Ch_16_-_Lymphatic_and_Immune_Systems__2024_

Chapter 16: The Lymphatic System & Immunity

Lymphatic Pathways

• Definition: Vast collection of cells and biochemicals traveling in lymphatic vessels.

• Network of vessels: Assists in circulating fluids and is closely associated with the cardiovascular system.

• Functions of the Lymphatic System:

  • Transport excess fluid away from interstitial spaces back to bloodstream.

  • Absorb lipids from the digestive system and transport them to the bloodstream.

  • Defense against diseases.

Lymphatic Pathway Explanation

• Mechanism: Interstitial fluid moves into lymphatic capillaries to become lymph.

• Structure:

  • Lymphatic capillaries are microscopic, thin-walled tubes that parallel blood capillaries and are formed from simple squamous epithelium.

  • They merge into larger lymphatic vessels.

Components of Lymphatic Vessels

• Structure:

  • Similar to veins but have thinner walls composed of three layers:

    1. Inner endothelial lining.

    2. Middle smooth muscle.

    3. Outer connective tissue.

  • Featuring semilunar valves that prevent backflow of lymph.

Lymphatic Trunks and Collecting Ducts

• Functionality: Drain lymph from lymphatic vessels, named for the regions they serve (e.g., Lumbar for lower limbs, Bronchomediastinal for lungs and heart).

• Ducts:

  • Thoracic Duct: Larger, drains a significant portion of the body.

  • Right Lymphatic Duct: Smaller, drains the right side of the head, neck, upper limb, and thorax.

Edema and Lymphatic Surgery

• Post-Surgery: Disruption in lymph nodes, like during cancer excision, can cause fluid retention (edema).

Tissue Fluid & Lymph

• Definition of Lymph: Tissue fluid that has entered the lymphatic capillaries. Formation is dependent on tissue fluid formation.

Mechanisms of Fluid Formation

• Tissue Fluid Formation:

  • Capillary blood pressure filters water and small molecules from plasma into interstitial fluid, similar to blood plasma minus proteins.

• Lymph Formation:

  • Excess tissue fluid from plasma filtration leads to hydrostatic pressure, forcing fluid into lymphatic capillaries and preventing edema.

Lymphatic Flow Mechanisms

• Low hydrostatic pressure: Similar to veins and requires:

  • Muscle activity from skeletal muscles and respiratory movements.

  • Smooth muscle contractions in larger vessels.

  • Presence of valves.

Lymphatic Function

• Diverse Roles:

  • Absorbing dietary fats, returning proteins to the bloodstream, and collecting excess fluid and foreign particles to deliver to lymph nodes.

Overview of Lymphatic Tissues & Organs

• Types of Lymphatic Tissues:

  • Mucosa-associated lymphoid tissue (MALT): Includes lymphatic tissues like tonsils and Peyer's patches in the intestines.

Lymph Nodes

• Characteristic Features:

  • Bean-shaped, < 2.5 cm long, contain lymphocytes and macrophages.

• Functions:

  • Filters harmful particles from lymph, immune surveillance, and sites for lymphocyte production.

Thymus

• Description: Soft, bi-lobed gland located in the mediastinum, involved in T-cell development and maturation.

• Changes over Lifespan: Larger in childhood, shrinks at puberty, and diminishes in functionality in elderly.

Spleen

• Largest lymphatic organ: Located in the upper-left abdomen, its functions resemble those of lymph nodes but filter blood instead of lymph.

• Components: Composed of white pulp (lymphocytes) and red pulp (RBCs, lymphocytes).

Immune Response Overview

• Pathogen Defense: The immune system differentiates between innate (non-specific) and adaptive (specific) defenses against pathogens.

• First Line of Defense: Mechanical barriers (skin), chemical barriers, and immune responses through inflammation and fever.

Types of Immune Responses

• Adaptive Immune Response:

  • T-Cell Activation: Requires antigen-presenting cells (APCs) and leads to cytokine secretion for enhanced immune response.

  • B-Cell Activation: Involves antigen-receptor binding and cytokine signaling from T-helper cells which leads B cells to proliferate into antibody-producing plasma cells.

Immunization and Hypersensitivity Types

• Immunization Types: Includes active (via vaccines) and passive immunization (via antibody transfer).

• Hypersensitivity Reactions: Four types ranging from immediate reactions to delayed reactions involving exaggerated immune responses.

Lifespan Changes in Immune Function

• Decreased Effectiveness: Immune system function declines early in life, increasing risk for infections and cancers as thymus size reduces and T- and B-cell responses slow down.

Conclusion

• Understanding the Lymphatic System and Immunity: Essential for grasping how the body defends against infections and maintains fluid balance.