Chapter 15: The Immune System - Innate Immunity

Overview

• Focus on the innate immune system, the body's first two lines of defense against microorganisms.
• Chapter 15 covers first and second lines of defense.
• Chapter 16 will focus on the third line of defense, or acquired immunity.

First Line of Defense

• Physical Barriers: Skin acts as a physical barrier to prevent pathogen entry.
• Chemical Barriers: Fluids and secretions like mucus, tears, ear wax, saliva, and stomach acid contribute to pathogen defense.
• Other Mechanisms: Mucociliary blanket in lungs helps remove pathogens.

Second Line of Defense

• Nonspecific Protective Cells and Fluids: Do not target specific pathogens.
  • Includes phagocytic cells such as neutrophils, macrophages, and dendritic cells.
  • Includes antimicrobial products like enzymes and proteins.
• Inflammatory Response: Involves increased blood flow, swelling, and increased temperature to combat infection.
• Fever: Elevates body temperature to inhibit microbial growth and improve immune response.
• Innate Immunity Characteristics:
  • Does not improve with repeated exposures.
  • Lacks memory and specific target recognition.

Innate Immune Responses

• Phagocytosis Steps:
  1. Chemotaxis: Directed movement of phagocytes to infection site.
  2. Ingestion: Phagocytes engulf pathogens.
  3. Phagolysosome Formation: Fusion of phagosome with lysosome to digest pathogens.
  4. Exocytosis: Expulsion of debris.
• Pattern Recognition: Phagocytes use Pattern Recognition Receptors (PRRs) to identify Pathogen-associated Molecular Patterns (PAMPs) like peptidoglycan.

Inflammation

• Signs: Redness, heat, swelling, pain, and sometimes loss of function.
• Functions: Mobilizes immune cells, repairs tissue damage, destroys microbes.
• Process: Involves histamine release, vasodilation, and immune cell recruitment.
• Acute vs. Chronic Inflammation: Acute is typically injury-related; chronic can indicate autoimmune issues.

Fever

• Causes: Triggered by pyrogens, which are released by phagocytic cells.
• Mechanism: Alters hypothalamus set-point, leading to increased body temperature.
• Benefits: Inhibits pathogen growth, reduces iron availability, enhances immune cell activity.

Additional Innate Mechanisms

• Interferon Response: Proteins released by virus-infected cells to protect neighboring cells by inducing antiviral states.
• Natural Killer Cells: Attack cells lacking MHC I proteins, a sign of infection or cancer.

Complement System

• Proteins: Over 50 complement proteins aid in immune responses like inflammation and pathogen destruction.
• Activation Pathways: Classical, lectin, and alternative pathways initiate complement activity.
• Membrane Attack Complex: Formed by complement proteins to lyse pathogens.

Key Definitions

• Antigens: Molecules recognizable by the immune system, can be self or foreign.
• PRRs and PAMPs: Ensure phagocytes can identify and destroy pathogens.
• Lymphatic System: Circulates lymph fluid containing immune cells, facilitating antigen encounters.

Study Recommendations

• Review phagocytosis and the role of PRRs and PAMPs.
• Understand inflammation and fever mechanisms.
• Study the functions of the complement system and natural killer cells.

Bonus Opportunity

• Summarize specific educational videos on the immune system for extra credit.