lymphatic

Lymphatic System Overview

  • Definition and General Structure

    • The lymphatic system is a network of vessels, nodes, and organs responsible for transporting lymph, a fluid containing infection-fighting white blood cells.

    • Components include:

    • Large veins

    • Small veins

    • Venous system

    • Heart

    • Arterial system

    • Large lymphatic vessels

    • Elastic arteries (conducting vessels)

    • Muscular arteries (distributing vessels)

    • Arterioles (resistance vessels)

    • Terminal arterioles

Functions of the Lymphatic System

  • Fluid Balance

    • Returns interstitial fluid and leaked plasma back to the cardiovascular (CV) system.

    • Once interstitial fluid enters lymphatics, it is termed lymph.

    • Collects plasma proteins and digested fat molecules.

  • Defense Mechanisms

    • Acts as a defense against harmful organisms and chemicals.

    • Types of immune defense:

    • Nonspecific Defense (Innate)

      • Includes skin, mucus membranes, fever, and phagocytes.

    • Specific Defense (Acquired Immunity)

      • Humoral Immunity: Involves B cells that become plasma cells, which produce antibodies to bind with specific antigens.

      • Cell-Mediated Immunity: Involves T cells that directly destroy foreign cells.

Parts of the Lymphatic System

  • Components include:

    • Lymphatic vessels

    • Lymphoid tissue

    • Red bone marrow (primary site for leukocyte production)

    • Leukocytes (white blood cells)

    • Lymph nodes

    • Lymphoid organs such as:

    • Tonsils

    • Thymus (primary lymphoid organ)

    • Spleen

    • Appendix

    • Peyer’s patches

Lymphatic Capillaries

  • Overview

    • Originate as blind-ended structures.

    • Highly permeable to larger particles (cell debris, pathogens).

    • Endothelial cells overlap to form minivalves.

    • All lymph flows toward the heart.

  • Unique Features

    • Not found in:

    • Teeth

    • Bones

    • Cartilage

    • Central Nervous System (CNS)

  • Lacteals

    • Present in intestinal mucosa, absorb digested fat and deliver fatty lymph (chyle) to the blood.

    • Transport fat to the liver and adipose tissue.

  • Vessel Structure

    • Capillaries unite to form larger vessels that resemble veins, but are thinner with more valves and anastomoses.

    • They contain:

    • Tunica externa

    • Tunica media

    • Tunica interna

    • Connect to lymph nodes at various intervals.

Lymphatic Trunks

  • Formation

    • Formed by the union of the largest collecting ducts.

  • Types

    • Paired trunks:

    • Lumbar

    • Bronchomediastinal

    • Subclavian

    • Jugular

    • A single intestinal trunk.

Major Channels of Lymph Delivery

  • Channels

    • Lymphatics ultimately deliver lymph into two main channels:

    • Right Lymphatic Duct:

      • Drains the right side of the head and neck, right arm, and right thorax.

      • Empties into the right subclavian vein.

    • Thoracic Duct:

      • Inferior terminus called the Cisterna Chyli.

      • Drains the rest of the body.

      • Empties into the left subclavian vein.

Lymph Movement

  • Mechanisms

    • The lymphatic system lacks a pumping mechanism.

    • Movement occurs via:

    • Skeletal muscle contractions (SMT) during activities, which push fluid through vessels.

    • Actions of breathing and arterial contractions during rest.

  • Valve Function

    • Numerous valves maintain one-way flow of lymph, always directed toward the heart.

Lymphoid Cells

  • Key Types of Cells

    • Lymphocytes are crucial for immune function.

    • Main varieties include:

    • T cells (T lymphocytes): Manage immune response, attack and destroy foreign cells, produced in red bone marrow and matured in the thymus.

    • B cells (B lymphocytes): Produce plasma cells that secrete antibodies, developed in red bone marrow.

  • Function of Lymphocytes

    • Protect against antigens:

    • Anything perceived as foreign, including bacteria, viruses, mismatched red blood cells (RBCs), or cancer cells.

Lymphocyte Maturation

  • Preparation during Maturation

    • Each cell type becomes:

    • Immunocompetent: Development of special receptors on cell surface to recognize specific antigens.

    • Self-tolerant: Ability to differentiate between normal and abnormal cells.

  • Clonal Selection

    • Travel to secondary organs to encounter antigens, allowing lymphocytes to bloom, forming memory and effector cells for faster future response.

Other Lymphoid Cells

  • Functions

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

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

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

Lymph Nodes

  • Overview

    • Principal lymphoid organs, typically bean-shaped and embedded within connective tissue.

