Study Notes on the Lymphatic System and Lymphoid Organs

Chapter 20: The Lymphatic System and Lymphoid Organs and Tissues

Lymph in Lymph Node

  • Immune scanning: Immune response is mounted.

  • Lymph exits the node as immune cells go to the site of infection.

Lymphatic System Overview

  • The lymphatic system plays a crucial role in maintaining fluid balance in the body and supporting the immune system.

  • Functions of the Lymphatic System:

    • Returns fluids that leaked from blood vessels back to the bloodstream.

  • Components of the Lymphatic System:

    • Lymphatic Vessels (Lymphatics): Network of vessels that transport lymph.

    • Lymph: Clear fluid that circulates within these vessels.

    • Lymph Nodes: Structures that cleanse lymph and filter pathogens.

Lymphoid Organs and Tissues

  • Lymphoid organs provide the structural basis of the immune system and house phagocytic cells and lymphocytes.

  • Main structures include:

    • Spleen

    • Thymus

    • Tonsils

    • Other Lymphoid Tissues scattered throughout the body.

Functions of the Lymphatic System

  • Lymphatic vessels return interstitial fluid and leaked plasma proteins back to blood (approximately 3L/day).

  • Once interstitial fluid enters lymphatic vessels, it is referred to as lymph.

Lymphatic Vessels: Structure and Distribution

  • One-way System: Lymph flows toward the heart through:

    • Lymphatic capillaries

    • Collecting lymphatic vessels

    • Lymphatic trunks

    • Lymphatic ducts

Lymphatic Capillaries

  • Lymphatic capillaries are:

    • Highly permeable: Able to take up proteins, cell debris, pathogens, and cancer cells.

    • Structure:

    • Endothelial cells overlap loosely to create one-way minivalves.

    • Anchored by collagen filaments, preventing collapse.

    • When tissue inflammation occurs, lymphatic capillaries increase uptake of larger particles such as debris and pathogens.

Unique Characteristics of Lymphatic Capillaries

  • Absent in:

    • Teeth

    • Bones (including bone marrow)

  • Present in limited locations in the central nervous system, mainly in meninges.

  • Lacteals: Specialized lymphatic capillaries that transport absorbed fats from the small intestine to the bloodstream, responsible for the milky white lymph called chyle.

Flow of Lymph

  • Lymph flows through increasingly larger and thicker-walled channels:

    • Sequence:

    • Collecting vessels

    • Lymphatic trunks

    • Lymphatic ducts

  • Collecting Lymphatic Vessels:

    • Features:

    • Similar to veins but with thinner walls and more internal valves.

    • Anastomose more frequently.

  • Collecting vessels in the skin travel with superficial veins, while deep vessels travel with arteries.

Lymphatic Trunks

  • Major lymphatic trunks that drain large areas of the body include:

    • Paired Lumbar Trunks

    • Paired Bronchomediastinal Trunks

    • Paired Subclavian Trunks

    • Paired Jugular Trunks

    • Single Intestinal Trunk

Major Lymphatic Ducts

  • Right Lymphatic Duct: Drains lymph from the right upper limb, and the right side of head and thorax; empties into blood at the junction of the right internal jugular and subclavian veins.

  • Thoracic Duct: (much larger)

    • Receives lymph from the rest of the body, including the two large lumbar trunks draining lower limbs and the intestinal trunk draining digestive organs.

    • Begins in about half of individuals as the enlarged sac known as the cisterna chyli.

    • Empties into the blood at the junction of the left internal jugular and subclavian veins

Lymph Transport Mechanisms

  • Lymph is transported by:

    • The milking action of skeletal muscle

    • Pressure changes during breathing

    • Valves to prevent backflow

    • Pulsations of nearby arteries

    • Contractions of smooth muscle in lymphatic vessel walls (except smallest vessels)

  • Lymph transport is sporadic and slow, enhanced by adjacent tissue movement during physical activity.

Lymphoid Cells

  • Lymphocytes: Main warriors of the immune system, generated in red bone marrow.

    • Types:

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

    • B Cells (B Lymphocytes): Produce plasma cells which secrete antibodies, marking antigens for destruction.

  • Other Lymphoid Cells:

    • Macrophages: Phagocytize foreign substances, aiding in T cell activation.

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

    • Reticular Cells: Produce reticular fiber stroma to support other cells in lymphoid organs and tissues.

