Chapter 20: Lymphatic System and Immunity

Overview

The lymphatic system is a vital component of the circulatory and immune systems, responsible for maintaining fluid balance, absorbing dietary fats, and defending the body against pathogens. This chapter explores the anatomy, functions, and clinical significance of the lymphatic system and immunity.

Functions of the Lymphatic System

Fluid Balance

• Interstitial Fluid Collection: Fluid that exits plasma enters the interstitial space and is picked up by lymphatic vessels, becoming lymph.

• Return to Circulation: Lymphatic vessels return fluid and solutes to the bloodstream, preventing tissue swelling (edema).

Fat Absorption

• Lacteals: Specialized lymphatic capillaries in the small intestine absorb lipid-soluble substances from the digestive tract.

Immune Functions

• Pathogen Filtration: Lymphatic tissues filter pathogens from lymph and blood.

• Lymphocyte Maturation: Lymphatic organs house and mature several types of lymphocytes (B cells and T cells).

Anatomy of the Lymphatic System

Main Components

• Lymphatic Vessels: Network of vessels transporting lymph throughout the body.

• Lymphatic Tissues/Organs:

  • Tonsils: Protect against pathogens entering through the mouth and nose.

  • Lymph Nodes: Filter lymph and house immune cells.

  • Spleen: Filters blood, stores platelets, and initiates immune responses.

  • Thymus: Site of T cell maturation.

• Lymph: Fluid containing white blood cells, proteins, and fats.

Lymphatic Vessels & Lymph Circulation

Pathway of Lymph Flow

1. Lymphatic capillaries

2. Lymph-collecting vessels

3. Lymph trunks

4. Lymphatic ducts

5. Venous circuit (subclavian vein)

Lymph is collected from tissues and returned to the bloodstream, maintaining fluid homeostasis.

Microscopic Structure and Function

Lymphatic Capillaries

• Structure: Thin-walled, highly permeable vessels with overlapping endothelial cells forming one-way flaps.

• Function: Allow entry of fluid, proteins, immune cells, and pathogens.

• Lacteals: Specialized capillaries in the small intestine for fat absorption.

Lymph Nodes

• Location: Found in clusters along lymphatic vessels (axillary, cervical, inguinal, mesenteric regions).

• Structure: Encapsulated with cortex (B cell-rich follicles) and medulla (mature B cells).

• Function: Filter lymph, trap pathogens, and facilitate immune cell activation.

Spleen

• Largest lymphoid organ: Located in the left upper quadrant (LUQ).

• Red pulp: Contains macrophages to destroy old RBCs.

• White pulp: Contains lymphocytes for immune response.

Thymus

• Location: Mediastinum, largest at puberty.

• Function: Secretes thymosins for T cell maturation.

Immune System Overview

Types of Immunity

• Innate (Non-specific) Immunity: Rapid response to all pathogens; includes surface barriers, phagocytes, inflammation, fever, and complement system.

• Adaptive (Specific) Immunity: Slower, targeted response to specific antigens; involves B cells (antibody-mediated) and T cells (cell-mediated).

Comparison Table: Innate vs. Adaptive Immunity

Innate Immunity: Defenses

Surface Barriers

• Skin: Physical barrier, acidic secretions.

• Mucous Membranes: Trap pathogens; cilia move them out.

• Stomach Acid: Destroys ingested pathogens.

Cellular Defenses

• Phagocytes: Macrophages, neutrophils, eosinophils ingest and destroy pathogens.

• Natural Killer (NK) Cells: Destroy virus-infected and cancerous cells.

• Basophils: Mediate inflammation.

Protein Defenses

• Complement System: Group of plasma proteins that enhance phagocytosis, lyse pathogens, and promote inflammation.

• Interferons: Proteins released by virus-infected cells to inhibit viral replication.

• Cytokines: Regulatory proteins affecting immune cell activity.

Inflammatory Response

• Cardinal Signs: Swelling, redness, heat, pain.

• Mediators: Histamine, heparin, cytokines, prostaglandins.

• Phagocyte Recruitment: Margination, diapedesis, chemotaxis, phagocytosis.

Fever

• Pyrogens: Cause hypothalamus to raise body temperature, enhancing immune response and inhibiting pathogen growth.

Adaptive Immunity: Specific Resistance

Antigens

• Definition: Substances that elicit an immune response; can be peptides, polysaccharides, lipids, or metals.

• Self-antigens: Normally ignored by the immune system.

Properties of Adaptive Immunity

• Specificity: T and B cells respond to specific antigens.

• Versatility: Immune system recognizes millions of antigens.

• Memory: Faster response upon re-exposure.

• Tolerance: Ignores self-antigens.

Major Histocompatibility Complex (MHC) Proteins

• Class I MHC: Found on all nucleated cells; present endogenous antigens to cytotoxic T cells (CD8+).

• Class II MHC: Found on antigen-presenting cells; present exogenous antigens to helper T cells (CD4+).

Cell-Mediated Immunity

• Cytotoxic T Cells (CD8+): Destroy infected or abnormal cells using perforin, lymphotoxin, and apoptosis-inducing signals.

• Helper T Cells (CD4+): Activate B cells and other T cells via cytokine secretion; form memory cells.

Antibody-Mediated (Humoral) Immunity

• B Cells: Produce antibodies specific to antigens; differentiate into plasma cells and memory B cells.

• Antibodies: Y-shaped proteins with variable and constant regions; neutralize pathogens, opsonize for phagocytosis, and activate complement.

Immunological Memory

• Memory B Cells: Enable rapid, efficient response upon re-exposure to the same antigen.

Forms of Immunity

• Active Immunity: Exposure to antigen (infection or vaccination) stimulates immune response and memory.

• Passive Immunity: Transfer of antibodies (maternal, therapeutic) provides temporary protection.

Clinical Connections

Lymphedema

• Cause: Removal or blockage of lymphatic vessels (surgery, infection).

• Effect: Accumulation of interstitial fluid, tissue swelling.

Disorders of the Immune System

• Hypersensitivity Disorders:

  • Type I: Immediate (allergies, anaphylaxis)

  • Type II: Antibody-mediated (blood transfusion reactions)

  • Type III: Immune complex (lupus, rheumatoid arthritis)

  • Type IV: Delayed (contact dermatitis, TB test)

• Immunodeficiency Disorders:

  • Primary: Genetic (SCID, hypogammaglobulinemia)

  • Secondary: Acquired (HIV/AIDS, cancer, medications)

• Autoimmune Disorders: Immune system attacks self-antigens (multiple sclerosis, type 1 diabetes, lupus).

Stress and Immune Response

• Chronic Stress: Glucocorticoids suppress immune function, increasing susceptibility to infection and disease.

Aging and Immunity

• Immunosenescence: Reduced thymic hormone production, fewer responsive T and B cells, decreased immune surveillance.

Summary Table: Lymphatic System Organs and Functions

Key Terms

• Lymph: Fluid in lymphatic vessels

• Lymphocyte: White blood cell (B cell, T cell)

• Antigen: Substance that triggers immune response

• Antibody: Protein produced by B cells to neutralize antigens

• MHC: Major histocompatibility complex, presents antigens to T cells

• Phagocyte: Cell that engulfs pathogens

Important Equations

• Osmotic Pressure (for fluid balance): where = osmotic pressure, = van't Hoff factor, = molarity, = gas constant, = temperature

Example

Example: A patient with lymphedema after lymph node removal experiences swelling in the affected limb due to impaired lymphatic drainage and accumulation of interstitial fluid.

Additional info: The notes have been expanded with academic context, definitions, and tables for clarity and completeness.