Ch.%2021%20The%20Lymphatic%20and%20Immune%20Systems

Chapter 21: The Lymphatic and Immune Systems

Immunity

• The immune system is not a specific organ system but a cell population that resides in all organs, defending the body against disease agents.

• It is especially concentrated in the lymphatic system, which plays a crucial role in immunity.

Lymphatic System Functions

• Fluid Recovery: Returns approximately 15% of fluid from tissues back to the bloodstream.

• Immunity: Filters lymph fluid to remove foreign matter and pathogens.

• Lipid Absorption: Lacteals in the gastrointestinal tract absorb dietary lipids.

Components of the Lymphatic System

• Lymph: The recovered fluid from tissues.

• Lymphatic Vessels: Transport lymph throughout the body.

• Lymphatic Tissues: Contain lymphocytes and macrophages that contribute to the immune response.

• Lymphatic Organs: Sites where defense cells are concentrated.

Lymph Overview

• Lymph is a clear fluid similar to plasma, originating as extracellular fluid and varies in composition based on the body location.

Lymphatic Vessels

• Capillaries: Have overlapping endothelial cells, are closed at one end, and form valve-like flaps that open with high interstitial fluid pressure.

• Structures: Composed of a three-layer structure:

  • Tunica Interna: Endothelium and valves.

  • Tunica Media: Contains elastic fibers and smooth muscle.

  • Tunica Externa: A thin outer layer.

• Lymphatic vessels increase in size to larger vessels and eventually drain into lymphatic trunks.

• Major collecting ducts: Right lymphatic duct and thoracic duct, both empty into left and right subclavian veins.

Flow of Lymph

• Lymphatic vessels enable lymph to flow at low pressure, assisted by:

  1. Smooth muscle contractions in tunica media.

  2. Pulsations from nearby arteries.

  3. Skeletal muscle contractions.

  4. Respiratory pressure changes (thoracic pump).

Lymphatic Cells

• Neutrophils: Phagocytic cells that help engulf pathogens.

• Lymphocytes:

  • T Cells: Mature in the thymus and are crucial for specific immune responses.

  • B Cells: Mature in bone marrow and are involved in producing antibodies.

• Natural Killer Cells: Large lymphocytes that attack bacteria, viruses, and cancer cells by releasing perforin and granzymes to induce apoptosis.

• Macrophages: Phagocytic cells that develop from monocytes; act as antigen-presenting cells (APCs) which display antigens to T cells.

• Dendritic Cells: Highly mobile, present in epidermis and mucous membranes; act as APCs.

• Reticular Cells: Stationary cells that help form the stroma of lymphatic organs.

Lymphatic Tissues

• Diffuse Lymphoid Tissue: Simplest form, found in all organs; abundant in areas open to the exterior (MALT).

• Lymphatic Nodules (Follicles): Masses of lymphocytes and macrophages that can be temporary or permanent.

Lymphatic Organs

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

• Secondary Organs: Lymph nodes, tonsils, and spleen; sites where immunocompetent cells operate.

Red Bone Marrow

• Plays a crucial role in hematopoiesis and immunity; contains highly vascular material that aids in the maturation of blood cells.

Thymus

• A bilobed organ where T cells mature; contains a cortex and medulla for T cell development.

• Degenerates with age (involution).

Lymph Nodes

• Numerous (around 450); cleanse lymph and serve as activation sites for T and B cells.

• Structure: Fibrous capsule with trabeculae, containing reticular fibers and lymphocyte-rich stroma divided into cortex and medulla.

Tonsils

• Function to guard against ingested or inhaled pathogens; part of MALT and can become inflamed (tonsillitis).

Spleen

• The largest lymphatic organ; has two types of tissue:

  • Red Pulp: Contains erythrocytes.

  • White Pulp: Contains lymphocytes and macrophages that recycle blood components.

Peyer’s Patches

• Clusters of lymphoid follicles located in the distal small intestine and appendix, part of MALT.

Immunity: Three Lines of Defense

1. First Line: Skin and mucous membranes as physical barriers.

2. Second Line: Innate defense mechanisms (leukocytes, macrophages, inflammation).

3. Third Line: Adaptive immunity which has a memory for quicker responses upon re-exposure to pathogens.

Innate Immunity

• Local, nonspecific responses lacking memory categorized into:

  • Physical and chemical barriers (e.g., skin, mucus).

  • Protective proteins (e.g., complement proteins, interferons).

  • Protective cells (e.g., natural killer cells).

  • Protective processes (e.g., fever, inflammation).

Protective Proteins

• Complement Proteins: Enhance inflammatory response and help eliminate pathogens.

• Interferons: Protect neighboring cells from virus infections and activate immune responses. A dying cell’s last contribution to society.

• Alert the unaffected cells that a infection is near.

Protective Cells

• Natural Killer Cells: Attack infected or malignant cells.

• Other leukocytes like dendritic cells, macrophages, and neutrophils are involved in phagocytosis and inflammation.

Protective Processes

• Fever: Inhibits bacterial replication and enhances activity of antimicrobial proteins. Innate Immunity

• Inflammation: Local response to injury involving vasodilation, increasing blood flow, and recruiting leukocytes for pathogen containment and cleanup.

Adaptive Immunity

• Characterized by systemic effects, specificity, and memory.

• Two types: Cellular (T lymphocytes) and Humoral (B lymphocytes).

Passive/Active – Natural/Artificial Immunity

• Natural Active: Antibody production following pathogen exposure (e.g., illness).

• Artificial Active: Antibody production due to vaccination.

• Natural Passive: Antibodies transferred from mother to child during pregnancy or breastfeeding.

• Artificial Passive: Temporary immunity through administered antibodies (e.g., tetanus).

Antigens

• Any molecule that can trigger an immune response, characterized by unique structures allowing for distinct recognition between self and non-self.

Antigen Presenting Cells (APCs)

• Required for T cell activation; include dendritic cells, macrophages, and B cells presenting antigens via major histocompatibility complex (MHC) proteins.

Lymphocytes

• NK Cells: Involved in immune surveillance and destruction of infected cells.

• T Lymphocytes: Integral in cellular immunity, distinguished into four classes: Cytotoxic, Helper, Regulatory, and Memory cells.

T Cell Activation

• Involves recognition of antigen-presenting cells, clonal selection for T-cell subclasses, and subsequent attack by cytotoxic T cells and helper T cells.

Helper T-Cells

• Activate macrophages and stimulate B cells while promoting inflammation.

Humoral (Antibody-Mediated) Immunity

• Mediated by B lymphocytes and involves antibodies that tag pathogens for destruction.

Antibodies

• Immunoglobulin (Ig): Defensive proteins produced in response to pathogens, consisting of monomers with heavy and light chains.

Antibody Classes

• IgA: Protects mucosal surfaces.

• IgD: Activates B cells.

• IgE: Involved in allergic reactions.

• IgG: Predominant antibody in circulation; confers long-term immunity.

• IgM: First antibody produced during an immune response.

Humoral Immune Response

1. Recognition: Antigen binds to B cell and is processed for T-helper cell recognition.

2. Attack: Antibodies mark pathogens.

3. Memory: Formation of memory B cells for faster future response.

Humoral Attack Mechanisms

• Neutralization: Antibodies block pathogen regions.

• Complement Fixation: Initiates destruction of pathogens through immune complexes.

• Agglutination: Clumping of pathogens for effective phagocytosis.

• Precipitation: Removal of soluble antigens that can be cleared by immune responses.

Humoral Memory

• First immune response establishes memory, leading to a faster and stronger response upon re-exposure to the same antigen.