Lymphatic System and Lymphoid Organs: Structure, Function, and Clinical Aspects

Lymphatic System and Lymphoid Organs and Tissues

Overview

The lymphatic system returns fluids leaked from blood vessels back to the blood and provides the structural basis of the immune system. It consists of three main components:

• Network of lymphatic vessels (lymphatics): elaborate drainage system

• Lymph: fluid contained within lymphatic vessels

• Lymph nodes: cleanse lymph and house immune cells

Lymphoid organs and tissues provide sites for immune cell proliferation and surveillance. Major structures include the spleen, thymus, tonsils, lymph nodes, and other lymphoid tissues.

20.1 Lymphatic System

Function and Fluid Movement

• Lymphatic vessels return interstitial fluid and leaked plasma proteins to the blood.

• Approximately 3 liters of interstitial fluid circulate per day.

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

Distribution and Structure of Lymphatic Vessels

Lymphatic Capillaries

• Form a one-way system, ensuring lymph flows only toward the heart.

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

• Absent from bones, teeth, and bone marrow; present in most tissues, including meninges (drain CSF).

• More permeable than blood capillaries, allowing uptake of larger molecules (proteins, cell debris, pathogens, cancer cells).

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

Special Features of Lymphatic Capillaries

• Increased permeability due to:

  • Endothelial cells overlap loosely to form one-way minivalves.

  • Minivalves anchored by collagen filaments; increased ECF volume opens minivalves, decreased ECF closes them.

• Lacteals: specialized lymph capillaries in intestinal mucosa that absorb digested fat and deliver fatty lymph (chyle) to the blood.

Larger Lymphatic Vessels

• Lymph capillaries drain into larger vessels called collecting lymphatic vessels (vessels, trunks, ducts).

• Similar structure to veins but with thinner walls and more internal valves; anastomose more frequently.

• Collecting vessels travel with superficial veins; deeper vessels travel with arteries.

• Lymphatic trunks are formed by union of largest collecting vessels and drain large areas of the body.

Lymphatic Trunk	Region Drained
Paired lumbar	Lower limbs, pelvis
Paired bronchomediastinal	Thoracic organs
Paired subclavian	Upper limbs
Paired jugular	Head and neck
Single intestinal trunk	Abdominal viscera

Lymphatic Ducts

• Right lymphatic duct: drains right upper arm and right side of head and thorax.

• Thoracic duct: drains rest of body; begins as cisterna chyli in about half of individuals.

• Each duct empties lymph into venous circulation at the junction of internal jugular and subclavian veins.

Clinical – Homeostatic Imbalance 20.1

Lymphangitis

• Condition in which lymphatic vessels appear as painful red lines under the skin.

• Caused by inflammation of larger lymphatic vessels containing vasa vasorum (small blood vessels supplying vessel walls).

Lymphedema

• Severe localized edema caused by anything preventing normal return of lymph to blood.

• Examples: tumors blocking lymphatics, removal of lymphatics during cancer surgery.

• Lymphedema may improve if some lymphatic pathways remain and enlarge.

Lymph Transport

Mechanisms of Lymph Movement

• Lymphatic system is a low-pressure system, similar to veins.

• Lymph is propelled by:

  • Milking action of skeletal muscle

  • Pressure changes in thorax during breathing

  • Valves to prevent backflow

  • Pulsations of nearby arteries

  • Contractions of smooth muscle in lymphatic vessel walls

• Physical activity increases lymph flow; immobilization keeps inflammatory material in area for faster healing.

20.2 Lymphoid Cells, Tissues, and Organs

Lymphoid Cells

• Consist of immune system cells and supporting cells.

• Lymphocytes: cells of the adaptive immune system; mature into:

  • T cells (T lymphocytes): manage immune response, attack infected cells.

  • B cells (B lymphocytes): produce plasma cells, which secrete antibodies.

• Antigens: anything the body perceives as foreign (bacteria, toxins, viruses, mismatched RBCs, cancer cells).

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

• Dendritic cells: capture antigens and deliver them to lymph nodes; help activate T cells.

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

Lymphoid Tissue

• Houses and provides proliferation sites for lymphocytes.

• Offers surveillance vantage points for lymphocytes and macrophages.

• Composed largely of reticular connective tissue (loose connective tissue).

• Spaces between fibers provide places for lymphocytes to occupy when returning from patrolling the body.

Lymphoid Organs

• Two main types of lymphoid tissues:

  • Diffuse lymphoid tissue: loose arrangement of lymphoid cells and some reticular fibers; found in virtually every body organ, especially in mucous membranes.

  • Lymphoid follicles (nodular tissue): solid, spherical bodies of tightly packed lymphoid cells and reticular fibers; contain germinal centers of proliferating B cells; may form part of larger lymphoid organs (nodes) or isolated aggregations (Peyer's patches, appendix).

