Immune System: Your Body's Defense Mechanism

Introduction

• Body's constant battle: Covered in bacteria (staph, strep, e coli).
• Immune system: Protects the body using various tissues, organ systems, and specialized cells.
• Three-part policy: Innate (nonspecific) defense, adaptive (specific) defense.

Innate Defense System

Physical Barriers

• Skin: Keratinized epithelial membrane acts as a wall.
• Mucous membranes: Line cavities open to the outside (respiratory, digestive, urinary, and reproductive tracts).

Chemical Defenses

• Stomach acid: Destroys pathogens in ingested food.
• Mucus: Traps pathogens in nasal passages.
• Enzymes: Bactericidal properties in saliva and lacrimal fluid.
• Defensins: Peptides in skin and membranes that hinder bacteria and fungi.

Second Line of Defense

Internal Defenses

• Phagocytes: "To eat," includes neutrophils and macrophages.
  • Neutrophils: Abundant, self-destruct after consuming pathogens.
  • Macrophages: Derived from monocytes, consume pathogens repeatedly.
• Natural Killer Cells (NK Cells): Detect infected or cancerous cells lacking MHC1 protein and induce apoptosis.

Inflammatory Response

• Initial signals: Mast cells release histamine.
  • Vasodilation: Redness and heat due to increased blood flow.
  • Increased permeability: Capillaries release proteins resulting in swelling.
  • Lymphatic response: Clears excess fluid, attracts phagocytes.
• Leukocytosis: Neutrophils released from bone marrow to the bloodstream, navigate to injury site.
• Monocytes become Macrophages: Arrive and clean up pathogen debris.

Fever as Systemic Response

• Pyrogen chemicals: Induce fever by influencing the hypothalamus.
• Effects of fever: Increases metabolic rate for faster healing, sequesters iron and zinc to inhibit bacterial growth.

Summary

• First-line defenses: Physical and chemical barriers (skin, mucous membranes, stomach acid, enzymes).
• Second-line defenses: Phagocytes (neutrophils, macrophages), natural killer cells, inflammatory response, fever.

Conclusion

• Preview: Adaptive (specific) defenses to be discussed in the next session.

Credits: Written, edited, directed, and produced by various contributors at Crash Course.