The Lymphatic System and Lymphoid Organs & Tissues: Structure, Function, and Cellular Composition

Chapter 20: The Lymphatic System and Lymphoid Organs and Tissues

Learning Objectives

• Describe the composition and primary functions of the lymphatic system and lymphoid organs.

• Describe the structure and distribution of lymphatic vessels, the source of lymph, and mechanisms of lymph transport.

• Describe the basic structure and cellular population of lymphoid tissue.

• Describe the location, histological structure, and function of lymph nodes.

• Describe the structure and function of the spleen.

The Lymphatic System: Composition and Primary Functions

Overview

The lymphatic system is a network of vessels and organs that returns fluids and proteins leaked from the cardiovascular system (CVS) back to the blood, absorbs fats from the intestine, and provides the structural basis for immune defense.

• Fluid Recovery: Returns excess interstitial fluid and leaked proteins to the bloodstream.

• Fat Absorption: Transports absorbed fats from the intestine to the blood via specialized lymphatic capillaries called lacteals.

• Immune Function: Lymph nodes and other lymphoid organs filter lymph and provide sites for immune cell activation.

The lymphatic system consists of three main components:

• Lymphatic vessels (lymphatics): Network that transports lymph.

• Lymph: Fluid contained within lymphatic vessels.

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

Lymphoid Organs and Tissues

Structural Basis of Immune System

• Includes spleen, thymus, tonsils, and other lymphoid tissues.

• Provide sites for immune cell proliferation and immune surveillance.

Structure and Distribution of Lymphatic Vessels

Lymphatic Vessel Organization

Lymphatic vessels form a one-way system in which lymph flows toward the heart.

• Lymphatic capillaries: Microscopic, blind-ended vessels located between tissue cells and blood capillaries. They are highly permeable and can take up proteins, cell debris, pathogens, and cancer cells.

• Structural Modifications:

  • Endothelial cells are not tightly joined, but overlap loosely to form one-way minivalves.

  • Anchored by collagen filaments, so increased interstitial fluid volume opens minivalves rather than causing capillaries to collapse.

• Collecting lymphatic vessels: Lymph flows from capillaries into larger, thicker-walled channels.

• Lymphatic trunks: Largest collecting vessels unite to form lymphatic trunks.

• Lymphatic ducts: Lymph is eventually delivered to one of two large ducts.

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 respective side of the body.

Lymph Transport Mechanisms

The lymphatic system lacks a central pump; lymph is propelled by mechanisms similar to those that aid venous return:

• Milking action of skeletal muscle

• Pressure changes in the thorax during breathing

• Squeezing of abdominal vessels as thoracic lymph vessels expand

• Valves to prevent backflow

Basic Structure and Cellular Population of Lymphoid Tissue

Lymphoid Cells

• Lymphocytes: Mature into two main types:

  • T cells (T lymphocytes): Manage immune response; 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.

• Reticular cells: Fibroblast-like cells that produce reticular fiber stroma supporting other cells in lymphoid organs.

Lymphoid Tissue

• Houses and provides proliferation sites for lymphocytes.

• Surveillance vantage point for lymphocytes and macrophages.

• Composed largely of reticular connective tissue (a type of loose connective tissue).

Lymphoid Follicles (Nodules)

• Solid, spherical bodies of tightly packed lymphoid cells and reticular fibers.

• Found in lymph nodes, mucosa-associated lymphoid tissue (MALT), Peyer's patches, and appendix.

Classification of Lymphoid Organs

• Primary lymphoid organs: Sites where B and T cells mature.

  • B cells mature in bone marrow.

  • T cells mature in thymus.

• Secondary lymphoid organs: Sites where mature lymphocytes first encounter antigens and are activated (e.g., lymph nodes, spleen, MALT, Peyer's patches).

Location, Histological Structure, and Function of Lymph Nodes

Location

• Most important lymphoid organs; cluster along lymphatic vessels.

• Found near body surface in inguinal, axillary, and cervical regions.

Functions

• Cleansing the lymph: Macrophages remove and destroy microorganisms and debris, preventing their entry into the bloodstream.

• Immune system activation: Lymphocytes encounter antigens and are activated to mount an immune response.

Structure

• Vary in shape and size, but most are bean-shaped.

• Surrounded by a dense, fibrous capsule.

• Connective tissue strands called trabeculae extend inward, dividing the node into compartments.

• Two histologically distinct regions:

  • Cortex: Contains lymphoid follicles with germinal centers (sites of B cell proliferation); deeper layers contain T cells.

  • Medulla: Medullary cords extend inward from cortex and contain both lymphocytes and plasma cells.

• Lymph sinuses: Large lymph capillaries containing macrophages that phagocytize foreign matter.

Circulation in Lymph Nodes

• Lymph enters via afferent lymphatic vessels, passes through subcapsular sinus and medullary sinuses, and exits at the hilum via efferent vessels.

• Fewer efferent vessels cause lymph to stagnate, allowing time for lymphocytes and macrophages to function.

Structure and Function of the Spleen

Overview

The spleen is the largest lymphoid organ, located in the left side of the abdominal cavity below the diaphragm. It provides immune surveillance and response, and removes old or defective blood cells.

• Immune function: Site for lymphocyte proliferation and immune surveillance.

• Blood cleansing: Macrophages remove and recycle old red blood cells and platelets.

• Storage: Stores breakdown products of RBCs (e.g., iron) for reuse.

Structure

• Surrounded by a fibrous capsule with trabeculae extending inward.

• Two distinct regions:

  • White pulp: Around central arteries; contains mostly lymphocytes on reticular fibers, involved in immune functions.

  • Red pulp: Surrounds white pulp; contains macrophages that engulf old RBCs and bloodborne pathogens, composed of splenic cords and sinusoids.

Summary Table: Major Lymphoid Organs and Their Functions

Organ	Location	Main Function
Lymph Nodes	Along lymphatic vessels (inguinal, axillary, cervical regions)	Filter lymph, immune activation
Spleen	Left abdominal cavity, below diaphragm	Immune surveillance, blood cleansing, RBC recycling
Thymus	Superior mediastinum	T cell maturation
Tonsils	Pharyngeal region	Trap pathogens, immune response initiation

Additional info: The lymphatic system is essential for maintaining fluid balance, immune defense, and fat absorption. Disorders of the lymphatic system can lead to edema, immune deficiencies, or lymphatic cancers.