• Lymphoid organs are grouped into:

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

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

20.3 Lymph Nodes

Structure and Function

• Principal secondary lymphoid organs; hundreds found throughout the body.

• Most are embedded deep in connective tissue; some are near body surface (inguinal, axillary, cervical regions).

• Two main functions:

  1. Cleansing the lymph: macrophages remove and destroy microorganisms and debris, preventing unwanted substances from being delivered to blood.

  2. Immune system activation: provide a place for lymphocytes to become activated and mount an attack against antigens.

Structure of a Lymph Node

• Most are bean-shaped, less than 2.5 cm in size.

• Surrounded by external fibrous capsule.

• Capsule fibers extend inward as trabeculae, dividing node into compartments.

• Two histologically distinct regions:

  • Cortex: contains follicles with germinal centers (dividing B cells); deep cortex houses T cells in transit; abundant dendritic cells activate both lymphocyte types.

  • Medulla: medullary cords contain B cells, T cells, plasma cells; lymph sinuses consist of large lymphatic capillaries spanned by reticular fibers, with macrophages residing on fibers.

Circulation in the Lymph Nodes

• Lymph enters via afferent lymphatic vessels on convex side.

• Travels through large subcapsular sinus and smaller sinuses in cortex and medulla.

• Exits at hilum via efferent lymphatic vessels on concave side.

• Fewer efferent vessels cause flow to slow, allowing lymphocytes and macrophages time to function.

• Lymph travels through several nodes before returning to circulation.

Clinical – Homeostatic Imbalance 20.3

• Buboes: inflamed, swollen, tender lymph nodes overwhelmed by what they are trying to destroy; sometimes pus-filled (e.g., bubonic plague).

• Lymph nodes can become secondary cancer sites if metastasizing cancer cells are trapped; cancer-infiltrated nodes are swollen but usually not painful.

20.4 Spleen

Structure and Function

• Blood-rich organ, about the size of a fist, located in the left side of the abdominal cavity, just below the stomach.

• Largest lymphoid organ; served by splenic artery and vein (enter/exit at hilum).

• Functions:

  • Site of lymphocyte proliferation and immune surveillance/response.

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

  • Stores breakdown products of RBCs (e.g., iron) for later reuse.

  • Stores blood platelets and monocytes for release into blood when needed.

  • May be site of fetal erythrocyte production.

• Encased by fibrous capsule and contains trabeculae.

• Histologically consists of:

  • White pulp: site of immune function; contains mostly lymphocytes on reticular fibers, found around central arteries.

  • Red pulp: site where old blood cells and bloodborne pathogens are destroyed; rich in RBCs and macrophages; composed of splenic cords and splenic sinusoids.

Clinical – Homeostatic Imbalance 20.4

• Spleen has a thin capsule; direct blow or severe infection may cause rupture, spilling blood into peritoneal cavity.

• Splenectomy: surgical removal of ruptured spleen; spleen can often repair itself, so emergency splenectomies have decreased.

• If removed, liver and bone marrow take over most functions; in children under 12, spleen will regenerate if a small part is left.

20.5 MALT (Mucosa-Associated Lymphoid Tissue)

Structure and Function

• Lymphoid tissues in mucous membranes throughout the body.

• Protects from pathogens trying to enter the body.

• Largest collections found in:

  • Tonsils: form ring of lymphatic tissue around pharynx; gather and remove pathogens from food or air.

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

  • Appendix: contains large concentration of lymphoid follicles; destroys bacteria and generates memory lymphocytes.

20.6 Thymus

Structure and Function

• Bilobed lymphoid organ found in the neck, extending into the mediastinum.

• Site where T cells mature; most active and largest in childhood, then gradually atrophies.

• Contains outer cortex (rapidly dividing lymphocytes and scattered macrophages) and inner medulla (fewer lymphocytes, thymic corpuscles where regulatory T cells develop).

• Regulatory T cells help prevent autoimmunity.

Developmental Aspects of the Lymphatic System

• Lymphatic vessels and main clusters of lymph nodes develop from lymph sacs derived from veins.

• Lymph organs (except thymus) arise from mesenchymal cells.

• Thymus (endodermal origin) forms as an outgrowth of the pharynx.

• At birth, high numbers of lymphocytes are present; development parallels immune system maturation.

Summary Table: Lymphoid Organs and Tissues

Organ/Tissue	Location	Main Function
Lymph nodes	Throughout body	Filter lymph, immune activation
Spleen	Left abdominal cavity	Filter blood, immune response
Thymus	Neck/mediastinum	T cell maturation
Tonsils	Pharynx	Trap pathogens from food/air
Peyer's patches	Small intestine	Destroy bacteria, memory cells
Appendix	Large intestine	Destroy bacteria, memory cells