The Lymphoid and Immune Systems

Chapter 21: The Lymphoid and Immune Systems

• Introduction

  • Microbiome

    • Microorganisms residing on/in the human body, some beneficial, others potentially harmful.

    • Constant exposure to harmful microorganisms from the environment.

    • Immune system protects the body.

  • Immune System

    • Not a traditional organ system; a cell population across all organs.

    • Concentrated in the lymphoid system.

    • Includes organs and vessels for fluid recovery, disease inspection, immune response activation, and fluid return to bloodstream.

The Lymphoid System

• Overview

  • Consists of lymphatic vessels and lymphoid tissues/organs producing immune cells.

  • Functions

    • Fluid Recovery:

      • 15% of filtered blood fluid enters lymphatic vessels after tissue filtering.

      • Lymphedema: Swelling due to lymphatic drainage interference.

    • Immune Surveillance:

      • Excess fluid captures foreign cells/chemicals, inspected in lymph nodes, activating immune responses.

    • Lipid Absorption:

      • Lacteals in the small intestine absorb dietary lipids not taken up by blood capillaries.

Components of the Lymphoid System

• Lymph

  • Clear, colorless fluid, similar to plasma but low in protein.

  • Variations in chemical composition depending on location (e.g., intestinal fluid).

  • Lymph flows at low pressure and a slower speed

• Lymphatic Vessels

  • Layers:

    • Tunica Interna: Endothelium and valves.

    • Tunica Media: Elastic fibers and smooth muscle.

    • Tunica Externa: Thin outer layer.

  • Lymphatic Capillaries: Collect tissue fluid and merge to form larger vessels and trunks.

  • Two collecting ducts:

    • Right Lymphatic Duct: Drains the right arm and side of head/thorax and empties into the right subclavian vein, facilitating the return of lymph to the circulatory system.

    • Thoracic Duct: Drains rest of the body and empties into the left subclavian vein, allowing for the reintroduction of lymph back into the bloodstream.

Lymphatic capillaries → collecting vessels → 11 lymphatic trunks→ two collecting ducts → subclavian veins

Flow of Lymph

• Driven by factors similar to venous blood return, without a central pump.

• Stimulated by:

  • Rhythmic contractions of lymphatic vessels.

  • Squeezing from skeletal muscles.

  • Pulsations of nearby arteries.

  • Thoracic (respiratory) pump.

Lymphoid Cells

• Types of Cells:

  • Neutrophils: Antibacterial.

  • Natural Killer (NK) Cells: Attack infected/cancerous cells.

  • T Lymphocytes (T cells): Mature in the thymus; respond selectively to pathogens.

  • B Lymphocytes (B cells): Differentiate into antibody-producing plasma cells upon activation.

  • Macrophages: Large phagocytic cells; antigen-presenting cells (APCs).

  • Dendritic Cells: APCs found in various tissues.

Lymphoid Organs

• Primary Lymphoid Organs: Red bone marrow and thymus; sites for T/B cell maturation.

• Secondary Lymphoid Organs: Lymph nodes, tonsils, spleen; sites for lymphocyte activity and immune challenges.

Major Lymphoid Organs

• Thymus:

  • Develops T cells and produces hormones regulating their activity.

  • Atrophies with age, diminishing new T cell production.

  • member of the endocrine, lymphoid, and immune system and plays a crucial role in the maturation of T lymphocytes, essential for adaptive immunity.

  • Thymus: Located in the upper chest, it is essential for the maturation of T cells, which play a crucial role in adaptive immunity.

• Lymph Nodes:

  • Located along lymphatic pathways.

  • Filter lymph; sites for immune responses to foreign antigens.

• Spleen:

  • Largest lymphoid organ; filters blood and monitors for pathogens.

  • Two types of pulp (white and red) serve specific functions in blood filtering and immune responses.

    • Red pulp: sinuses filled with erythrocytes

    • White pulp: lymphocytes, macrophages surrounding small branches of splenic artery

    • Functions of the spleen include recycling iron, producing antibodies, and serving as a reservoir for blood.

    • Location: located in the upper left quadrant of the abdomen, the spleen is positioned posterior to the stomach and lateral to the left kidney.

• Red Bone Marrow:

  • Primary site of hematopoiesis; produces red blood cells, white blood cells, and platelets, playing a crucial role in the immune system by generating lymphocytes.

• Lymphadenitis: swollen, painful lymph node responding to foreign antigen

• Lymphadenopathy: collective term for all lymph node diseases

Lymph Nodes and Metastatic Cancer

• Metastasizing cells easily enter lymphatic vessels

• Cancerous nodes are usually swollen, firm, painless

Immune Response

• Types of Immunity:

  • Innate Immunity:

    • First line: skin and mucous membranes.

    • Second line: leukocytes, macrophages, fever;

    • Third line: adaptive immunity, memory capabilities.

  • Immune System: widely distributed throughout the body, it includes various organs and tissues such as the thymus, spleen, and lymph nodes, which play crucial roles in the production and activation of immune cells.

  • Adaptive Immunity:

    • Specific responses to pathogens; include natural and artificial forms of active/passive immunity.

Fever and Inflammation

• Fever:

  • Body's systemic response to infection/injury; beneficial effects include promoting repair and inhibiting pathogen reproduction.

• Inflammation:

  • Local response to tissue injury/infection; involves increased blood flow, recruitment of immune cells, and containment/degradation of pathogens.

  • Neutrophils is to kill bacteria

  • Neutrophilla 5,000 cells/uL to 25000 cells/uL

Hypersensitivities and Autoimmunities

• Hypersensitivity Types:

  • Categorized by response mechanisms (Type I - IV).

  • Examples include allergies, asthma, and anaphylactic reactions.

• Autoimmune Diseases:

  • Self-tolerance failures lead to tissues being attacked by the immune system, resulting in a variety of autoimmune disorders.

Immunodeficiency Diseases

• Two major types:

  • Primary (hereditary) and Acquired (e.g., AIDS).

  • Immunodeficiency characteristics include vulnerability to infections and inadequate immune responses.

COVID-19 Pandemic

• Caused by SARS-CoV-2; infection characterized by respiratory issues and potential for severe inflammatory reaction.

• Vaccination is primary prevention method against severe illness and spread.