The Lymphatic System and Immunity – Study Notes

The Lymphatic System and Immunity

Introduction

The lymphatic system is a crucial component of the human body's defense mechanisms, responsible for maintaining fluid balance and protecting against infection and disease. This chapter explores the anatomy and physiology of the lymphatic system, its role in immunity, and the integration of innate and adaptive immune responses.

Anatomy of the Lymphatic System

Lymphatic System Components

• Lymphocytes: Primary cells of the lymphatic system, responsible for immune responses to pathogens, abnormal cells, and foreign proteins.

• Lymph: Interstitial fluid that enters lymphatic vessels.

• Lymphatic Vessels: Network that transports lymph from peripheral tissues to the venous system.

• Lymphoid Tissues and Organs: Include primary (red bone marrow, thymus) and secondary (lymph nodes, tonsils, MALT, appendix, spleen) lymphoid structures.

Lymphatic Vessels

• Lymphatic Capillaries: Closed at one end, larger diameters, thinner walls, and incomplete basement membranes. Overlapping endothelial cells act as one-way valves, allowing entry of fluid, solutes, and pathogens but preventing their return to intercellular spaces.

• Valves: Present in small to medium-sized vessels, prevent backflow of lymph, and are aided by skeletal muscle contraction.

• Lacteals: Specialized lymphatic capillaries in the small intestine, important for lipid absorption.

• Distribution: Absent in areas without blood supply (e.g., cornea).

Lymphatic Ducts and Drainage

• Superficial Lymphatics: Located in subcutaneous layers and mucous/serous membranes.

• Deep Lymphatics: Accompany deep arteries and veins.

• Collecting Vessels: Superficial and deep lymphatics converge to form lymphatic trunks, which empty into the thoracic duct (drains most of the body) and right lymphatic duct (drains right upper quadrant).

• Cisterna Chyli: Sac-like chamber at the base of the thoracic duct, receiving lymph from lower body regions.

• Lymphedema: Swelling due to blocked lymphatic drainage, leading to accumulation of interstitial fluid and increased risk of infection.

Lymphocytes and Immune Function

Classes of Lymphocytes

• T Cells: Cell-mediated immunity. Types include cytotoxic (destroy infected/abnormal cells), helper (activate T and B cells), regulatory (moderate immune response), and memory T cells (respond to previously encountered antigens).

• B Cells: Antibody-mediated (humoral) immunity. Differentiate into plasma cells that secrete antibodies and memory B cells.

• NK (Natural Killer) Cells: Immune surveillance, destroy abnormal cells, including cancer cells.

Lymphopoiesis

• Occurs in red bone marrow (all lymphocytes originate here), thymus (T cell maturation), and peripheral lymphoid tissues.

• T cells undergo selection in the thymus to prevent autoimmunity; most are eliminated if self-reactive.

Lymphoid Tissues and Organs

Lymphoid Tissues

• Lymphoid Nodules: Densely packed lymphocytes in areolar tissue, may cluster as Peyer's patches in intestines.

• MALT (Mucosa-Associated Lymphoid Tissue): Protects mucosal surfaces throughout the body.

• Tonsils: Large lymphoid nodules in the pharynx (pharyngeal, palatine, lingual). Inflammation is called tonsillitis.

Lymph Nodes

• Small, bean-shaped organs that filter lymph, removing pathogens before lymph returns to the bloodstream.

• Path of lymph: Afferent lymphatics → subcapsular space (dendritic cells) → outer cortex (B cells) → paracortex (T cells) → medullary sinus (B cells, plasma cells) → efferent lymphatics at hilum.

Thymus

• Located in the mediastinum, posterior to the sternum. Largest before puberty, shrinks with age (involution).

• Divided into lobules with cortex (T cell development) and medulla (T cells enter circulation).

• Produces thymosins, hormones essential for T cell maturation.

• Disorders: Myasthenia gravis (autoimmune, associated with thymus dysfunction).

Spleen

• Largest lymphoid organ, filters blood, removes abnormal RBCs, stores iron, and initiates immune responses to blood-borne antigens.

• Contains red pulp (RBCs, macrophages) and white pulp (lymphocytes).

• Rupture is a medical emergency; removal (splenectomy) increases infection risk.

Innate (Nonspecific) Immunity

Overview

• Present at birth, provides general defense against pathogens.

• Components: Physical barriers, phagocytes, immune surveillance (NK cells), interferons, complement system, inflammation, and fever.

