The Lymphatic System

Overview and Functions

The lymphatic system is a network of vessels, organs, and tissues that returns fluids leaked from blood vessels back to the bloodstream and provides the structural basis for the immune system. It consists of lymphatic vessels, lymphoid organs, and lymphoid tissues.

• Fluid Recovery: Returns interstitial fluid and leaked plasma proteins to the blood.

• Immune Defense: Houses phagocytic cells and lymphocytes that defend against pathogens.

• Fat Absorption: Specialized lymphatic capillaries (lacteals) absorb dietary fats from the intestine.

Lymphoid organs and tissues include the spleen, thymus, tonsils, lymph nodes, and other diffuse lymphoid tissues.

Lymphatic Vessels (Lymphatics)

Structure and Function

Lymphatic vessels form a one-way system that ensures lymph flows only toward the heart. They include lymphatic capillaries, collecting lymphatic vessels, lymphatic trunks, and ducts.

• Lymphatic Capillaries: Open-ended vessels that weave between tissue cells and blood capillaries. They are more permeable than blood capillaries, allowing uptake of large molecules (e.g., proteins, cell debris, pathogens, cancer cells).

• Specialized Structures:

  • Overlapping endothelial cells form one-way minivalves.

  • Anchored by collagen filaments; increased interstitial fluid opens minivalves further.

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

• Collecting Lymphatic Vessels: Lymph capillaries drain into these larger vessels, which have structures and tunics similar to veins but with thinner walls and more internal valves.

• Lymphatic Trunks: Formed by the union of the largest collecting vessels; named for the regions they drain (e.g., lumbar, intestinal, bronchomediastinal, subclavian, jugular).

• Lymphatic Ducts:

  • Right Lymphatic Duct: Drains right upper arm and right side of head and thorax.

  • Thoracic Duct: Drains lymph from the rest of the body.

  • Each duct empties lymph into venous circulation at the junction of the internal jugular and subclavian veins on its own side of the body.

Lymph Transport

Lymph transport is a low-pressure system, similar to the venous system. Lymph is propelled by:

• Milking action of skeletal muscles

• Pressure changes during breathing

• Valves to prevent backflow

• Pulsations of nearby arteries

• Contraction of smooth muscle in lymphatic vessel walls

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

Lymphoid Cells

Types and Functions

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

• Lymphocytes: Main cells of the adaptive immune system; mature into T cells and B cells.

  • T cells (T lymphocytes): Manage immune response; some directly attack and destroy infected cells.

  • B cells (B lymphocytes): Produce plasma cells, which secrete antibodies that mark antigens for destruction.

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

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

• Reticular Cells: Produce the stroma (network of reticular fibers) that supports other cells in lymphoid organs.

Lymphoid Tissue

Structure and Types

Lymphoid tissue provides a site for lymphocyte proliferation and surveillance. It is largely composed of reticular connective tissue.

• Lymphocytes live on reticular fibers; spaces between fibers allow lymphocytes to occupy the tissue when returning from patrol.

• Two main types:

  • Diffuse Lymphoid Tissue: Loose arrangement of lymphoid cells and some reticular fibers; found in nearly every organ, especially in mucous membranes.

  • Lymphoid Follicles (Nodules): Solid, spherical bodies of tightly packed lymphoid cells and reticular fibers; contain germinal centers of proliferating B cells.

Lymphoid Organs

Primary and Secondary Organs

• 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).

Lymph Nodes

Structure and Function

Lymph nodes are the principal secondary lymphoid organs, found throughout the body, often in clusters along lymphatic vessels (e.g., inguinal, axillary, and cervical regions).

• Functions:

  1. Filter lymph: Macrophages remove and destroy microorganisms and debris.

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

• Structure:

  • Surrounded by a fibrous capsule; capsule fibers extend inward as trabeculae, dividing the node into compartments.

  • Two regions:

    • Cortex: Contains lymphoid follicles with germinal centers (rich in dividing B cells); deep cortex houses T cells in transit; dendritic cells are abundant.

    • Medulla: Medullary cords contain B cells, T cells, and plasma cells; medullary sinuses contain macrophages on reticular fibers.

• Circulation:

  • Lymph enters via afferent lymphatic vessels, passes through subcapsular sinus, cortex, and medulla, and exits via fewer efferent lymphatic vessels at the hilum. This slows lymph flow, allowing time for immune functions.

• Clinical Note: Inflamed, swollen, and tender lymph nodes (lymphadenitis) occur when nodes are overwhelmed by pathogens. Buboes (as in the plague) are pus-filled lymph nodes. Cancer-infiltrated nodes are swollen but usually not painful.

Spleen

Structure and Functions

The spleen is the largest lymphoid organ, located in the left upper abdomen. It is served by the splenic artery and vein at the hilum.

• Functions:

  • Site of lymphocyte proliferation and immune surveillance/response

  • Cleanses blood of aged cells and platelets; removes debris

  • Stores blood platelets and monocytes

  • May produce erythrocytes in the fetus

• Histology:

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

  • Red pulp: Site of destruction of old blood cells and pathogens; rich in macrophages; consists of splenic cords (reticular tissue) and blood-filled sinusoids.

Mucosa-Associated Lymphoid Tissue (MALT)

Distribution and Function

MALT consists of lymphoid tissues in mucous membranes throughout the body, protecting against pathogens entering through mucosa.

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

• Largest collections in:

  • Tonsils: Form a ring around the pharynx; contain follicles with germinal centers and scattered lymphocytes; crypts trap and destroy bacteria, helping build immune memory.

  • Peyer’s Patches: Clusters of lymphoid follicles in the distal small intestine; similar to tonsils; location aids in monitoring intestinal bacteria and generating immune responses.

  • Appendix: Offshoot of the large intestine; contains many lymphoid follicles; location aids in immune functions similar to Peyer’s patches.

Thymus

Structure and Function

The thymus is a bilobed lymphoid organ in the inferior neck, extending into the mediastinum. It is the site of T cell maturation.

• Most active and largest during childhood; atrophies after adolescence but continues to produce immunocompetent cells.

• Divided into lobules with outer cortex (densely packed lymphocytes and some macrophages) and inner medulla (fewer lymphocytes, thymic corpuscles).

• Thymic corpuscles: Involved in development of regulatory T cells, which help prevent autoimmunity.

• Unique Features:

  1. No follicles (lacks B cells).

  2. Does not directly fight antigens; contains a blood-thymus barrier to prevent premature activation of immature T cells.

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