The Lymphatic System and Body Defenses: Structured Study Notes

The Lymphatic System and Body Defenses

Introduction

The lymphatic system is a crucial component of the human body's defense mechanisms, working alongside the cardiovascular and immune systems to maintain fluid balance and protect against disease. This chapter explores the structure and function of the lymphatic system, the mechanisms of body defenses, and common disorders related to immunity.

The Lymphatic System

Components of the Lymphatic System

• Lymphatic vessels: Network of vessels that transport lymph, a fluid containing excess tissue fluid and plasma proteins, back to the bloodstream.

• Lymphoid tissues and organs: Includes lymph nodes, spleen, thymus, tonsils, Peyer’s patches, and appendix.

Functions:

• Returns escaped fluids from the cardiovascular system to the blood.

• Essential in body defense and resistance to disease.

Lymphatic Vessels

• Lymph: Excess tissue fluid and plasma proteins carried by lymphatic vessels.

• If not collected, fluid accumulates in tissues, causing edema.

• Lymphatic vessels form a one-way system, with lymph flowing only toward the heart.

• Lymph capillaries: Weave between tissue cells and blood capillaries, have overlapping walls forming minivalves, and are anchored by filaments to connective tissue.

• Lymphatic collecting vessels: Collect lymph from capillaries and return it to veins near the heart via the right lymphatic duct (right arm, right side of head/thorax) and thoracic duct (rest of body).

• Transport is aided by skeletal muscle action, thoracic pressure changes during breathing, and smooth muscle in vessel walls.

Lymph Nodes

• Filter lymph before it returns to the blood, removing bacteria, viruses, cancer cells, and foreign substances.

• Defense cells:

  • Macrophages: Engulf and destroy pathogens.

  • Lymphocytes: Respond to foreign substances.

• Structure: Kidney-shaped, less than 1 inch long, surrounded by a capsule, divided into cortex (lymphocyte follicles) and medulla (macrophages).

• Lymph enters via afferent vessels, flows through sinuses, and exits via fewer efferent vessels, slowing flow for filtration.

Other Lymphoid Organs

• Spleen: Filters blood, destroys old red blood cells, stores platelets, and provides immune surveillance.

• Thymus: Overlies the heart, functions mainly in youth, programs T lymphocytes.

• Tonsils: Trap and remove pathogens entering the throat; inflammation is called tonsillitis.

• Peyer’s patches & Appendix: Located in the intestine, contain macrophages to destroy bacteria.

• Mucosa-associated lymphoid tissue (MALT): Includes Peyer’s patches, tonsils, and appendix; protects respiratory and digestive tracts.

Body Defenses

Overview of Body Defenses

• Innate (nonspecific) defense system: Immediate, general protection (skin, mucous membranes, inflammation, proteins).

• Adaptive (specific) defense system: Targets specific invaders, provides immunity, and has memory.

Innate (Nonspecific) Body Defenses

• First line of defense: Surface barriers (skin, mucous membranes) and their secretions (acidic secretions, mucus, gastric juice, lysozyme in saliva/tears).

• Second line of defense: Cellular and chemical defenses (natural killer cells, phagocytes, inflammation, antimicrobial proteins, fever).

Inflammatory Response

• Triggered by tissue injury; cardinal signs: redness, heat, pain, swelling.

• Involves release of histamine and kinins, vasodilation, increased capillary permeability, and attraction of phagocytes.

• Functions: Prevents spread of damage, disposes of debris/pathogens, sets stage for repair.

Phagocytosis

• Neutrophils and macrophages engulf and digest foreign material.

• Phagocytic vesicle fuses with lysosome for digestion.

Antimicrobial Proteins

• Complement proteins: At least 20 plasma proteins; activated by complement fixation, form membrane attack complexes (MACs) to lyse cells, enhance inflammation.

• Interferons: Secreted by virus-infected cells, protect neighboring cells from viral multiplication.

Fever

• Systemic response to infection; hypothalamus resets body temperature higher via pyrogens.

• High temperature inhibits bacterial growth and speeds repair.