Lymphoid Tissue

  • This tissue is critical for effective immune function:

    1. Houses and provides a proliferation site for lymphocytes.

    2. Offers a surveillance vantage point for lymphocytes and macrophages.

  • Composed largely of reticular connective tissue (except thymus).

  • Types of Lymphoid Tissue:

    • Diffuse Lymphoid Tissue: Loose arrangement of lymphoid cells and reticular fibers in almost every organ.

    • Lymphoid Follicles (Nodules): Spherical structures with tightly packed lymphoid cells and reticular fibers; often contain germinal centers of B cells.

Lymphoid Organs

  • Grouped into two categories:

    • Primary Lymphoid Organs: Where B and T cells mature (Red Bone Marrow and Thymus).

    • Secondary Lymphoid Organs: Where mature lymphocytes first encounter antigens (Lymph Nodes, Spleen, Tonsils, Peyer’s Patches, Appendix).

Lymph Nodes

  • The most important secondary lymphoid organs, clustered along lymphatic vessels:

    • Functions:

    • Cleansing lymph by macrophages destroying microorganisms and debris.

    • Activation of the immune system through lymphocyte activation.

Structure of a Lymph Node

  • Shape and size can vary; generally bean-shaped.

  • Features:

    • External Fibrous Capsule

    • Trabeculae: Extend inward to divide nodes into compartments.

    • Regions: Cortex and Medulla (distinct histological regions).

Cortex and Medulla

  • Cortex:

    • Contains lymphoid follicles with germinal centers, heavily populated with B cells.

    • Dendritic cells are abundant in this area, crucial for preparing B and T cells for action.

    • Deep cortex contains T cells in transit, continually circulating to perform surveillance.

  • Medulla:

    • Composed of medullary cords and sinuses, containing macrophages and reticular fibers which support lymphocytes.

Circulation in Lymph Nodes

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

  • The arrangement of veins allows for stagnation, giving lymphocytes and macrophages time to act on pathogens before lymph exits the node.

Spleen

  • Largest lymphoid organ, with critical functions:

    • Site of lymphocyte proliferation and immune response.

    • Cleanses blood of aged cells and pathogens.

    • Recycles breakdown products of RBCs (e.g., iron).

    • Stores blood platelets and monocytes; at times, may produce erythrocytes in the fetus.

Structure of the Spleen

  • Contained within a fibrous capsule; has trabeculae.

  • Two distinct areas:

    • White Pulp: Immune functions take place, primarily composed of lymphocytes.

    • Red Pulp: Destroys aged RBCs and contains numerous macrophages and erythrocytes.

Mucosa-associated Lymphoid Tissue (MALT)

  • Guards against pathogens at body entry points, located in mucous membranes:

    • Largest collections found in tonsils, Peyer's patches, appendix, and mucosa of respiratory, genitourinary organs, and the digestive tract.

Tonsils

  • Form a protective ring of lymphatic tissue around the pharynx:

    • Types of Tonsils:

    • Palatine Tonsils: Largest and most commonly infected.

    • Lingual Tonsils: Located at the base of the tongue.

    • Pharyngeal Tonsil: Enlarged termed adenoids, located in nasopharynx.

    • Tubal Tonsils: Surround auditory tube openings.

  • Contain lymphoid follicles with germinal centers and crypts for trapping and destroying pathogens; promote a versatile immune response through early exposure.

Peyer's Patches and Appendix

  • Peyer's Patches: Large clusters of lymphoid follicles found in the wall of the distal small intestine, function similarly to tonsils.

  • Appendix: Offshoot of the large intestine containing lymphoid follicles, functions to destroy bacteria and generate memory lymphocytes.

Thymus

  • Bilobed organ essential for T lymphocyte maturation, located in the inferior neck and mediastinum:

    • Most active early in life, increasing in size, and atrophies after puberty.

    • Consists of two regions:

    • Cortex: Contains rapidly dividing lymphocytes (T cells) and macrophages.

    • Medulla: Fewer lymphocytes; involved in regulatory T cell development to prevent autoimmunity.

    • Unique Characteristics:

    1. Lacks B cells and follicles.

    2. Does not directly fight antigens; functions in T cell maturation only.

    3. Contains epithelial cells in stroma instead of reticular fibers, providing an environment for T cell maturation.