Chapter 22: The Lymphatic System and Immunity – Structured Study Notes

Lymphatic System Anatomy and Physiology

Lymph Composition

The lymphatic system is a network of vessels and organs that helps maintain fluid balance and supports immune function. Lymph is a clear fluid that closely resembles plasma but contains fewer proteins.

• Lymph transports immune cells, nutrients, and waste products.

• It is collected from interstitial fluid and returned to the bloodstream.

Primary Lymphoid Organs

Primary lymphoid organs are where lymphocytes mature and become immunocompetent.

• Thymus: Site of T cell maturation. T cells complete their development here before entering circulation.

Lymphatic Ducts

Lymphatic ducts collect lymph from various regions and return it to the venous system.

• Thoracic duct: Drains lymph from the left side of the body above the diaphragm and the entire body below the diaphragm.

• Right lymphatic duct: Drains lymph from the right side of the head, neck, arm, and thorax.

• Cisterna chyli: A sac-like structure that collects lymph from the lower body and intestines, leading into the thoracic duct.

Lymphatic vs. Blood Capillaries

Lymphatic capillaries differ structurally and functionally from blood capillaries.

• Lymphatic capillaries originate as closed-ended tubes (pockets), while blood capillaries are open at both ends.

• Lymphatic capillaries have a complete basement membrane.

Lymphedema

Lymphedema is swelling caused by the accumulation of lymph due to obstruction of lymphatic vessels.

• Obstruction prevents normal drainage, leading to fluid buildup in tissues.

Secondary Lymphoid Organs

Secondary lymphoid organs are sites where immune responses are initiated.

• Spleen: Filters blood, removes abnormal blood cells, and stores iron.

• Lymph nodes: Filter lymph and are sites for immune cell activation.

• Tonsils: Trap pathogens entering the respiratory and digestive tracts.

Lymph Node Structure

Lymph nodes are organized into regions with specialized immune cells.

• Outer cortex: Contains B cells and macrophages.

• Paracortex: Contains T cells and dendritic cells.

• Medulla: Contains plasma cells and macrophages.

Mucosa-Associated Lymphoid Tissue (MALT)

MALT protects the epithelial linings of the digestive, respiratory, urinary, and reproductive tracts from pathogens.

Immune Surveillance

Natural Killer (NK) cells are responsible for immune surveillance, attacking foreign cells, virus-infected cells, and cancer cells without prior sensitization.

Thymus Involution

After puberty, the thymus reaches its maximum size and then begins to atrophy (shrink), reducing T cell production with age.

Innate Immunity

Antiviral Defense

Interferons are small proteins that trigger the production of antiviral proteins in the cytoplasm, helping cells resist viral infection.

Inflammatory Response

Inflammation is a localized tissue response to injury or infection.

• Mast cells release histamine (increases blood flow) and heparin (prevents local clotting).

Physical Barriers

Physical barriers are the first line of defense against pathogens.

• Desmosomes and tight junctions in epithelial cells form a continuous barrier.

• Other barriers include the skin and mucous membranes.

Benefits of Fever

Fever is an elevation of body temperature that enhances immune response.

• Increases the speed of immune cell movement.

• Inhibits the growth of some microorganisms.

Macrophage Classifications

Macrophages are classified based on their location and mobility.

• Fixed macrophages: Stationary within tissues.

• Free macrophages: Migrate throughout the body.

Complement System Activation

The complement system is a cascade of proteins that enhances the ability of antibodies and phagocytic cells to clear pathogens.

• Activated by three pathways: classical, lectin, and alternative.

Adaptive Immunity and Immune Cells

Properties of Adaptive Immunity

Adaptive immunity is characterized by four main properties:

• Tolerance: Immune system does not attack "self" antigens.

• Memory: Remembers previous encounters for faster, stronger responses.

• Specificity: Targets specific antigens.

• Versatility: Can respond to a wide variety of antigens.

Types of Immunity

Type	How Acquired	Example
Naturally acquired active	Exposure to live pathogen	Recovery from chickenpox
Artificially acquired active	Vaccination	MMR vaccine
Naturally acquired passive	Maternal antibodies	Antibodies in breast milk
Artificially acquired passive	Injection of antibodies	Rabies immunoglobulin

MHC Proteins

Major Histocompatibility Complex (MHC) proteins present antigens to T cells.

• Class I MHC: Found on all nucleated cells; recognized by CD8 T cells.

• Class II MHC: Found on antigen-presenting cells; recognized by CD4 T cells.

T Cell Function and Types

T cells are central to adaptive immunity and have specialized roles.

• Helper T cells (CD4): Stimulate activation and function of T and B cells.

• Cytotoxic T cells (CD8): Directly kill infected or tumor cells.

• Regulatory T cells: Moderate immune response, prevent autoimmunity.

T Cell Activation

T cells require two signals for activation: antigen recognition and costimulation (binding to a second site on the stimulating cell).

B Cell Sensitization and Activation

B cells are sensitized and activated in lymph nodes near the site of infection.

• Once activated, B cells differentiate into plasma cells that secrete antibodies.

Antibody Structure and Function

Antibodies are Y-shaped proteins with variable regions that bind specific antigens.

• Variable region: Binds to a specific antigenic determinant (epitope).

• Antibody actions:

  • Neutralization: Blocks harmful effects of pathogens or toxins.

  • Opsonization: Coats pathogens to enhance phagocytosis.

  • Agglutination: Clumps pathogens together.

  • Complement activation: Initiates the complement cascade.

Immune System and Aging

Aging affects immune function:

• B cells become less responsive; antibody levels rise more slowly.

• T cell function declines.

Immune Disorders and Modulators

Autoimmune Disorders

Autoimmune disorders occur when the immune system attacks the body's own tissues.

• Rheumatoid arthritis: Autoantibodies attack connective tissues around joints.

• Type 1 diabetes: Immune attack on pancreatic beta cells.

• Multiple sclerosis: Immune attack on myelin sheath of nerve cells.

Stress and Immunity

Chronic stress can suppress immune function through the long-term secretion of glucocorticoids, which have immunosuppressive effects.

Additional info: The above notes expand on the original points with definitions, examples, and context for clarity and completeness. Table added for types of immunity. For equations, none were present or required in this section.