Chapter 21 Part A&B - BLG 102 - The Lymphatic System and Lymphoid Organs and Tissues

Lymph Nodes

  • Two main functions:

    1. Cleansing the lymph:

    • Act as lymph "filters."

    • Macrophages remove and destroy microorganisms and debris entering lymph.

    • Prevent unwanted substances from being delivered to blood.

    1. Immune system activation:

    • Provide a site for lymphocytes to become activated and initiate attack against antigens.

Overview of The Immune System

  • Resistance to disease:

    • Composed of two intrinsic systems:

    1. Innate (nonspecific) defense system:

      • First and second lines of defense.

        • First line of defense: External body membranes (skin and mucosae).

        • Second line of defense: Antimicrobial proteins, phagocytes, and other cells inhibit the spread of invaders; inflammation is the most important mechanism.

    2. Adaptive (specific) defense system:

      • Third line of defense that targets specific foreign substances (takes longer to react than innate).

Simplified Overview of Innate and Adaptive Defenses

  • Surface barriers:

    • Skin

    • Mucous membranes

  • Internal defenses:

    • Phagocytes

    • Natural killer cells

    • Inflammation

    • Antimicrobial proteins

    • Fever

  • Adaptive defenses:

    • Humoral immunity: B cells

    • Cellular immunity: T cells

Part 1 – Innate Defenses

  • Innate system: Uses the first and/or second lines of defense to stop attacks by pathogens (disease-causing microorganisms).

First Line of Defense: Surface Barriers

Definitions and Functions

  • Surface barriers:

    • Skin and mucous membranes, along with their secretions, form a physical barrier to most microorganisms.

    • Keratin: A substance resistant to weak acids, bases, bacterial enzymes, and toxins.

    • Mucosae provide similar mechanical barriers.

Protective Chemicals

  • Skin and mucous membranes produce protective chemicals that inhibit or destroy microorganisms:

    • Acid: The acidity of skin and certain mucous secretions inhibits growth; referred to as the acid mantle.

    • Enzymes:

    • Lysozyme found in saliva, respiratory mucus, and lacrimal fluid (eyes) kills many microorganisms.

    • Enzymes in the stomach similarly kill pathogens.

    • Mucin: Sticky mucus lining the digestive and respiratory tracts traps microorganisms.

    • Defensins: Antimicrobial peptides inhibiting microbial growth.

    • Other chemicals: Lipids in sebum and dermicidin in sweat are toxic to some bacteria.

Modifications in the Respiratory System

  • The respiratory system contains modifications to prevent pathogen entry.

    • Mucus-coated hairs in the nose: Trap inhaled particles.

    • Cilia of upper respiratory tract: Sweep dust- and bacteria-laden mucus toward the mouth.

  • Surface barrier breaches (e.g., nicks or cuts) trigger the internal second line of defense protecting deeper tissues.

Second Line of Defense: Cells and Chemicals

  • Innate system required if microorganisms invade deeper tissues; includes:

    • Phagocytes

    • Natural killer (NK) cells

    • Inflammatory response:

    • Involves macrophages, mast cells, white blood cells (WBCs), and inflammatory chemicals.

    • Antimicrobial proteins:

    • Interferons and complement proteins.

    • Fever

Phagocytes

Definition and Types

  • Phagocytes:

    • White blood cells ingesting and digesting foreign invaders.

    • Neutrophils:

    • Most abundant phagocytes; become phagocytic upon exposure to infectious material.

    • Use chemotaxis to track chemical "scent" left by pathogens (chemoattractants).

    • Macrophages:

    • Develop from monocytes and are chief phagocytic cells; known for their robust phagocytic capability.

    • Free macrophages: Wander through tissue spaces (e.g., alveolar macrophages).

    • Fixed macrophages: Permanent residents of specific organs (e.g., stellate macrophages in the liver, microglia in the brain).

Phagocytosis Process

  1. Recognition and Adherence:

    • Begins when a phagocyte recognizes and adheres to the pathogen's carbohydrate "signature."

