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

Chapter 20: The Lymphatic System and Lymphoid Organs and Tissues

20.1 The Lymphatic System: Structure and Function

The lymphatic system is a network of vessels, nodes, and organs that returns excess tissue fluid and leaked plasma proteins to the blood, transports absorbed fats, and plays a critical role in immune defense.

• Components: Lymphatic vessels, lymph (the fluid), and lymph nodes.

• Primary Functions:

  • Return excess tissue fluid to the blood

  • Return leaked plasma proteins to the blood

  • Carry pathogens to lymph nodes for immune surveillance

  • Transport absorbed fats from the intestine to the blood

Distribution and Structure of Lymphatic Vessels

• One-way system: Lymph flows only toward the heart.

• Lymphatic Capillaries: Found among tissue cells and blood capillaries, especially in loose connective tissue. Absent from bones, teeth, and bone marrow.

Structure of Lymphatic Capillaries

• Microscopic, blind-ended tubes

• Endothelial cells overlap to form flaplike mini-valves

• Mini-valves open when interstitial pressure is greater than lymphatic capillary pressure, allowing fluid entry

• Collagen filaments anchor endothelial cells to surrounding tissues

Function of Lymphatic Capillaries

• Easily absorb excess interstitial fluid, plasma proteins, cell debris, pathogens, and cancer cells

• Transport these materials to lymph nodes for immune processing

Lacteals

• Specialized lymphatic capillaries in the small intestine

• Transport absorbed fats (chyle) to the blood

Larger Lymphatic Vessels and Flow Pathway

• Lymphatic capillaries → Collecting lymphatic vessels → Lymphatic trunks → Lymph nodes → Lymphatic ducts → Venous circulation

• Major lymphatic trunks: lumbar, bronchomediastinal, subclavian, jugular (paired); intestinal (single)

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

• Cisterna chyli: enlarged sac at the beginning of the thoracic duct, receives lymph from lumbar and intestinal trunks

Lymph Transport Mechanisms

• No central pump; relies on skeletal muscle contractions, pressure changes during breathing, one-way valves, pulsations of nearby arteries, and rhythmic contraction of smooth muscle in vessel walls

20.2 Lymphoid Cells and Tissues

Lymphoid cells and tissues are distributed throughout the body, forming the basis of immune defense.

• Lymphocytes: Main warriors of the immune system; include T cells (cell-mediated immunity) and B cells (produce antibodies).

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

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

• Reticular cells: Produce reticular fiber stroma, forming the supporting network in lymphoid organs.

Lymphoid Tissue Structure and Function

• Composed of reticular connective tissue

• Houses lymphocytes and provides a site for their activation and proliferation

• Continuous circulation allows rapid immune response

Types of Lymphoid Tissue

• Diffuse lymphoid tissue: Loose arrangement, found in virtually every organ

• Lymphoid follicles (nodules): Solid, spherical bodies with tightly packed lymphoid cells, especially B cells; found in lymph nodes, Peyer's patches, and appendix

Lymphoid Organs

• Primary lymphoid organs: Sites where lymphocytes mature (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)

20.3 Lymph Nodes: Structure and Function

Lymph nodes are the principal secondary lymphoid organs, filtering lymph and providing sites for immune activation.

• Hundreds of lymph nodes are clustered along lymphatic vessels, especially where collecting vessels converge to form trunks

• Structure: Surrounded by a dense fibrous capsule; trabeculae divide the node into compartments

• Cortex: Contains lymphoid follicles with dividing B cells, T cells in transit, and dendritic cells

• Medulla: Contains medullary cords with B and T lymphocytes

Functions of Lymph Nodes

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

• Immune system activation: Lymphocytes encounter antigens and initiate immune responses

Circulation Through a Lymph Node

• Lymph enters via afferent vessels → subcapsular sinus → cortical sinuses → medullary sinuses → exits at hilum via efferent vessels

• Fewer efferent than afferent vessels slows lymph flow, allowing more time for immune processing

20.4 The Spleen: Structure and Function

The spleen is the largest lymphoid organ, filtering blood and providing immune surveillance.

• Located on the left side of the abdominal cavity beneath the diaphragm

• Functions:

  • Lymphocyte proliferation

  • Removal of bloodborne pathogens, debris, and aged RBCs/platelets

  • Recycles breakdown products of RBCs and stores iron

  • Stores platelets and monocytes

20.5 MALT: Mucosa-Associated Lymphoid Tissue

MALT is a collection of lymphoid tissues strategically located in mucous membranes to guard entryways against pathogens.

• Includes tonsils, Peyer's patches, appendix, and lymphoid tissue in the mucosa of the respiratory, genitourinary, and digestive tracts

Tonsils

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

• Trap and destroy bacteria and particulate matter entering in food or air

• Exposure to pathogens helps produce immune cells with memory

Peyer's Patches

• Large clusters of lymphoid follicles in the small intestine

• Prevent pathogens from breaching the intestinal wall

Appendix

• Tubular lymphoid organ after the first part of the large intestine

• Contains high concentration of lymphoid follicles

• Generates memory lymphocytes for long-term immunity

20.6 The Thymus: Site of T Lymphocyte Maturation

The thymus is a primary lymphoid organ where T lymphocytes mature and become immunocompetent.

• Located in the inferior neck, deep to the sternum, partially overlying the heart

• Most active during childhood; atrophies after puberty but continues to produce T cells at a declining rate

• Does not directly fight antigens; functions only as a maturation site for T lymphocytes

• Blood-thymus barrier prevents premature activation of T cells

Comparative Table: Lymphoid Organs

Organ	Major Functions	Capsule	Cortex and Medulla	Lymphoid Follicles	Stroma	Special Features
Lymph nodes	Cleanse lymph; site for lymphocyte activation and proliferation	Yes	Yes	Yes (in cortex)	Reticular connective tissue	Both afferent and efferent vessels
Spleen	Clears blood, removes aged/defective RBCs; site for lymphocyte activation and proliferation; stores platelets, monocytes, iron	Yes	Yes (white pulp)	Yes (in white pulp)	Reticular connective tissue	Red and white pulp
MALT	Prevents pathogens from penetrating mucous membranes; site for lymphocyte activation and proliferation	No	No	Yes	Reticular connective tissue	Diffuse lymphoid tissue in addition to follicles
Thymus	Site of T cell maturation	Yes	Yes	No	Epithelial tissue	Thymic corpuscles