Chapter_20__Part_II__-_Innate_Immune_System

Chapter 20 (Part II): The Innate Immune System

The Immune System

• Definition: A group of specialized cells known as leukocytes (white blood cells) that protect the body from infections, toxins, and other harmful substances.

• Leukocytes: These cells are found circulating in the bloodstream, patrolling tissues, and residing in lymphoid organs or lymph.

• Collaboration with Lymphatic System: The Immune System works alongside the lymphatic system in responding to infections.

• Host: The individual protected by the immune system is referred to as the host.

Three Lines of Defense for the Host

1. Surface Barriers

2. Innate Immune System (Innate Immunity)

3. Adaptive Immune System (Adaptive Immunity)

Surface Barriers (1st Line of Defense)

• Definition: Physical and secretory mechanisms that prevent pathogens from initiating infections.

• Physical Barriers:

  • Skin: Largest organ, acts as a barrier against pathogens.

  • Hair: Prevents entry of pathogens at body openings.

• Protective Secretions:

  • Sebum: Acidic oil deterring pathogen growth.

  • Mucus: Traps microorganisms in the gastrointestinal and respiratory tracts.

  • Sweat: Contains antimicrobial proteins.

  • Cerumen (Ear Wax): Protects against pathogens in ears.

  • Stomach Acid: Destroys pathogens via acidic conditions.

  • Tears and Saliva: Contain antibodies helpful in pathogen elimination.

Innate Immunity (2nd Line of Defense)

• Definition: Immune response present at birth.

• Components of Innate Immune System:

  1. Complement System: Contains antimicrobial proteins that eliminate pathogens.

  2. Inflammatory Response: A reaction to cellular damage from various causes.

  3. Innate Immune Cells: Includes neutrophils, monocytes, macrophages, dendritic cells, etc.

  4. Cytokines: Communication proteins among immune cells, enhancing responses.

• Interaction with Adaptive Immunity: Innate immunity provides a broad defense but works in conjunction with adaptive immunity.

Innate vs. Adaptive Immunity

• Innate Immunity: Non-specific, differentiates self from foreign material using pathogen-associated molecular patterns (PAMPs).

• PAMPs: Conservatively found on pathogens, recognized by receptors on innate immune cells.

• Adaptive Immunity: Specific, with memory for complex infections, acting when the innate response is insufficient.

Cells of the Innate Immune System

Neutrophils

• Role: Primary soldiers of the immune system. Most abundant leukocytes (~60%).

• Function: Destroy pathogens via phagocytosis; release antimicrobial proteins; major component of pus.

Monocytes

• Role: Largest leukocytes; account for 4-8% of WBCs.

• Function: Transition into macrophages or dendritic cells after entering tissues.

Macrophages

• Role: Officers of the immune system.

• Function: Coordinate immune response, conduct phagocytosis, release cytokines, and act as antigen-presenting cells (APCs).

Dendritic Cells

• Role: Sentinels that efficiently present antigens.

• Function: Phagocytose pathogens, process their antigens, and activate T-cells in lymph nodes.

Eosinophils

• Role: Target parasites predominantly.

• Function: Phagocytose pathogens; account for about 3% of WBCs.

Mast Cells and Basophils

• Mast Cells: Trigger inflammatory response upon allergen detection; found in tissues.

• Basophils: Rare granulocytes (<1% of WBCs); release mediators during allergic responses.

Natural Killer (NK) Cells

• Role: Hybrid function in innate and adaptive immunity.

• Function: Kill aberrant cells (tumor cells, infected cells) by releasing cytotoxic molecules.

• Mechanism: Employ perforins and granzymes to induce apoptosis in target cells.

Cytokines

• Definition: Proteins facilitating communication among immune cells.

• Categories:

  • Tumor Necrosis Factor (TNF): Facilitates inflammatory response, attracts phagocytes; can induce septic shock.

  • Interferons (IFNs): Inhibit viral replication, activate immune responses against intracellular agents.

  • Interleukins (IL): Promote proliferation and activation of leukocytes.

  • Chemokines: Involved in the movement of immune cells to sites of infection.

The Complement System

• Definition: A series of antimicrobial proteins vital for pathogen destruction.

• Pathways:

  1. Classical Pathway: Antibody-dependent activation.

  2. Lectin Pathway: Antibody-independent activation via mannose-binding lectin.

  3. Alternative Pathway: Activation from spontaneously hydrolyzed complement proteins.

Functions of Pathways and C3b

• C3b Functions:

  • Opsonization: Tags pathogens for easier phagocytosis.

  • Neutralization: Prevents viral infectivity by binding.

  • Immune Clearance: Aids in clearing immune complexes through phagocytosis.

Inflammatory Response

• Definition: Reaction to cellular injury; involves the release of inflammatory mediators.

• Signs (FWPPRS): Fever, Warmth, Pain, Pus, Redness, Swelling.

• Role of Mediators: Increase blood flow, enhance capillary permeability, attract leukocytes, and promote pain signaling.

Inflammatory Response Mechanism

• Vasodilation: Increases blood flow to the area (hyperemia); leads to warmth and redness.

• Increased Permeability: “Leaky” capillaries allow proteins and leukocytes to enter the tissue to combat infection.

• Chemotaxis: Attracts more immune cells to the site.

• Cellular Response: Macrophages are key first responders, calling for reinforcements through cytokine signaling.

Summary

• The innate immune system is crucial for immediate defense. It is non-specific and provides broad protection, working in synergy with the adaptive immune system for enhanced responses against specific pathogens.