    • Some pathogens have capsules hiding their surface carbohydrates, helping them evade phagocytosis.

  2. Opsonization:

    • Immune system uses antibodies or complement proteins as opsonins that coat pathogens.

    • Opsonins act as "handles" for phagocytes, enhancing their ability to grab pathogens.

  3. Engulfing: Form pseudopods that engulf the particle forming a phagosome.

  4. Phagosome And Lysosome Fusion: The phagosome fuses with a lysosome forming a phagolysosome.

  5. Destruction of Pathogen:

    • The phagolysosome is acidified, and lysosomal enzymes digest the particles.

    • Indigestible and residual waste is exocytosed from the phagocyte.

Phagocytosis (Continued)

  • Pathogens resistant to acidified lysosomal enzymes:

    • An example is tuberculosis bacteria.

    • Helper T cells can trigger macrophages to produce a respiratory burst which kills those pathogens by:

    • Releasing cell-killing free radicals.

    • Producing oxidizing chemicals (e.g., H2O2).

    • Increasing pH and osmolarity of the phagolysosome.

    • Defensins in neutrophils also aid by piercing the membrane of pathogens.

Natural Killer (NK) Cells

  • Definition:

    • Nonphagocytic, large granular lymphocytes that patrol blood and lymph.

  • Function:

    • Kill cancerous and virus-infected cells before the adaptive immune system is activated.

    • Attack cells lacking “self” cell-surface receptors.

    • Kill by inducing apoptosis (programmed cell death) in target cells.

    • Secrete chemicals that enhance the inflammatory response.

Inflammation: Tissue Response to Injury

  • Trigger: Inflammation occurs whenever body tissues are injured due to trauma, heat, irritating chemicals, or infections.

Benefits of Inflammation

  1. Prevents the spread of damaging agents.

  2. Disposes of cell debris and pathogens.

  3. Alerts the adaptive immune system.

  4. Sets the stage for tissue repair.

Cardinal Signs of Acute Inflammation

  • Four main signs:

  1. Redness

  2. Heat

  3. Swelling

  4. Pain

  • A fifth sign, impairment of function, may be present if movement in the area is hampered.

Stages of Inflammation

  1. Inflammatory chemical release.

  2. Vasodilation and increased vascular permeability.

  3. Phagocyte mobilization.

Inflammatory Chemical Release

  • Chemicals released into extracellular fluid (ECF) by injured tissues or immune cells, such as:

    • Histamine: Released by mast cells, crucial for inflammation.

    • Other inflammatory mediators:

    • Kinins, prostaglandins (PGs), cytokines.

    • Enhance vasodilation and permeability, attracting phagocytes to the affected area.

Chemical

Source

Physiological Effects

Histamine

Granules of mast cells and basophils.

Promotes vasodilation and increases permeability of local capillaries.

Kinins

Released in response to injury and microorganisms.

Same effects as histamine, also induces chemotaxis of leukocytes and enhances kinins generation.

Prostaglandins (PGs)

Plasma protein kininogen broken down into active kinins.

Induce pain and neutrophil chemotaxis.

Phagocyte Mobilization Process

  1. Leukocytosis: Release of neutrophils from bone marrow in response to leukocytosis-inducing factors from injured cells.

  2. Chemotaxis: Neutrophils follow chemical trails from inflamed sites.

  3. Diapedesis: Neutrophils flatten and squeeze through capillaries.

  4. Margination: Neutrophils cling to the capillary wall.

Antimicrobial Proteins

  • Function: Enhance innate defense by attacking microorganisms directly or hindering their ability to reproduce.

  • Major types of antimicrobial proteins:

    • Interferons: Proteins secreted by virus-infected cells that warn neighboring healthy cells.

    • Complement proteins: Approximately 20 blood proteins activated to destroy foreign substances.

  • Fever: An abnormally high body temperature that serves as a systemic response to invading microorganisms, raising the body’s thermostat and providing further defenses.