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Introduction to the Immune System

• The human body is exposed to thousands of microorganisms daily.

• A powerful immune system exists to prevent infections from most of these microorganisms.

Components of the Immune System

• The immune system is divided into:

  • Nonspecific Immunity (Innate Immunity):

  • Present at birth.

  • Provides immediate defense against a wide variety of pathogens without targeting specific microorganisms.

  • Specific Immunity (Adaptive Immunity):

  • Develops upon exposure to specific pathogens over time.

  • Involves targeted responses to specific antigens.

Nonspecific Immunity: First Line Defenses

• The first line of defense includes barriers that prevent microorganisms from entering the body.

  • Physical Barriers:

  • Skin:

    • Acts as a primary barrier; damage (e.g., cuts) increases susceptibility to infections.

  • Body Fluids:

    • Tears: Help wash out bacteria and debris from the eyes.

    • Structural connection between eyes and nasal cavity explains why tear production leads to a runny nose.

    • Saliva: Reduces microbial growth; reduced saliva flow can lead to bad breath.

    • Ciliated Epithelial Cells:

    • Present in the respiratory tract; covered in mucus to trap and expel microorganisms, preventing them from reaching the lungs.

    • Influenza virus can damage these cells, leading to increased risk for bacterial infections like pneumonia during recovery.

    • Smoking impairs cilia function, increasing susceptibility to respiratory infections.

  • Chemical Barriers:

  • Stomach Acidity (Low pH):

    • Kills harmful microorganisms ingested with food or water.

  • Lysozyme:

    • Found in various body fluids; destroys peptidoglycan in gram-positive bacteria but not accessible to gram-negative bacteria due to their outer membrane.

    • Common fluids containing lysozyme include tears, saliva, and mucus.

Nonspecific Immunity: Second Line Defenses

• Activated when microorganisms breach initial barriers; also nonspecific.

• Leveraged by several components:

  • Leukocytes (White Blood Cells):

  • Key players in immune response.

    • Phagocytic Cells:

    • Neutrophils: First responders to infection.

    • Macrophages: Engulf and digest pathogens and dead cells.

    • Platelets: Assist in blood clotting to prevent blood loss.

    • Red blood cells transport oxygen but are not involved in the immune response.

  • Interstitial Fluid and Lymph:

  • Blood serum leaks into tissues and becomes interstitial fluid, later forming lymph within lymphatic vessels.

  • The lymphatic system plays a role in immune function by draining and filtering pathogens from the fluid.

Lymphatic System Structure and Function

• Lymphatic Vessels: Present in all tissues except the CNS and cartilage; transport lymph.

• Functions:

  • Maintains fluid balance via drainage and cleansing.

  • Absorbs fats with specialized vessels known as lacteals in the digestive system.

  • Provides immunological protections against pathogens.

• Movement of Lymph:

  • Lymph is moved by skeletal muscle contractions, smooth muscle contractions in larger vessels, and pressure changes during breathing.

  • One-way valves prevent backflow.

• Medical Condition - Edema:

  • Results from failure of the lymphatic system, leading to fluid buildup in tissues (e.g., during pregnancy or elephantiasis caused by parasites).

Lymph Nodes and Their Functions

• Lymph Nodes:

  • Functions:

  • As testing stations, they monitor blood plasma for invaders.

  • Produce lymphocytes to fight invaders.

  • Filter lymph before it returns to the bloodstream.

• Tonsils: Groups of lymph nodules that trap pathogens; can become inflamed (tonsillitis); their removal is known as a tonsillectomy.

• Peyer’s Patches:

  • Clusters of lymphatic tissue in the small intestine; role in immune response similar to tonsils.

Spleen: Structure and Functions

• Spleen:

  • Filters blood, removing worn-out red blood cells, and helps mount immune responses.

• Functions:

  • Cleans blood of pathogens.

  • Reservoir for extra oxygen-rich blood; responds to hemorrhage by releasing stored blood.

  • Despite its importance, removal (splenectomy) is possible; compensatory functions are taken over by the liver and other lymphatic tissues.

Thymus Gland and T Lymphocyte Maturation

• Thymus:

  • Largest relative to body size at youth; deteriorates after puberty.

• Function:

  • Responsible for maturation of T lymphocytes (T cells) through thymic education, eliminating potentially harmful lymphocytes.

  • Produces hormone thymosin, influencing immune response.

Role of Phagocytes and Inflammation

• Phagocytic Cells:

  • Key role in engulfing and digesting pathogens.

  • Nonspecific; respond to a wide array of microorganisms.

• Inflammation:

  • Increases blood flow to affected tissues, allowing immune cells to migrate into the area.

  • Involves increased vessel permeability and recruitment of neutrophils and macrophages.

Fever and the Complement System

• Fever:

  • Helpful in speeding up immune responses; does not directly kill pathogens at moderate temperatures.

• Complement System:

  • A series of proteins activated to enhance immune responses via:

  • Opsonization: Coating of microbes to facilitate phagocytosis.

  • Inflammation: Induction through histamine release.

  • Membrane Attack Complex: Formation of holes in bacterial membranes, leading to lysis.

Activation of the Complement System

• Activation triggers via:

  • Antibody-Antigen Complex: Antibodies binding to antigens can activate complement proteins.

  • Direct Interaction: Certain microorganisms can directly trigger complement proteins.

  • Pathogen-Specific Compounds: Various compounds on microbes may also activate the complement system.

• Results: Regardless of the activation pathway, outcomes remain the same: opsonization, inflammation, and membrane lysis.

Antiviral Response

• Interferons:

  • Produced by infected cells; warn surrounding cells of the viral threat.

  • Neighboring cells produce antiviral proteins in anticipation of an impending viral infection, slowing down replication until specific immunity activates.

Conclusion

• The immune system integrates nonspecific and specific components to protect against a broad spectrum of pathogens, working effectively to maintain health and respond dynamically to infections.