Physical Barriers

• Skin and mucous membranes prevent pathogen entry.

• Secretions (sweat, sebum, mucus, enzymes) destroy or wash away pathogens.

• Hair and stratified squamous epithelium provide mechanical protection.

Phagocytes

• Engulf and destroy pathogens and debris.

• Types: Neutrophils (abundant, fast-acting), eosinophils (target antibody-coated pathogens), macrophages (fixed and free).

• Key processes: Emigration (leave capillaries), chemotaxis (move toward chemicals), adhesion (bind to targets).

Immune Surveillance (NK Cells)

• NK cells monitor tissues, recognize abnormal cells (e.g., cancer), and destroy them using perforins to induce cell lysis.

• Cancer cells may evade detection (immunological escape).

Interferons and Complement System

• Interferons (IFNs): Proteins released by infected cells, stimulate antiviral defenses in neighboring cells. Types: alpha (stimulate NK cells), beta (slow inflammation), gamma (stimulate macrophages).

• Complement System: Over 30 proteins that enhance antibody action. Pathways: classical (antibody-dependent), lectin (carbohydrate recognition), alternative (direct pathogen recognition). Effects: cell lysis, enhanced phagocytosis (opsonization), inflammation.

Inflammation and Fever

• Inflammation: Localized response to injury, characterized by redness, swelling, heat, pain, and sometimes loss of function. Involves release of prostaglandins, proteins, and ions.

• Fever: Systemic response, induced by pyrogens, raises body temperature to inhibit pathogens and accelerate repair.

Adaptive (Specific) Immunity

Types and Properties

• Active Immunity: Develops after exposure to antigens (naturally or via vaccination).

• Passive Immunity: Acquired by receiving antibodies (maternal or artificial).

• Properties: Specificity, versatility, memory, and tolerance (distinguishing self from nonself).

Antigen Recognition and Presentation

• Antigens are presented on cell membranes by MHC proteins (Class I: all nucleated cells; Class II: antigen-presenting cells).

• Triggers activation of T cells (cell-mediated) and B cells (antibody-mediated).

T Cell Activation and Differentiation

• CD Markers: CD8 (cytotoxic, regulatory T cells) respond to Class I MHC; CD4 (helper T cells) respond to Class II MHC.

• Activation requires antigen recognition and costimulation.

• Cytotoxic T cells destroy infected/abnormal cells; memory T cells provide rapid future responses; regulatory T cells moderate immune activity.

B Cell Activation and Antibody Production

• B cells are sensitized by antigen binding and require helper T cell cytokines for full activation.

• Activated B cells differentiate into plasma cells (secrete antibodies) and memory B cells.

Antibody Structure and Classes

• Antibodies (immunoglobulins) are Y-shaped proteins with variable (antigen-binding) and constant regions.

• Classes: IgG (most abundant), IgE (allergies), IgD (B cell sensitization), IgM (first produced), IgA (secretions).

Primary and Secondary Responses

• Primary Response: Slow, initial antibody production after first exposure.

• Secondary Response: Faster, stronger response upon re-exposure due to memory cells.

Mechanisms of Antibody Action

• Neutralization, prevention of pathogen adhesion, activation of complement, stimulation of inflammation, opsonization, attraction of phagocytes, precipitation, and agglutination.

Clinical Aspects of Immunity

Hypersensitivities and Anaphylaxis

• Excessive immune responses to antigens (allergens) can cause localized or systemic reactions.

• Immediate hypersensitivity (e.g., allergic rhinitis) involves IgE and mast cell activation.

• Anaphylaxis is a severe, systemic reaction that can lead to anaphylactic shock.

Immune Disorders

• Autoimmune Disorders: Immune system attacks self-antigens (e.g., rheumatoid arthritis, type 1 diabetes, multiple sclerosis).

• Transplant Rejection: Cytotoxic T cells attack transplanted tissue; immunosuppressive drugs can help prevent rejection.

• Immunodeficiency Diseases: Inadequate immune response (e.g., AIDS caused by HIV, which targets CD4 helper T cells).

• Age-Related Changes: Immune function declines with age, increasing susceptibility to infection and cancer.

Integration of Innate and Adaptive Immunity

• Both systems work together to defend against pathogens. Innate responses (e.g., phagocytosis, antigen presentation) often initiate and enhance adaptive responses (e.g., T and B cell activation).

• Effective immunity requires coordination between both branches.