The Lymphatic System and Lymphoid Organs and Tissues

Overview

The lymphatic system is a crucial component of the circulatory and immune systems. It returns excess tissue fluid to the bloodstream, absorbs dietary fats, and provides the structural basis for immune function through lymphoid organs and tissues.

Composition and Primary Functions of the Lymphatic System

Lymphatic System

• Fluid Recovery: Returns fluids that have leaked from the cardiovascular system (CVS) back to the blood.

• Protein Return: Returns leaked proteins to the blood.

• Fat Absorption: Carries absorbed fat from the intestine to the blood via lacteals (specialized lymphatic capillaries in the intestinal mucosa).

• Components:

  • Lymphatic vessels (lymphatics): Network of vessels transporting lymph.

  • Lymph: Fluid contained within lymphatic vessels.

  • Lymph nodes: Filter and cleanse lymph as it passes through.

Lymphoid Organs and Tissues

• Provide the structural basis of the immune system.

• Include the spleen, thymus, tonsils, and other lymphoid tissues.

Structure and Distribution of Lymphatic Vessels

Lymphatic Vessels

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.

  • Highly permeable, allowing uptake of proteins, cell debris, pathogens, and cancer cells.

  • Specialized structure:

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

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

• The lymphatic system lacks a central pump; lymph is propelled by:

  • Milking action of skeletal muscle

  • Pressure changes in the thorax during breathing

  • Contraction of smooth muscle in lymphatic vessel walls

  • Valves to prevent backflow

Basic Structure and Cellular Population of Lymphoid Tissue

Lymphoid Cells

• Lymphocytes: Main cells of the immune response; 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 for marking antigens for destruction.

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

• Reticular cells: Produce reticular fiber stroma that supports other cells in lymphoid organs.

Lymphoid Tissue

• Houses and provides a proliferation site for lymphocytes.

• Surveillance vantage point for lymphocytes and macrophages.

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

Lymphoid Organs

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

  • B cells mature in the bone marrow.

  • T cells mature in the 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

Lymph Nodes

• Most important lymphoid organs; clustered along lymphatic vessels.

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

Functions

• Cleansing the lymph: Macrophages in nodes 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.

• Trabeculae (connective tissue strands) extend inward, dividing the node into compartments.

• Two histologically distinct regions:

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

  • Medulla: Contains medullary cords (extensions of cortical tissue) with both B and T cells, and medullary sinuses with macrophages.

• Lymph flows through the node via afferent vessels, subcapsular sinus, medullary sinuses, and exits at the hilum via fewer efferent vessels, slowing flow for effective filtration.

Structure and Function of the Spleen

Spleen

• Largest lymphoid organ; located in the left side of the abdominal cavity, just below the diaphragm.

• Functions:

  • Site of lymphocyte proliferation and immune surveillance/response.

  • Removes old and defective blood cells and platelets from the blood.

  • Stores breakdown products of red blood cells (e.g., iron) for later reuse.

  • Stores blood platelets and monocytes for release into the blood when needed.

  • In the fetus, the spleen is involved in erythrocyte production.

• Surrounded by a fibrous capsule with trabeculae extending inward.

• Two distinct areas:

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

  • Red pulp: Surrounds white pulp; rich in macrophages that engulf old erythrocytes and bloodborne pathogens.