Chapter 21 Study Notes: The Lymphoid and Immune Systems

Chapter 21: The Lymphoid and Immune Systems

Introduction

Microbiome
  • Definition: Microorganisms that reside on and in the human body.
  • Classification: Some are beneficial, while others can be harmful.
  • Exposure: Constant exposure to potentially harmful microorganisms from the external environment.
  • Protection: The immune system protects the body against these harmful agents.
Immune System
  • Definition: Not a traditional organ system; instead, it is a population of cells that inhabit all organs to defend the body from disease agents.
  • Concentration of Immune Cells: Immune cells are particularly concentrated in the lymphoid system, which is a network of organs and vein-like vessels.

21.1 The Lymphoid System

Expected Learning Outcomes
  • List the functions of the lymphoid system.
  • Explain how lymph forms and returns to the bloodstream.
  • Name the major cells of the lymphoid system and state their functions.
  • Name and describe the types of lymphoid tissue.
  • Describe the structure and function of the red bone marrow, thymus, lymph nodes, tonsils, and spleen.
Overview of the Lymphoid System
  • Definition: The lymphoid (lymphatic) system comprises lymphatic vessels that penetrate nearly every tissue and a collection of lymphoid tissues and organs that produce immune cells.
  • Functions:
    • Fluid Recovery: Filters fluid from blood capillaries into tissue spaces; blood capillaries reabsorb about 85% of this fluid; the remaining 15% enters the lymphatic vessels.
    • Lymphedema: Swelling caused by interference in lymphatic drainage.
Functions of the Lymphoid System
  1. Fluid Recovery: Constant filtration leads to the absorption of excess fluid into lymphatic vessels.
  2. Immune Surveillance: Excess filtered fluid carries foreign cells and chemicals to lymph nodes, triggering immune responses.
  3. Lipid Absorption: Lacteals in the small intestine absorb dietary lipids not taken up by blood capillaries.
Components of the Lymphoid System
  • Lymph: The recovered fluid.
  • Lymphatic Vessels: Transport lymph throughout the body.
  • Lymphoid Tissue: Aggregates of lymphocytes and macrophages found in various organs.
  • Lymphoid Organs: Well-defined structures concentrated with lymphoid cells, covered by connective tissue capsules.
Anatomy of the Lymphoid System