    • Hundreds exist along lymphatic vessels, with large collections near the body surface in regions like:

    • Inguinal

    • Axillary

    • Cervical region

  • Functions

    • Filter lymph - macrophages destroy microorganisms and debris.

    • Activate lymphocytes in the immune system.

Structure of a Lymph Node

  • General Structure

    • Bean-shaped with an external fibrous capsule.

    • Trabeculae (partitions) extend inward, creating compartments.

  • Histological Regions

    • Divided into two main regions:

    • Cortex:

      • Contains follicles with germinal centers rich in dividing B cells; dendritic cells nearly encircle these follicles.

      • Deep cortex houses T cells.

    • Medulla:

      • Contains medullary cords with B cells, T cells, and plasma cells.

      • Lymph sinuses contain macrophages.

Circulation in the Lymph Nodes

  • Pathway of Lymph Flow

    • Lymph enters via afferent lymphatic vessels.

    • Travels through a large subcapsular sinus and smaller sinuses.

    • Exits at the hilus via efferent lymphatic vessels, with fewer efferent vessels allowing lymph flow to stagnate for immune functions.

Spleen

  • Description

    • Largest lymphoid organ, served by the splenic artery and vein entering/exiting at the hilus.

  • Functions

    • Site for lymphocyte proliferation and immune surveillance and response.

    • Cleanses blood of aged cells, platelets, and debris.

  • Storage

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

    • Stores blood platelets.

    • Site of fetal erythrocyte production, which usually ceases after birth.

Structure of the Spleen

  • Areas of the Spleen

    • Two distinct areas:

    • White Pulp:

      • Surrounds central arteries, primarily consists of lymphocytes on reticular fibers, involved in immune functions.

    • Red Pulp:

      • Comprises venous sinuses and splenic cords, rich in macrophages for disposal of worn-out RBCs and pathogens.

Thymus

  • Size Changes with Age

    • Located in the inferior neck, extending into the mediastinum, overlapping the heart.

    • Increases in size and activity during childhood; stops growing in adolescence and gradually atrophies afterward.

    • Atrophy correlates with poorer immune responses in older adults.

  • Structure

    • Contains lobes with an outer cortex and an inner medulla.

    • Cortex: Densely packed with lymphocytes and scattered macrophages.

    • Medulla: Fewer lymphocytes, contains thymic (Hassall’s) corpuscles involved in T cell maturation.

  • Functionality

    • Differentiates from other lymphoid organs as it strictly functions in T lymphocyte maturation and does not directly fight antigens.

    • Stroma consists of star-shaped epithelial cells providing an environment for T lymphocyte development.

Tonsils

  • Overview

    • Simplest lymphoid organs forming a ring of lymphatic tissue around the pharynx.

  • Types

    • Palatine Tonsils: Located at the posterior end of the oral cavity.

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

    • Pharyngeal Tonsil: Found in the posterior wall of the nasopharynx.

    • Tubal Tonsils: Surround openings of the auditory tubes into the pharynx.

  • Functionality

    • Contain follicles with germinal centers and are not fully encapsulated.

    • Epithelial tissue overlying tonsil masses invaginates, creating tonsillar crypts that trap and destroy bacteria and particulate matter.

Aggregates of Lymphoid Follicles

  • Peyer’s Patches

    • Clusters of lymphoid follicles found in the wall of the distal small intestine; similar structures are in the appendix.

    • Function to destroy bacteria, preventing breaches in the intestinal wall and generate “memory” lymphocytes for future immunological memory.

Immunoglobulins (Antibodies)

  • Types and Functions

    • IgG: Smallest but most common; found in all body fluids, crucial against bacterial and viral infections; the only antibody that can cross the placenta.

    • IgA: Protects surfaces exposed to the external environment (e.g., nose, trachea, digestive tract), also present in saliva, tears, and blood.

    • IgM: Largest antibody; acts as first responders in infections; found in blood and lymph fluid.

    • IgE: Found in lungs, skin, and mucous membranes; causes reactions against foreign substances like pollen, causing allergies.

    • IgD: Found in small amounts in tissues lining the abdominal and thoracic cavities; function remains unclear.

Leukocytes (White Blood Cells)

  • Classification

    • Lymphocytes: 20-25% of leukocytes; play a pivotal role in the immune system.

    • Agranular leukocytes include:

    • Monocytes: 3-8%

    • Lymphocytes: 20-25%

    • Granular leukocytes include:

    • Neutrophils: 60-70%

    • Eosinophils: 2-4%

    • Basophils: 0.5-1%