bone marrow take over its functions. In children under of age, the spleen can regenerate if a small part is left.
Mucosa-Associated Lymphoid Tissue (MALT)
Definition: Lymphoid tissues located in mucous membranes throughout the body (respiratory, digestive, and genitourinary tracts) to protect against entering pathogens.
Tonsils: The simplest lymphoid organs, forming a ring around the pharynx. They are not fully encapsulated and feature tonsillar crypts that trap and destroy bacteria to build immune memory. * Palatine Tonsils: Located at the posterior end of the oral cavity; largest and most frequently infected. * Lingual Tonsil: Located at the base of the tongue. * Pharyngeal Tonsil (Adenoids): Located in the posterior wall of the nasopharynx. * Tubal Tonsils: Surround the openings of the auditory tubes.
Peyer’s Patches: Clusters of lymphoid follicles (aggregated lymphoid nodules) in the wall of the distal small intestine. They destroy bacteria and generate memory lymphocytes.
Appendix: An offshoot of the first part of the large intestine containing high concentrations of lymphoid follicles.
The Thymus
Overview: A bilobed organ in the inferior neck, extending into the mediastinum. It is most active and largest during childhood, gradually atrophying after adolescence.
Function: Strictly functions in lymphocyte maturation; it does not directly fight antigens.
Internal Anatomy: * Cortex: Contains rapidly dividing lymphocytes and scattered macrophages. * Medulla: Contains fewer lymphocytes and thymic corpuscles, which are involved in the development of regulatory cells (important for preventing autoimmunity).
Distinct Differences from Other Lymphoid Organs: * It lacks cells and thus has no follicles. * It contains a blood thymus barrier to keep immature cells isolated from antigens. * Its stroma consists of epithelial cells rather than reticular fibers, providing the specific environment for cell immunocompetence.