Chapter 20: The Lymphatic System and Lymphoid Organs and Tissues

Components and Primary Functions of the Lymphatic System

  • The Three Integral Parts of the Lymphatic System:

    • Lymphatic Vessels: A network of drainage tubes.
    • Lymph: The fluid contained within these vessels.
    • Lymph Nodes: Specialized organs that cleanse the lymph as it passes through.

  • Associated Lymphoid Organs and Tissues:

    • Specific organs include the Spleen, Thymus, and Red Bone Marrow.
    • Other tissues include the Palatine Tonsils, Lingual Tonsils, Pharyngeal Tonsils (Adenoids), Peyer's Patches, and the Appendix.

  • Functions of Lymphatic Vessels:

    • To drain lymph and return it to the bloodstream.
    • To ensure a one-way flow of fluid toward the heart.

  • Definition and Flow of Lymph:

    • Lymph is derived from interstitial fluid.
    • It flows in a one-way direction toward the heart and does not circulate back into the blood directly from the interstitial space.

  • Function of Lymphoid Organs and Tissues:

    • To collect lymph, house specialized immune cells, and provide a site for monitoring the body for pathogens.

Structure and Distribution of Lymphatic Vessels

  • Tissues Lacking Lymphatic Capillaries:

    • Teeth
    • Bone
    • Brain
    • Muscle

  • Hierarchical Order of Lymphatic Transport:

    • Fluid moves from the tissues toward the heart in the following sequence:
      1. Lymphatic Capillaries
      2. Collecting Vessels
      3. Lymphatic Trunks
      4. Lymphatic Ducts

  • Structural Adaptations for Fluid Uptake:

    • Anchoring Filaments: These structures anchor the endothelial cells to surrounding tissue, holding the vessel open when interstitial pressure increases.
    • Minivalves: The edges of adjacent endothelial cells overlap loosely, forming flaplike minivalves. These are high-permeability structures that open under low pressure to allow fluid in but close to prevent backflow into the interstitial space.

  • Lacteals:

    • Specialized lymphatic capillaries located in the finger-like villi of the intestinal mucosa. Their primary function is to transport absorbed fat from the small intestine to the bloodstream.

  • Lymphatic Trunks and Ducts:

    • Unpaired Trunk: The Intestinal Trunk is the only unpaired lymphatic trunk.
    • Paired Trunks: Bronchomediastinal, Subclavian, and Jugular trunks.
    • Drainage Patterns:
      • The Thoracic Duct drains lymph from the left side of the head, the left side of the thorax, the entire left upper limb, and both lower limbs (including the right lower limb).
      • The Right Lymphatic Duct empties its lymph into the venous circulation at the junction of the Right Subclavian Vein and the right internal jugular vein.

Lymphoid Cells, Tissues, and Organs

  • Functional Classification of Lymphoid Cells:

    • T Lymphocytes (T Cells): Cells that direct the immune response; when activated, some directly attack and destroy infected cells.
    • B Lymphocytes (B Cells): Cells that, when activated, produce effector cells that carry out an antibody-mediated immune response.
    • Plasma Cells: The progeny of activated B cells; they are responsible for producing and releasing antibodies.
    • Macrophages: Cells that phagocytize tissue cells, bacteria, and other foreign substances; they also help activate T cells.
    • Dendritic Cells: Spiny-looking cells that capture antigens and transport them to the lymph nodes.
    • Reticular Cells: Fibroblast-like cells that produce the reticular fiber stroma, which forms the structural framework of lymphoid organs.

  • Composition of Lymphoid Tissue:

    • Lymphoid tissue is largely composed of Reticular Connective Tissue.
    • Diffuse Lymphoid Tissue: A loose arrangement of lymphoid cells and some reticular fibers.
    • Lymphoid Follicles (Nodules): Solid, spherical bodies consisting of tightly packed lymphoid cells and reticular fibers.

  • Distinction Between Primary and Secondary Lymphoid Organs:

    • Primary Lymphoid Organs: Include the Thymus and Red Bone Marrow (RBM). These are the locations where B and T cells mature.
    • Secondary Lymphoid Organs: Include the Spleen, Lymph Nodes, Tonsils, Peyer’s Patches, and Appendix. These are the sites where lymphocytes first encounter antigens and become activated.