Adaptive (Specific) Body Defenses

• Third line of defense; immune response targets specific antigens.

• Characteristics: Antigen-specific, systemic, memory.

• Two arms:

  • Humoral immunity: Antibody-mediated, B cells produce antibodies in body fluids.

  • Cellular immunity: Cell-mediated, T cells target infected or abnormal cells.

Antigens

• Any substance provoking an immune response (nonself antigens: proteins, nucleic acids, carbohydrates, lipids, pollen, microorganisms).

• Self-antigens: Body’s own molecules, usually not antigenic to self but can be to others (basis for transplant rejection).

• Haptens: Incomplete antigens, become antigenic when combined with body proteins (e.g., poison ivy, detergents).

Cells of the Adaptive Defense System

• Lymphocytes: B cells (humoral immunity, mature in bone marrow), T cells (cell-mediated immunity, mature in thymus).

• Antigen-presenting cells (APCs): Dendritic cells, macrophages, and B cells; present antigens to T cells.

• Immunocompetence: Ability to recognize and bind specific antigens, genetically determined.

Humoral (Antibody-Mediated) Immune Response

• B cells bind antigens, undergo clonal selection, and differentiate into plasma cells (produce antibodies) and memory cells.

• Active immunity: Body produces antibodies (natural: infection; artificial: vaccination).

• Passive immunity: Antibodies obtained from another source (natural: mother to fetus; artificial: immune serum); no memory, short-lived.

• Monoclonal antibodies: Laboratory-produced, specific for one antigen; used in diagnostics and therapy.

Antibody Structure and Classes

• Four polypeptide chains (T- or Y-shaped); variable regions bind antigens, constant regions determine class.

• Five classes: IgM (fixes complement), IgA (secretions), IgD (B cell activation), IgG (crosses placenta, most abundant), IgE (allergies).

Class	Main Function
IgM	First antibody released, fixes complement
IgA	Found in secretions (mucus, tears)
IgD	Activates B cells
IgG	Most abundant, crosses placenta, fixes complement
IgE	Involved in allergic responses

Antibody Functions

• Complement fixation, opsonization, neutralization, agglutination, precipitation.

Cellular (Cell-Mediated) Immune Response

• T cells activated by antigen presentation (APCs) and double recognition (antigen + self protein).

• Effector T cells:

  • Helper T cells: Stimulate B and T cells, release cytokines.

  • Cytotoxic (Killer) T cells: Destroy infected/cancer cells by releasing perforin and granzymes.

  • Regulatory T cells: Suppress immune response to prevent overactivity.

  • Memory cells: Provide long-term immunity.

Organ Transplants and Rejection

• Types of grafts: Autografts (self), isografts (identical twin), allografts (same species), xenografts (different species).

• Success depends on tissue matching; immunosuppressive therapy is required to prevent rejection.

Disorders of Immunity

Allergies (Hypersensitivities)

• Abnormal, vigorous immune responses to harmless antigens.

• Immediate (acute) hypersensitivity: IgE-mediated, rapid onset (hives, anaphylaxis).

• Delayed hypersensitivity: T cell-mediated, slower onset (contact dermatitis).

Autoimmune Diseases

• Immune system attacks self-tissues due to breakdown of self-tolerance.

• Examples: Rheumatoid arthritis, myasthenia gravis, multiple sclerosis, Graves’ disease, type 1 diabetes mellitus, systemic lupus erythematosus, glomerulonephritis.

• Causes: New self-antigens, foreign antigens resembling self-antigens.

Immunodeficiencies

• Congenital or acquired defects in immune function.

• Examples: Severe combined immunodeficiency disease (SCID), AIDS (caused by HIV attacking helper T cells).

Developmental Aspects of the Lymphatic System and Body Defenses

• Lymphatic vessels form from veins; lymph nodes present by week 5 of development.

• Thymus and spleen are first lymphoid organs to appear; other organs develop later.

• Immune response develops around birth; efficiency declines with age, increasing susceptibility to infection, cancer, and autoimmune diseases.