The Lymphatic System and Lymphoid Organs and Tissues

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 the immune system, and plays a key role in defending the body against disease.

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

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

• Lymph nodes: Structures that cleanse lymph as it passes through.

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

Components of the Lymphatic System

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

• Lymphatic vessels collect excess protein-containing 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

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 tissues.

• Highly permeable due to unique endothelial cell modifications.

• Minivalves allow one-way entry of interstitial fluid.

• Anchored by collagen filaments; increases in interstitial fluid volume open minivalves.

• In the brain, astrocytes form channels connecting to lymphatics in meninges, forming the glymphatic system.

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

Larger Lymphatic Vessels

Lymphatic capillaries drain into larger, thicker-walled channels called collecting lymphatic vessels, which have three tunics like veins but thinner walls and more valves.

• Largest vessels unite to form lymphatic trunks (nine major trunks, named for regions they drain).

• Lymph is delivered from trunks into one of two large lymphatic ducts: right lymphatic duct and thoracic duct.

• Right lymphatic duct drains right upper limb and right side of head and thorax; thoracic duct drains the rest of the body.

• Thoracic duct often begins as an enlarged sac called cisterna chyli.

Body-wide Distribution

Clinical Note: Lymphangitis

• Inflamed lymphatics appear as red streaks and are tender to the touch.

• Caused by congestion with blood during severe inflammation.

Lymph Transport

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

• Milking action of skeletal muscles

• Pressure changes during breathing

• Pulse pressure waves from adjacent arteries

• Rhythmic contractions of smooth muscle in vessel walls

• Physical activity increases lymph flow

Clinical Note: Lymphedema

• Insufficient return of lymph to blood results in severe localized edema.

• May be caused by tumors or removal of lymphatics during surgery.

Lymphoid Cells and Tissues

Lymphoid Cells

Lymphoid cells include immune system cells and supporting cells that form the scaffolding of lymphoid tissues.

• 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 an important component of the immune system, providing sites for activation and proliferation of lymphocytes.

• Composed largely of loose reticular connective tissue.

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

• Comes in two main forms: diffuse lymphoid tissue (loose arrangement) and lymphoid follicles (solid, spherical bodies).

• Lymphoid follicles often have germinal centers of proliferating B cells.

• Isolated aggregations found in Peyer’s patches (ileum) and appendix.

Lymphoid Organs

Primary and Secondary Lymphoid Organs

Lymphoid organs are grouped into two functional categories:

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

• Secondary lymphoid organs: Sites where mature lymphocytes encounter antigens and become activated (lymph nodes, spleen, tonsils, Peyer’s patches, appendix).

Lymph Nodes

Structure and Function

Lymph nodes are clustered along lymphatic vessels and are the most important secondary lymphoid organs. They cleanse lymph and house lymphocytes.

• Bean-shaped, surrounded by a fibrous capsule.

• Divided into cortex (contains follicles with germinal centers) and medulla (contains medullary cords).

• Lymph sinuses are large capillaries spanned by reticular fibers; macrophages reside here.

Circulation in Lymph Nodes

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

• Fewer efferent vessels than afferent vessels, causing slow flow and allowing time for immune functions.

Clinical Note: Buboes and Cancer

• Buboes: Infected, pus-filled lymph nodes (e.g., bubonic plague).

• Lymphadenopathy: Swollen, tender nodes due to infection.

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

The Spleen

Structure and Function

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

• Stores iron, recycles breakdown products of RBCs, stores platelets and monocytes, and may produce erythrocytes in the fetus.

• Surrounded by a fibrous capsule and has trabeculae.

• Consists of white pulp (immune function) and red pulp (destruction of old RBCs and pathogens).

Clinical Note: Splenectomy

• Rupture of the spleen can cause life-threatening hemorrhage.

• Liver and bone marrow can take over most functions if spleen is removed.

• In children, the spleen can regenerate if a small part remains.

Mucosa-Associated Lymphoid Tissue (MALT)

Overview

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

• Form a ring of lymphoid tissue around the entrance to the pharynx.

• Gather and remove pathogens in food or inhaled air.

• Contain follicles with germinal centers and tonsillar crypts that trap debris and bacteria.

Peyer's Patches

• Large clusters of lymphoid follicles in the wall of the distal small intestine.

• Prevent bacteria from breaching the intestinal wall and generate memory lymphocytes.

Appendix

• Tubular offshoot of the large intestine containing many lymphoid follicles.

• Functions similar to Peyer's patches.

The Thymus

Structure and Function

The thymus is a bilobed lymphoid organ in the inferior neck and mediastinum, prominent in newborns and most active during childhood. It is the site of T lymphocyte development.

• Divided into thymic lobules with cortex (rapidly dividing lymphocytes) and medulla (fewer lymphocytes, thymic corpuscles).

• No follicles (lacks B cells), does not directly fight antigens, functions strictly in T cell maturation.

• Blood thymus barrier prevents premature activation of T cells.

• Stroma consists of epithelial cells, not reticular fibers.

Summary Table: Lymphoid Organs and Tissues

Developmental Aspects

Lymphatic vessels and main clusters of lymph nodes develop by the fifth week of embryonic development. Lymphoid organs (except thymus) develop from mesodermal mesenchymal cells. The thymus is an endodermal derivative and the first lymphoid organ to appear.

Key Terms and Concepts

• Lymph: Interstitial fluid that enters lymphatic vessels.

• Lymphatic capillaries: Highly permeable, blind-ended vessels.

• Lymph nodes: Filter lymph and house immune cells.

• Spleen: Filters blood, recycles RBCs, immune surveillance.

• MALT: Mucosa-associated lymphoid tissue, protects entryways.

• Thymus: Site of T cell maturation.

Equations and Formulas

Fluid movement across capillaries is governed by Starling's law:

Where NFP determines the direction and amount of fluid movement.

Additional info:

• The glymphatic system in the brain is a recently discovered pathway for waste clearance, potentially relevant in neurodegenerative diseases.

• Immunocompetent cells are those capable of mounting an immune response.