Ch.20 The Lymphatic System and Lymphoid Organs: Structure and Function

The Lymphatic System

Overview of the Lymphatic System

The lymphatic system is a vital component of the circulatory and immune systems, responsible for returning fluids leaked from blood vessels back to the bloodstream and providing the structural basis for immune defense. It consists of a network of vessels, lymph fluid, and lymphoid organs and tissues.

• Lymphatic vessels (lymphatics): An elaborate network of drainage vessels that collect and transport lymph.

• Lymph: The fluid found within lymphatic vessels, derived from interstitial fluid.

• Lymph nodes: Small organs that cleanse lymph and house immune cells.

Lymphoid organs and tissues provide the structural basis of the immune system by housing phagocytic cells and lymphocytes. Major structures include the spleen, thymus, tonsils, lymph nodes, and other lymphoid tissues.

Functions of the Lymphatic System

• Returns interstitial fluid and leaked plasma proteins back to the blood.

• Circulates approximately 3 liters of interstitial fluid per day.

• Once interstitial fluid enters lymphatic vessels, it is called lymph.

Structure and Distribution of Lymphatic Vessels

Lymphatic Vessel Organization

Lymphatic vessels form a one-way system, ensuring lymph flows only toward the heart. They include lymphatic capillaries and larger lymphatic vessels.

• Lymphatic capillaries: Blind-ended vessels that weave between tissue cells and blood capillaries.

• Absent from bones, teeth, and bone marrow.

• Recently discovered in the central nervous system (CNS), particularly in the meninges, where they help drain interstitial fluid and cerebrospinal fluid (CSF).

• More permeable than blood capillaries, allowing uptake of larger molecules and particles such as proteins, cell debris, pathogens, and cancer cells.

• Can act as routes for pathogens or cancer cells to travel throughout the body.

Special Features of Lymphatic Capillaries

Lymphatic capillaries possess unique structural adaptations that enhance their permeability and function.

• Endothelial cells overlap loosely to form one-way minivalves, allowing fluid entry but preventing backflow.

• Minivalves are anchored by collagen filaments to the surrounding matrix; increased extracellular fluid (ECF) volume opens minivalves further, while decreased ECF closes them.

• Lacteals: Specialized lymphatic capillaries located in the intestinal mucosa that absorb digested fats and deliver chyle (fat-rich lymph) to the bloodstream.

Pathway of Lymph Flow

Lymphatic capillaries drain into increasingly larger vessels:

• Collecting lymphatic vessels: These have thinner walls and more internal valves than veins, and they anastomose (connect) more frequently.

• Superficial collecting vessels travel with veins; deep vessels travel with arteries.

• Lymphatic trunks: Formed by the union of the largest collecting vessels, each trunk drains a specific region of the body (e.g., lumbar, bronchomediastinal, subclavian, jugular, and intestinal trunks).

• Lymphatic ducts: The right lymphatic duct drains the right upper arm and right side of the head and thorax; the thoracic duct drains the rest of the body, beginning as the cisterna chyli in many individuals.

Lymph Transport Mechanisms

Movement of Lymph

Lymph transport is a low-pressure system similar to veins, relying on several mechanisms:

• 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 lymphatic vessels

Physical activity increases lymph flow, while immobilization slows it, allowing inflammatory materials to remain in the area for faster healing.

Lymphoid Cells, Tissues, and Organs

Lymphoid Cells

Lymphoid cells include immune system cells found in lymphoid tissue and supporting cells that form the structural framework.

• Lymphocytes: Key cells of the adaptive immune system, maturing into T cells (T lymphocytes) and B cells (B lymphocytes).

• T cells manage immune responses and can directly attack infected cells.

• B cells produce plasma cells, which secrete antibodies to mark antigens for destruction.

• Macrophages: Phagocytize foreign substances and help activate T cells.

• Dendritic cells: Capture antigens and deliver them to lymph nodes, also aiding in T cell activation.

• Reticular cells: Produce reticular fibers (stroma) that act as scaffolding for immune cells in lymphoid organs.

Lymphoid Tissue

Lymphoid tissue houses and provides proliferation sites for lymphocytes and offers surveillance vantage points for lymphocytes and macrophages as they filter lymph.

• Composed mainly of reticular connective tissue.

• Macrophages live on reticular fibers; lymphocytes occupy spaces between fibers when patrolling the body.

Lymphoid Organs

Lymphoid organs are classified into primary and secondary categories:

• Primary lymphoid organs: Sites where T and B cells mature (red bone marrow and thymus).

• Secondary lymphoid organs: Sites where mature lymphocytes first encounter antigens and become activated (lymph nodes, spleen, MALT, and diffuse lymphoid tissues).

Diffuse lymphoid tissue is found in virtually every body organ, with larger collections in the lamina propria of mucous membranes. Lymphoid follicles (nodules) are solid, spherical bodies containing germinal centers of proliferating B cells, found in lymph nodes and as isolated aggregations (e.g., Peyer's patches, appendix).

