Chapter:20 Lymphatic System and Lymphoid Organs

Lymphatic System Overview

  • Definition and Function

    • The lymphatic system is responsible for returning fluids that have leaked from blood vessels back into the bloodstream.

    • Components include:

    • Lymphatic vessels (drainage system)

    • Lymph (fluid in vessels)

    • Lymph nodes (cleanse lymph)

  • Role in Immune System

    • Lymphoid organs and tissues support the immune system’s structure and functions, crucial for the body’s defense against diseases.

    • Important components:

    • Spleen

    • Thymus

    • Tonsils

    • Lymph nodes

    • Other lymphoid tissues

Structure of the Lymphatic System

20.1 The Lymphatic System

  • Hydrostatic and Colloid Osmotic Pressures

    • Forces fluid out of blood capillaries at the arterial end and reabsorbs most at the venous end.

    • The remaining fluid (about 3 L daily) becomes interstitial fluid.

  • Lymphatic Vessels

    • Collect interstitial fluid, which contains proteins, and return it to the bloodstream.

    • Transport pathogens to lymph nodes and fats from the intestines.

Distribution and Structure of Lymphatic Vessels

  • Lymphatic Capillaries

    • Blind-ended vessels between tissue cells and blood capillaries.

    • Absent in bones and teeth; special adaptations:

    • Overlapping endothelial cells form minivalves allowing fluid entry.

    • Anchored by collagen filaments to the matrix.

    • More permeable than blood capillaries.

    • Special lacteals present in the small intestine for fat absorption.

  • Larger Lymphatic Vessels

    • Drain into collecting lymphatic vessels, which have three tunics like veins but thinner walls and more valves.

    • Major trunks include lumbar, bronchomediastinal, subclavian, jugular, and a single intestinal trunk.

  • Lymph Ducts

    • Thoracic duct and right lymphatic duct receive lymph from trunks:

    • Thoracic duct drains the rest of the body; empties into venous circulation at the junction of left internal jugular and left subclavian veins.

    • Right lymphatic duct drains right upper limb and right side of head and thorax at the junction of right internal jugular and right subclavian veins.

Lymph Transport

  • Lymphatic vessels are low-pressure conduits. The flow is facilitated by:

    • Skeletal muscle contraction (milking action)

    • Thoracic pressure changes during breathing

    • Pulse pressure from adjacent arteries

    • Rhythmic smooth muscle contractions in vessel walls.

    • Physical activity boosts lymph flow.

Lymphoid Cells and Tissues

20.2 Lymphoid Cells and Tissues

  • Composition of Lymphoid Cells

    • Immune system cells (e.g., lymphocytes - T and B cells) and supporting cells (reticular cells).

    • Antigens trigger immune responses and include bacteria, toxins, viruses, and cancer cells.

    • Functionality:

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

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

    • Macrophages: Phagocytize foreign substances and activate T cells.

    • Dendritic Cells: Capture and present antigens to T cells in lymph nodes.

  • Supporting Cells

    • Reticular Cells: Produce reticular fibers forming the supportive stroma in lymphoid tissues.

Lymphoid Tissue

  • Definition and Importance

    • Lymphoid tissue supports immune function and provides activation/proliferation spaces for lymphocytes.

    • Mainly consists of loose reticular connective tissue found in various organs, except thymus.

  • Types of Lymphoid Tissue

    • Diffuse Lymphoid Tissue: Loose arrangement in almost every body organ, mainly in mucous membranes.

    • Lymphoid Follicles (Nodules): Spherical bodies with tightly packed lymphocytes, often containing germinal centers (B cell proliferation sites). Found in:

    • Peyer’s patches in intestinal wall

    • Appendix

Lymphoid Organs

  • Functional Categories

    • Primary Lymphoid Organ: Site where T and B cells mature.

    • B cells mature in red bone marrow.

    • T cells mature in thymus.

    • Secondary Lymphoid Organs: Sites for activation of mature lymphocytes upon antigen encounter, including:

    • Lymph nodes

    • Spleen

    • Collections of MALT: tonsils, Peyer’s patches, appendix.