Figure 21.1

  • Major structures include the thymus, spleen, and various lymph nodes located in key regions such as the cervical and axillary areas.
Lymphatic Vessels
  • Lymphatic Capillaries: Microscopic vessels that penetrate nearly all tissues of the body except for cartilage, cornea, bone, and bone marrow.
  • Structure: Capillary walls made up of overlapping endothelial cells, closed at one end. Gaps allow bacteria and cells to enter.
Types of Lymphatic Capillaries
  • Lymphatic Capillary Structure includes:
    • Tissue fluid accumulation.
    • Endothelial configuration similar to shingles due to overlapping.
    • Anchoring filaments tether cells to surrounding tissue.
Function of Larger Lymphatic Vessels
  • Larger vessels consist of three layers:
    • Tunica Interna: Endothelium and valves.
    • Tunica Media: Elastic fibers and smooth muscle.
    • Tunica Externa: A thin outer layer.
  • Flow Path: Lymphatic capillaries → collecting vessels → lymphatic trunks → collecting ducts → subclavian veins.
  • Lymphatic trunks include intestinal, right and left jugular, and subclavian trunks.
Flow of Lymph
  • Lymph does not flow through a pump (heart), but under low pressure through rhythmic contractions of vessels, skeletal muscle movement, thoracic (respiratory) pump activity, and rapid blood flow in subclavian veins.
Glymphatic System of the Brain
  • Glymphatic System: A lymphatic-like mechanism associated with neuroglia. Fluid flow driven by arterial pulsations promotes waste clearance.
  • Function: Drains metabolic wastes and delivers nutrients. Active during sleep, helps remove beta-amyloid linked to Alzheimer’s disease.
Lymphoid Cells
  • Types of Immune Cells:
    • Neutrophils: Antibacterial functions.
    • Natural Killer Cells (NK Cells): Target infected host cells and tumors.
    • T Lymphocytes (T cells): Mature in the thymus and play multiple roles in immune responses.
    • B Lymphocytes (B cells): Differentiate into plasma cells that produce antibodies.
    • Macrophages: Phagocytic cells that also present antigens to other immune cells.
    • Dendritic Cells: Antigen-presenting cells located in skin and mucous membranes.
Lymphoid Tissues
  • A. Structure: Aggregation of lymphocytes within connective tissues, such as found in mucous membranes
  • B. Types of Lymphoid Tissue:
    • Diffuse Lymphoid Tissue: Scattered lymphocytes, prevalent in body passages open to the exterior.
    • Lymphoid Nodules (Follicles): Dense clusters of lymphocytes, can be temporary or permanent.
    • Aggregated Lymphoid Nodules (Peyer’s Patches): Large clusters found in the small intestine.
Lymphoid Organs
  • Definition: Anatomically well-defined structures, with a connective tissue capsule separating lymphatic tissue from various tissues.
  • Primary Lymphoid Organs: Red bone marrow and thymus, where lymphocytes mature.
  • Secondary Lymphoid Organs: Lymph nodes, tonsils, and spleen, sites for immune activation.
Red Bone Marrow
  • Structure: Highly vascular, involved in hematopoiesis and immunity.
  • Function: Produces blood cells; secretes colony-stimulating factors to stimulate stem cells.
Thymus
  • Location: Bilobed organ in the mediastinum.
  • Function: Houses developing T cells; secrets hormones regulating T cell activity.
  • Involution: The thymus shrinks with age, impairing T cell production.
Lymph Nodes
  • Structure: Bean-shaped organs that cleanse lymph. Approximately 450 nodes in the body, features include cortex and medulla.
  • Function: Activate immune response through germinal centers in the cortex where B cells proliferate.
  • Lymphadenitis: Swelling in response to an infection.
Tonsils
  • Location: At the entrance of the pharynx.
  • Function: Guard against pathogens that are inhaled or ingested.
  • Types: Palatine (pair), pharyngeal (single), and lingual (numerous).
Spleen
  • Location: Largest lymphoid organ, beneath the diaphragm.
  • Function: Monitors and filters blood for foreign antigens. Involved in blood cell production in fetuses and certain adults.
  • Structures: Red pulp (erythrocyte sinuses) and white pulp (lymphocytes and macrophages).

21.2 Innate Immunity

Expected Learning Outcomes
  • Define pathogen and immune system.
  • Identify the body’s three lines of defense against pathogens.
  • Contrast innate and adaptive immunity.
  • Describe defensive functions of various leukocytes and macrophages.
  • Explain the immune roles of interferons and the complement system.
  • Explain how fever serves a protective role against pathogens.
  • Describe the process of inflammation and its cardinal signs.
Pathogens
  • Definition: Agents capable of producing disease, including bacteria, viruses, and fungi.
Three Lines of Defense
  1. First Line: Skin and mucous membranes, serve as barriers to pathogens.
  2. Second Line: Involves defenses such as leukocytes, macrophages, and fever; initiates if pathogens infiltrate first defenses.
  3. Third Line: Adaptive immunity; specific defenses against known pathogens.
Innate Immunity
  • Characteristics:
    • Localized effect; nonspecific; lacks memory.
  • In contrast with adaptive immunity, which is systemic, specific, and retains memory of pathogens.
External Barriers
  • Skin: Tough, dry, contains antimicrobial peptides (e.g., defensins), and acid mantle.
  • Mucous Membranes: Sticky mucus traps microbes; lysozyme destroys bacterial cell walls.
Leukocytes and Macrophages
  • Phagocytes: Types include neutrophils (kill bacteria), eosinophils (guard against parasites), basophils (release inflammatory chemicals), and monocytes (form macrophages).
Antimicrobial Proteins
  • Interferons: Proteins secreted in response to viral infections.
  • Complement System: Group of proteins activated sequentially during an immune response, aiding in infection response.
  • Complement Activation: Occurs via three pathways: classical, alternative, and lectin, leading to inflammation, immune clearance, phagocytosis, and cytolysis.
Fever
  • Definition: Abnormal elevation of body temperature.
  • Mechanism: Induced by pyrogens, enhances immune function, inhibits pathogen replication.
Inflammation
  • Definition: Local defensive response to tissue injury.
  • Cardinal Signs: Redness, swelling, heat, and pain.
  • Purpose: Contain pathogens, remove debris and initiate tissue repair.