The Lymphoid and Immune Systems

Anatomy and Physiology of the Lymphoid and Immune Systems

  • Introduction
    • The immune system is not a distinct organ system; rather, it consists of a population of cells defending the body from diseases.
    • Immune cells are primarily concentrated within the lymphoid system, which includes a network of organs and vessels responsible for fluid recovery and monitoring for disease agents.

The Lymphoid System

  • Functions

    • Fluid Recovery: Collects interstitial fluid (filtrate) from tissues, which becomes lymph, and returns it to the bloodstream.
    • Filtering Pathogens: Specialized cells in lymphoid tissues filter pathogens and initiate immune responses.
    • Transport of Lipids: Transports lipids and fat-soluble vitamins from the digestive tract.
    • Lymphedema: Condition caused by impaired lymphatic drainage, resulting in swelling.

  • Structure

    • Composed of lymphatic vessels, immune cells, and specialized tissues and organs (e.g., lymph nodes, spleen).

  • Lymphatic Capillaries

    • Entry point of interstitial fluid into the lymphatic system, present in nearly all tissues except the CNS, bone marrow, and teeth; characterized by a one-cell thick structure with valves.

  • Larger Lymphatic Vessels

    • Right Lymphatic Duct: Drains lymph from the right arm and right side of the head/thorax, emptying into the right subclavian vein.
    • Thoracic Duct: Drains lymph from the left side of the body, begin in the abdomen (cisterna chyli), and empties into the left subclavian vein.
    • Pathway summary: Lymphatic capillaries → collecting vessels → 11 lymphatic trunks → 2 collecting ducts → subclavian veins.

  • Secondary Lymphoid Organs: Include lymph nodes (filter lymph), spleen (filters blood), and lymphoid nodules (cluster of lymphocytes).

Immune Function and Types of Immune Cells

  • Barrier Defenses

    • Skin and mucous membranes that act as physical barriers to pathogen entry.

  • Types of Immune Responses

    • Innate Immune Response:
    • Rapid response, nonspecific, involving phagocytes and inflammation.
    • Adaptive Immune Response:
    • Slower, specific, involves white blood cells (WBCs) such as lymphocytes.
      • Humoral Immunity: B cells produce antibodies.
      • Cell-Mediated Immunity: T cells attack infected cells.

  • Types of Lymphocytes:

    • B Lymphocytes (B cells): Produce antibodies, 10-15% of WBCs.
    • T Lymphocytes (T cells): 80% of WBCs, includes:
    • Cytotoxic T cells (killer T cells) that destroy infected or cancer cells.
    • Helper T cells that secrete cytokines to stimulate other immune cells.
    • Suppressor T cells that regulate immune response.
    • Memory T cells that quickly respond to previously encountered pathogens.
    • Natural Killer (NK) Cells: 5-10% of WBCs, destroy abnormal cells, including tumor and virally infected cells.

Development of Lymphocytes

  • Primary Lymphoid Organs:

    • Bone Marrow: Site of B cell development and immature T cells release.
    • Thymus: Site of T cell maturation.

  • Secondary Lymphoid Organs:

    • Include lymph nodes and spleen contains lymphoid follicles and germinal centers for B cell proliferation.

  • T-cell Development:

    • Positive selection ensures T cells can recognize MHC molecules, contributing to immunocompetence.

Immune Responses Against Pathogens

  • Definition of Pathogens

    • Agents that cause disease, including viruses, bacteria, and fungi.

  • Mechanisms of Innate vs Adaptive Immunity:

    • Innate Immunity: First line of defense, non-specific, immediate response without memory.
    • Adaptive Immunity: Highly specific, develops after exposure, retains memory for faster subsequent responses.

  • Steps in Immune Response:

    • Clonal Selection and Expansion: T and B cells that recognize a specific pathogen undergo rapid division, creating a pool of cells against that pathogen.
    • B cell Activation: Requires a signal from antigens and helper T cells, leading to the production of memory B cells and plasma cells that secrete antibodies.

  • Antibody Functions:

    • Neutralization: Antibodies block pathogen binding.
    • Opsonization: Tag pathogens for destruction by phagocytes.
    • Activation of the Complement System: Enhances inflammation, leads to cell lysis.

  • Immunological Memory:

    • Secondary immune responses are more rapid and robust due to memory cells.

    • Antibodies are classified into five classes (IgM, IgG, IgA, IgE, IgD) based on structure and function.