The Lymphatic System and Lymphoid Organs and Tissues

20.1 Overview of the Lymphatic System

The lymphatic system is a vital component of the circulatory and immune systems. It returns fluids leaked from blood vessels back to the blood, provides structural support for immune responses, and helps defend the body against disease.

• Lymphatic vessels: (Lymphatics) Network of drainage vessels that collect excess interstitial fluid.

  • Return excess tissue fluid to the blood

  • Return leaked proteins to the blood

  • Carry pathogens to lymph nodes

  • Carry absorbed fat from intestines to blood

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

• Glymphatic system: Waste clearing pathway in the brain; Astrocyte network acts a lymphatic vessels and connects to meningeal lymphatics. System clears extracellular fluid (interstitial + cerebrospinal) waste products. Waste accumulation maybe connected to Alzheimer's.

• Lymph nodes: Structures that filter lymph and house immune cells.

• Lymphoid organs and tissues: Include spleen, thymus, tonsils, lymph nodes, and other tissues; provide the basis for immune function.

Components of the Lymphatic System

• Hydrostatic and colloid osmotic pressures at blood capillaries force fluid out and reabsorb most of it; the remainder becomes interstitial fluid.

• Lymphatic vessels collect excess protein-rich interstitial fluid and return it to the blood.

• Transport pathogens to lymph nodes and absorbed fats from the intestine to the blood.

Distribution and Structure of Lymphatic Vessels

Lymphatic capillaries are blind-ended vessels that weave between tissue cells and blood capillaries. They are absent from bones and teeth, but present in most other tissues.

• Highly permeable due to overlapping endothelial cells forming one-way minivalves.

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

• Specialized capillaries in the small intestine, called lacteals, absorb and transport digested fats.

• Fatty lymph (chyle) from lacteals eventually enters the blood.

Larger Lymphatic Vessels

Lymphatic capillaries drain into larger collecting vessels, which have thinner walls, more valves, and more anastomoses than veins. These vessels unite to form nine major lymphatic trunks, most named for the regions they drain.

• Paired lumbar, bronchomediastinal, subclavian, and jugular trunks

• Single intestinal trunk

Lymphatic Ducts

• Right lymphatic duct: Drains right upper limb, right side of head and thorax.

• Thoracic duct: Drains the rest of the body; often begins as the cisterna chylin (T12-L2)

Body-wide Distribution of Lymphatic System

The lymphatic system is distributed throughout the body, with regional lymph nodes located in the cervical, axillary, and inguinal regions.

Lymph Transport Mechanisms

Lymphatic vessels are low-pressure conduits, relying on mechanisms similar to veins for unidirectional flow.

• Milking action of skeletal muscles

• Pressure changes during breathing

• Pulsations from adjacent arteries

• Rhythmic contractions of smooth muscle in vessel walls

• Physical activity increases lymph flow

Clinical Imbalances

• Lymphangitis: Inflammation of lymphatic vessels, appearing as red streaks.

• Lymphedema: Severe localized edema due to insufficient lymph return, often caused by tumors or surgical removal of lymphatics.

20.2 Lymphoid Cells and Tissues

Lymphoid Cells

Lymphoid tissues contain immune system cells and supporting cells that form the structural framework.

• Lymphocytes: T cells (manage immune response, attack infected cells) and B cells (produce plasma cells, secrete antibodies).

• Macrophages: Phagocytize foreign substances and activate T cells.

• Dendritic cells: Capture and deliver antigens to lymph nodes.

• Reticular cells: Produce reticular fiber stroma, supporting other cell types.

Lymphoid Tissue

Lymphoid tissue is a key component of the immune system, providing sites for lymphocyte activation and proliferation.

• Composed mainly of loose reticular connective tissue.

• Macrophages reside on reticular fibers; lymphocytes occupy spaces between fibers.

• Two main types: Diffuse lymphoid tissue (loose arrangement) and lymphoid follicles (solid, spherical bodies with germinal centers; b cells predominate).

• Follicles found in intestinal wall (Peyer's patches) and appendix.

Lymphoid Organs

Primary and Secondary Lymphoid Organs

Lymphoid organs are classified by their function in lymphocyte development and activation.

• Primary lymphoid organs: Red bone marrow (B cell maturation) and thymus (T cell maturation).

• Secondary lymphoid organs: Lymph nodes, spleen, (MALT) tonsils, Peyer's patches, appendix, and diffuse lymphoid tissues.

20.3 Lymph Nodes

Structure and Function of Lymph Nodes

Lymph nodes are bean-shaped, small structures clustered along lymphatic vessels. They filter lymph and house lymphocytes.

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

• Immune activation: Lymphocytes encounter antigens and mount immune responses.

• Located in clusters near body surface (inguinal, axillary, cervical regions).

Histological Regions

• Cortex: Contains follicles with germinal centers (B cells) and T cells in transit.

• Medulla: Medullary cords with B cells, T cells, plasma cells; lymph sinuses with macrophages.

Clinical Imbalances

• Buboes: Infected, swollen lymph nodes (bubonic plague).

• Lymphadenopathy: Swollen, tender nodes due to infection.

• Cancer-infiltrated nodes: Swollen but not painful, distinguishing from infection.

The Spleen

Structure and Functions of the Spleen

The spleen is the largest lymphoid organ, located in the left abdominal cavity. It provides sites for lymphocyte proliferation, immune surveillance, and response, and cleanses blood of aged cells and platelets.

• Stores iron and recycles breakdown products of red blood cells.

• Stores blood platelets and monocytes for release when needed.

• May produce erythrocytes in the fetus.

Histological Components

• White pulp: Islands of lymphocytes around central arteries; site of immune function.

• Red pulp: Contains RBCs and macrophages; site of destruction of old blood cells and pathogens.

Clinical Imbalances

• Splenic rupture: May require splenectomy; liver and bone marrow compensate for lost functions.

• Children's spleen can regenerate if a small part remains.

Mucosa-Associated Lymphoid Tissue (MALT)

Role and Locations of MALT

MALT consists of lymphoid tissues in mucous membranes throughout the body, protecting against pathogens entering via respiratory, genitourinary, and digestive tracts.

• Largest collections: Tonsils, Peyer's patches, appendix.

Tonsils

Tonsils form a ring of lymphoid tissue around the entrance to the pharynx, gathering and removing pathogens from food and air.

• Types: Paired palatine tonsils, lingual tonsil, pharyngeal tonsil (adenoid), tubal tonsils.

• Contain follicles with germinal centers; epithelium forms deep tonsillar crypts.

• Immune cells build memory against pathogens.

Peyer's Patches

Peyer's patches are clusters of lymphoid follicles in the wall of the distal small intestine, preventing bacterial invasion and generating memory lymphocytes.

Appendix

The appendix is a tubular offshoot of the large intestine containing many lymphoid follicles, helping prevent bacterial invasion and generating memory lymphocytes.

The Thymus

Structure and Function of the Thymus

The thymus is a bilobed lymphoid organ in the inferior neck and mediastinum, serving as the site of T lymphocyte development. It is most active during childhood and gradually atrophies after puberty, but continues to produce immunocompetent cells.

Summary Table: Major Lymphoid Organs and Their Functions