Anatomy and Function of Lymph Nodes

  • Protective Functions:

    • Filtering and cleansing the lymph (removing pathogens).
    • Housing and providing a site for lymphocyte activation.

  • Internal Structure of a Lymph Node:

    • Capsule: The fibrous outer covering.
    • Trabeculae: Connective tissue strands that extend inward from the capsule to divide the node into compartments.
    • Cortex: Contains Lymphoid Follicles with Germinal Centers, which are most abundant in B lymphocytes.
    • Medulla: Contains Medullary Cords (inward extensions of cortical tissue containing B and T cells) and Medullary Sinuses (large lymph capillaries).
    • Subcapsular Sinus: The space immediately beneath the capsule where lymph first enters.
    • Hilum: The concave region where lymph exits the node.

  • Lymphatic Circulation Through the Node:

    • Lymph enters the Convex side of the node through multiple Afferent Lymphatic Vessels.
    • Lymph exits the node at the Hilum on the Concave side via Efferent Lymphatic Vessels.
    • Flow Dynamics: The flow of lymph slows down within the node because there are fewer efferent vessels draining the node than there are afferent vessels feeding it. This stagnation allows time for macrophages and lymphocytes to perform their protective functions.

  • Major Regional Clusters:

    • Large clusters of lymph nodes occur near the body surface in the Cervical, Axillary, and Inguinal regions.

Anatomy and Function of the Spleen

  • Location: Situated on the left side of the body, directly under the diaphragm, curling around the anterior aspect of the stomach.

  • Primary Functions:

    • Removal of blood-borne pathogens and aged/damaged Red Blood Cells (RBCs).
    • Storage of platelets, monocytes, and iron.
    • Site for lymphocyte activation and proliferation.

  • Histology: White Pulp vs. Red Pulp:

    • White Pulp: Clusters around central arteries; composed mostly of lymphocytes; the site where immune functions take place.
    • Red Pulp: Consists of Splenic Cords and Splenic Sinusoids; contains large numbers of erythrocytes and macrophages; the site where worn-out RBCs are destroyed.

  • Functional Comparisons:

    • Filters Lymph: Lymph node only.
    • Filters Blood: Spleen only.
    • Has Lymphoid Follicles: Both spleen and lymph node.
    • Surrounded by Fibrous Capsule: Both spleen and lymph node.
    • Has Cortical and Medullary Regions: Lymph node only.
    • Site for Lymphocyte Activation: Both spleen and lymph node.

  • Surgical Note: If the spleen is removed, the Liver and Bone Marrow take over most of its blood-cleansing functions.

MALT (Mucosa-Associated Lymphoid Tissue)

  • Definition: A set of distributed lymphoid tissues located in mucous membranes throughout the body to protect entryways from pathogens.

  • Largest Collections of MALT:

    • Tonsils (Pharyngeal, Palatine, Lingual)
    • Peyer's Patches (in the small intestine)
    • Appendix

  • Tonsils:

    • Pharyngeal Tonsil: Located in the posterior wall of the nasopharynx; referred to as the Adenoid if enlarged.
    • Tonsillar Crypts: Deep pits in the tonsils that trap and filter out bacteria and particulate matter.

Anatomy and Function of the Thymus

  • Location: Found in the mediastinum, overlying the heart.

  • Role in Immunity:

    • Functions as the primary maturation site for T Lymphocytes.
    • Highly functional during youth and gradually atrophies with age.

  • Distinct Structural Features:

    • Unlike other lymphoid organs, the Stroma of the thymus consists of Epithelial Tissue, not reticular connective tissue.
    • Cortex: Contains large numbers of rapidly dividing lymphocytes.
    • Medulla: Contains Thymic (Hassall's) Corpuscles.
    • Blood-Thymus Barrier: A protective barrier that prevents immature T cells from being prematurely activated by antigens in the blood.

Questions & Discussion

  • Question: What happens to the flow of lymph when there are more afferent vessels than efferent vessels?

  • Answer: The flow slows down, which is essential to provide time for the lymph node to filter the fluid and for immune cells to encounter antigens.

  • Question: Which trunk is responsible for draining the majority of the body's lymph?

  • Answer: The Thoracic Duct, as it handles the entire lower body and the left upper half.

  • Question: What is the specific tissue type that forms the structural framework of most lymphoid organs?

  • Answer: Reticular connective tissue, except in the thymus where the stroma is epithelial.