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

CH 20 The Lymphatic System: Overview

Introduction to 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 the structural basis for the immune system, and plays a key role in defending the body against disease.

• Lymphatic vessels: An elaborate network of drainage vessels that collect excess interstitial fluid.

• Lymph: The fluid contained within lymphatic vessels.

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

• Lymphoid organs and tissues: Include the spleen, thymus, tonsils, lymph nodes, and other tissues that support immune function.

Structure and Distribution of Lymphatic Vessels

Lymphatic Capillaries

Lymphatic capillaries are blind-ended vessels that weave between tissue cells and blood capillaries. They are highly permeable, allowing them to absorb proteins, cell debris, pathogens, and even cancer cells from the interstitial fluid.

• Absent from bones, teeth, and most of the central nervous system (CNS).

• In the brain, astrocytes form channels that connect to lymphatics in the meninges, forming the glymphatic system for draining cerebrospinal fluid and wastes.

• Unique structure: Endothelial cells overlap loosely to form one-way minivalves, anchored by collagen filaments. Increased interstitial fluid volume opens these minivalves, allowing fluid entry.

• Specialized capillaries in the small intestine, called lacteals, absorb digested fats, forming fatty lymph known as chyle.

Larger Lymphatic Vessels

Lymphatic capillaries drain into larger collecting vessels, which have thinner walls and more valves than veins. These vessels unite to form nine major lymphatic trunks, named for the regions they drain (e.g., lumbar, bronchomediastinal, subclavian, jugular, and intestinal trunks).

• Lymph is ultimately delivered to one of two large ducts:

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

  • Thoracic duct: Drains the rest of the body, often beginning as the cisterna chyli.

• Both ducts empty into the venous circulation at the junctions of the internal jugular and subclavian veins.

Lymph Transport and Clinical Considerations

Lymph Flow Mechanisms

Lymphatic vessels are low-pressure conduits. Lymph flow is maintained by:

• Milking action of skeletal muscles

• Pressure changes during breathing

• Pulsations of adjacent arteries

• Rhythmic contractions of smooth muscle in vessel walls

• One-way valves prevent backflow

Physical activity increases lymph flow, while immobilization restricts it, which can help contain infections.

Clinical Imbalances

• Lymphangitis: Inflammation of lymphatic vessels, appearing as red streaks due to congestion with blood.

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

Lymphoid Cells and Tissues

Lymphoid Cells

Lymphoid tissues contain immune system cells and supporting cells:

• Lymphocytes: Main warriors of the immune system.

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

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

• Macrophages: Phagocytize foreign substances and activate T cells.

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

• Reticular cells: Produce reticular fiber stroma, forming the network that supports other cell types.

Lymphoid Tissue Structure

Lymphoid tissue provides sites for lymphocyte activation and proliferation. It is largely composed of loose reticular connective tissue and comes in two main forms:

• Diffuse lymphoid tissue: Loose arrangement found in most organs.

• Lymphoid follicles (nodules): Solid, spherical bodies with germinal centers of proliferating B cells. Found in lymph nodes, Peyer’s patches, and the appendix.

Lymphoid Organs

Primary and Secondary Lymphoid Organs

Lymphoid organs are classified by their function:

• Primary lymphoid organs: Sites of lymphocyte maturation (red bone marrow for B cells, thymus for T cells).

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

Lymph Nodes: Structure and Function

Role and Location

Lymph nodes are the most important secondary lymphoid organs, clustered along lymphatic vessels, especially in the inguinal, axillary, and cervical regions. They filter lymph and activate the immune system.

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

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

Structure of a Lymph Node

Lymph nodes are bean-shaped and surrounded by a fibrous capsule. They have two main regions:

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

• Medulla: Contains medullary cords (B cells, T cells, plasma cells) and lymph sinuses with macrophages.

Circulation in Lymph Nodes

Lymph enters via afferent vessels, passes through sinuses, and exits via fewer efferent vessels at the hilum. This slow flow allows time for immune cells to act.

Clinical Note: Lymphadenopathy and Cancer

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

• Cancer metastasis: Cancer cells may become trapped in lymph nodes, causing painless swelling.

The Spleen: Structure and Function

Functions of the Spleen

The spleen is the largest lymphoid organ, located in the left upper abdomen. It provides a site for lymphocyte proliferation, immune surveillance, and response. It also removes old blood cells and platelets, stores iron, and serves as a blood reservoir.

Histology of the Spleen

The spleen is surrounded by a fibrous capsule and divided into:

• White pulp: Lymphocytes on reticular fibers around central arteries (immune function).

• Red pulp: Venous sinuses and splenic cords where old RBCs and pathogens are destroyed.

Clinical Note: Splenic Injury and Splenectomy

• The spleen's thin capsule makes it vulnerable to rupture. Removal (splenectomy) is less common now, as the spleen can often repair itself and other organs can compensate for its loss.

Mucosa-Associated Lymphoid Tissue (MALT)

Overview of MALT

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

• Major collections: Tonsils, Peyer’s patches, and appendix.

Tonsils

Tonsils form a ring of lymphoid tissue around the pharynx, trapping and removing pathogens from food and air. They contain follicles with germinal centers and deep crypts that trap debris and bacteria.

Peyer’s Patches

Peyer’s patches are clusters of lymphoid follicles in the distal small intestine. They prevent bacterial invasion and generate memory lymphocytes for long-term immunity.

Appendix

The appendix is a tubular offshoot of the large intestine, rich in lymphoid follicles. It helps prevent bacterial invasion and supports immune memory.

The Thymus: T Lymphocyte Maturation

Structure and Function of the Thymus

The thymus is a bilobed organ in the neck and mediastinum, most active during childhood. It is the site of T lymphocyte maturation and gradually atrophies after puberty.

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

• Differs from other lymphoid organs: lacks B cells, does not directly fight antigens, and has a blood-thymus barrier to prevent premature activation of T cells.

• Stroma consists of epithelial cells, not reticular fibers.

Summary Table: Lymphoid Organs and Tissues

The following table summarizes the main lymphoid organs and tissues, their locations, and primary functions.

Organ/Tissue	Location	Function
Lymph nodes	Throughout body, clustered in cervical, axillary, inguinal regions	Filter lymph, immune activation
Spleen	Left upper abdomen	Filters blood, immune surveillance, recycles RBCs
Tonsils	Pharynx	Trap and remove pathogens from food/air
Peyer’s patches	Distal small intestine	Prevent bacterial invasion, immune memory
Appendix	Large intestine	Immune surveillance, memory lymphocytes
Thymus	Neck/mediastinum	T cell maturation

Developmental Aspects of the Lymphatic System

Lymphatic vessels and main clusters of lymph nodes develop by the fifth week of embryonic life, arising from lymph sacs derived from veins. The thymus develops from the pharynx lining and is the first lymphoid organ to appear, followed by the colonization of other organs by lymphocytes after birth.