Lymph Nodes

Structure and Function of Lymph Nodes

Lymph nodes are the principal secondary lymphoid organs, distributed throughout the body, often embedded deep in connective tissue and clustered along lymphatic vessels.

• Main functions:

  • Cleansing the lymph: Macrophages remove and destroy microorganisms and debris.

  • Immune system activation: Provide a site for lymphocytes to become activated and mount an immune response.

• Bean-shaped, less than 2.5 cm in size, surrounded by a fibrous capsule.

• Internal structure includes cortex (with follicles and germinal centers) and medulla (with medullary cords and sinuses).

• Lymph enters via afferent vessels, passes through sinuses, and exits via efferent vessels at the hilum.

• Fewer efferent vessels cause lymph flow to stagnate, allowing time for immune cells to function.

Clinical Homeostatic Imbalances

• Lymphangitis: Inflammation of lymphatic vessels, visible as painful red lines.

• Lymphedema: Severe localized swelling due to blocked or removed lymphatics.

• Buboes: Swollen, tender lymph nodes due to overwhelming infection; characteristic of bubonic plague.

• Cancer-infiltrated lymph nodes are swollen but usually not painful.

The Spleen

Structure and Functions of the Spleen

The spleen is the largest lymphoid organ, located in the left side of the abdominal cavity, just below the stomach. It is supplied by the splenic artery and vein.

• Site of lymphocyte proliferation and immune response.

• Cleanses blood of aged blood cells and platelets; macrophages remove debris.

• Stores breakdown products of red blood cells (e.g., iron) for reuse.

• Stores blood platelets and monocytes for release when needed.

• May be the site of fetal erythrocyte production.

• Encased by a fibrous capsule and divided into white pulp and red pulp.

White Pulp vs. Red Pulp

• White pulp: Site of immune function, containing lymphocytes on reticular fibers, clustered around central arteries.

• Red pulp: Site where old blood cells and pathogens are destroyed; rich in red blood cells and macrophages, composed of splenic cords and sinusoids.

Clinical Homeostatic Imbalances

• Splenic rupture: Can cause severe internal bleeding; splenectomy (surgical removal) may be required, but the spleen can sometimes repair itself.

• Liver and bone marrow can take over most splenic functions if removed; in children, the spleen can regenerate if a small part remains.

Mucosa-Associated Lymphoid Tissue (MALT)

Overview of MALT

MALT consists of lymphoid tissues located in mucous membranes throughout the body, protecting against pathogens attempting to enter.

• Found in the mucosa of the respiratory tract, genitourinary organs, and digestive tract.

• Main locations: tonsils, Peyer's patches, and appendix.

Tonsils

• Form a ring of lymphatic tissue around the pharynx.

• Types:

  • Palatine tonsils: Largest and most often infected, located at the posterior end of the oral cavity.

  • Lingual tonsil: Collection of follicles at the base of the tongue.

  • Pharyngeal tonsil (adenoids): Located in the posterior wall of the nasopharynx.

  • Tubal tonsils: Surround openings of auditory tubes into the pharynx.

• Gather and remove pathogens in food or air.

• Contain follicles with germinal centers and scattered lymphocytes; not fully encapsulated.

• Epithelium invaginates to form tonsillar crypts, trapping bacteria and particulate matter for immune activation.

Peyer's Patches and Appendix

• Peyer's patches: Clusters of lymphoid follicles in the wall of the distal small intestine; destroy bacteria and generate memory lymphocytes.

• Appendix: Offshoot of the first part of the large intestine; contains many lymphoid follicles, aiding in bacterial destruction and memory lymphocyte generation.

The Thymus

Structure and Function of the Thymus

The thymus is a bilobed lymphoid organ located in the inferior neck, extending into the mediastinum and partially overlying the heart. It is most active and largest during childhood, then gradually atrophies with age.

• Site of T cell maturation; does not directly fight antigens.

• Broken into lobules containing an outer cortex (rapidly dividing lymphocytes and macrophages) and inner medulla (fewer lymphocytes and thymic corpuscles).

• Thymic corpuscles: Sites where regulatory T cells develop, helping prevent autoimmunity.

• No follicles (lacks B cells); contains a blood-thymus barrier to prevent premature activation of immature T cells.

• Stroma is made of epithelial cells, not reticular fibers, providing an environment for T cell maturation.

Summary Table: Lymphoid Organs and Tissues

Organ/Tissue	Main Function	Key Features
Lymph Nodes	Filter lymph, immune activation	Cortex and medulla; afferent/efferent vessels
Spleen	Filter blood, immune response, RBC recycling	White pulp (immune), red pulp (RBC destruction)
Tonsils	Trap pathogens from food/air	Crypts, germinal centers
Peyer's Patches	Destroy bacteria in intestine	Aggregated lymphoid nodules
Appendix	Destroy bacteria, memory lymphocytes	Many lymphoid follicles
Thymus	T cell maturation	Lobules, cortex/medulla, thymic corpuscles