Lymph Nodes

20.3 Lymph Nodes

  • Definition and Function

    • Lymph nodes are vital secondary lymphoid organs located along lymphatic vessels, serving two main functions:

    • Cleansing lymph as it flows back to blood (filtering). Macrophages destroy pathogens.

    • Activation of the immune system, where lymphocytes encounter antigens.

  • Location and Structure

    • Hundreds of lymph nodes located mostly in connective tissue, with clusters in regions: inguinal, axillary, cervical.

    • Structure: Bean-shaped (length < 2.5 cm); surrounded by fibrous capsule with compartments created by trabeculae.

  • Histological Regions

    • Cortex:

    • Densely packed follicles, many with germinal centers (B cells).

    • Houses T cells and abundant dendritic cells.

    • Medulla:

    • Contains medullary cords with B cells, T cells, plasma cells, and lymph sinuses with macrophages for filtration.

Lymph Node Circulation

  • Flow: Lymph enters convex surface through afferent vessels, passes through sinuses, and exits via efferent vessels at the hilum.

  • Functionality: Fewer efferent vessels than afferent slow lymph flow, allowing for thorough filtration and immune response activation.

Clinical Significance

  • Buboes: Infected lymph nodes, possibly pus-filled, indicating overwhelmed lymphatic response (e.g., bubonic plague).

  • Lymphadenopathy: Swollen lymph nodes, which can signal infection or cancerous processes; cancer-infiltrated nodes are painful but swollen.

Spleen

20.4 Spleen Functions

  • Description

    • Largest lymphoid organ, located in the left abdominal cavity beneath the diaphragm, rich in blood supply.

    • Site for lymphocyte proliferation and immune responses.

    • Functions in cleansing the blood of aged/defective cells and platelets through macrophages.

  • Additional Functions

    • Iron storage and recycling of breakdown products from red blood cells.

    • Storage and release of platelets and monocytes as needed.

    • Potential site for fetal erythrocyte production.

  • Histological Components

    • Surrounded by fibrous capsule, divided into:

    • White Pulp: Immune function region.

    • Red Pulp: Where old blood cells are destroyed; consists of splenic cords and sinusoids filled with blood.

Clinical Significance

  • Rupture Risk: The spleen can rupture easily due to its thin capsule, leading to hemorrhage.

  • Splenectomy: Surgical spleen removal is less common now, due to the organ's self-repair capability. If removed, the liver and bone marrow often compensate for its functions.

Mucosa-Associated Lymphoid Tissue (MALT)

20.5 MALT Functions

  • MALT comprises lymphoid tissues found in mucous membranes in various body sites, including respiratory, genitourinary, and digestive tracts.

  • Acts as a barrier against pathogens attempting to enter.

  • Major collections include:

    • Tonsils

    • Peyer’s patches

    • Appendix

Tonsils

  • Form a ring around the pharynx, trapping pathogens from food/inhaled air.

  • Types of tonsils:

    • Palatine Tonsils: Largest and most commonly infected (posterior oral cavity).

    • Lingual Tonsils: At the base of the tongue.

    • Pharyngeal Tonsils (Adenoids): Located in nasopharynx.

    • Tubal Tonsils: Surrounding the openings of auditory tubes.

  • Partially encapsulated with deep crypts trapping debris and bacteria for immune response.

Peyer’s Patches

  • Clusters in the small intestine wall, similar in structure to tonsils; essential for preventing bacteria from breaching the intestinal wall.

Appendix

  • Tubular offshoot of the large intestine, also rich in lymphoid follicles, contributing to memory lymphocyte generation and defense against bacteria.

T Lymphocytes

20.6 T Lymphocytes

  • Thymus: A bilobed organ located in the neck extending into the mediastinum, primarily involved in T lymphocyte development.

  • Active during childhood; atrophies after puberty, replaced mostly by connective tissues.

  • Components: Divided into lobules containing:

    • Cortex: Rapidly dividing lymphocytes and macrophages.

    • Medulla: Contains fewer lymphocytes, features thymic corpuscles for regulatory T cell development.

  • Key Differences from secondary lymphoid organs:

    • Contains no follicles (no B cells).

    • Does not fight antigens directly, only facilitates T cell maturation.

    • Blood-thymus barrier prevents premature activation of T cells during maturation.