The Innate Immune System
Learning Objectives for Topic 5A - The Innate Immune System
Difference between Innate and Adaptive Defense Systems
- Innate: Immediate, nonspecific response
- Adaptive: Delayed, specific response based on previous encounters
Surface Barriers for the Innate Immune System (P782-783)
- First line of defense includes skin and mucous membranes
- Keratinized skin offers resilience against various environmental factors
Internal Defense Mechanisms (P784-789):
- Phagocytes and Phagocytosis
- Natural Killer (NK) Cells
- Inflammation Process (P785-787) - including Fig 21.4
- Complement Proteins (P788-789)
- Fever (P789)
Immune System Overview
- Provides resistance to disease and consists of two primary systems:
- Innate (Nonspecific) Defense System:
- First and Second Lines of Defense:
- First Line: External body membranes (skin and mucous membranes)
- Second Line: Phagocytes, antimicrobial proteins, NK cells, inflammation
- Adaptive (Specific) Defense System:
- Third Line of Defense: Attacks specific foreign substances
Innate Defenses: Surface Barriers
- Mechanical Barriers:
- Skin and mucous membranes protect against pathogens
- Chemical Barriers:
- Acidity: Acid mantle inhibits bacterial growth
- Enzymes:
- Lysozymes in saliva and respiratory mucus kill microorganisms
- Digestive enzymes in the stomach destroy pathogens
- Mucin: Traps microorganisms within mucosal linings
- Antimicrobial Peptides:
- Defensins and Dermicidin, secreted by skin and mucous membranes
Protective Mechanisms of Surface Barriers
- Intact Skin:
- Forms a mechanical barrier to pathogens
- Skin secretions create an acidic environment, hindering bacterial growth
- Intact Mucous Membranes:
- Prevent entry of pathogens
- Mucus traps microorganisms in respiratory and digestive tracts
Cells and Chemicals: Second Line of Defense
- Phagocytes: White blood cells that ingest pathogens
- Neutrophils:
- Most abundant WBCs, phagocytic upon exposure to infection
- Macrophages:
- Derived from monocytes; can be free or fixed
- Natural Killer (NK) Cells:
- Recognize and destroy abnormal cells (cancerous or virus-infected)
Inflammation
- Stages of Inflammation:
- Release of inflammatory chemicals (e.g., histamine)
- Vasodilation: Increased blood flow leads to redness and heat
- Phagocyte Mobilization: Neutrophils and macrophages respond rapidly
Phagocytosis Process
- Adherence: Phagocyte recognizes and binds to pathogens
- Engulfment: Pseudopods form a phagosome
- Fusion: Phagosome fuses with lysosome to form phagolysosome
- Destruction: Pathogens are destroyed via enzymes and oxidative bursts
Antimicrobial Proteins
- Interferons: secreted by infected cells to protect neighboring cells
- Complement Proteins: Enhance defense via lysis of pathogens and amplification of inflammation
Fever and Its Benefits
- Pyrogens from leukocytes act on hypothalamus, leading to increased body temperature
- Benefits:
- Sequesters iron and zinc
- Enhances metabolic rate for faster recovery
Summary of Second Line Defenses
| Category | Protective Mechanism |
|---|---|
| Phagocytes | Engulf and destroy pathogens; contribute to adaptive immune responses |
| Natural Killer Cells | Induce apoptosis in infected or cancerous cells; recognize abnormal cells without specific antigens |
| Inflammation | Prevents spread of pathogens; disposes of pathogens and promotes tissue repair |
| Antimicrobial Proteins | Interferons mobilize immune response; complement proteins lyse pathogens and enhance responses |
| Fever | Raises temperature to inhibit microbes and enhance repair processes |