Lecture on the Immune System

Overview

• The body is in a constant battle against microbes like staph, strep, and E. coli.
• The immune system acts as a defense mechanism, engaging various defense cells and strategies to protect the body.

Immune System Components

• Innate (nonspecific) defense system
  • First line of defense
  • Includes skin, mucous membranes, phagocytes, antimicrobial proteins, and other attack cells
• Adaptive (specific) defense system
  • Second line of defense, like specialized forces
  • Takes time to activate, targets specific threats

Focus on the Innate Immune System

• Uses physical and chemical barriers, killer cells, and fever

Physical Barriers

• Skin
  • Acts as a tough barrier against pathogens
• Mucous Membranes
  • Line cavities that open to the external environment (respiratory, digestive tracts)

Chemical Defenses

• Stomach acid neutralizes ingested pathogens
• Mucus in nasal passages traps inhaled pathogens
• Saliva and eye fluid contain bacteria-fighting enzymes
• Skin and mucosa produce defensins, peptides that prevent microbial colonization

Internal Defenses

• Phagocytes
  • Neutrophils: Abundant white blood cells that self-destruct after attacking pathogens
  • Macrophages: Derived from monocytes, engulf and digest pathogens repeatedly
• Natural Killer Cells (NK Cells)
  • Target and destroy infected or cancerous cells lacking MHC1 protein

Inflammatory Response

• Triggered by pathogens breaching barriers (e.g., cuts)
• Histamines released, causing:
  • Vasodilation, increased temperature, and capillary permeability
  • Attracting phagocytes to the injury site
• Formation of pus from dead neutrophils

Fever

• Systemic response to overwhelming pathogens
• Pyrogen chemicals increase body temperature, accelerating healing

Conclusion

• Physical barriers and phagocytes form the first line of defense.
• Natural killer cells and inflammatory response act in support.
• Fever acts as a systemic response when local defenses are insufficient.

Credits

• Thanks to Linnea Boyev, Patreon patrons, and the Crash Course team for making the lecture possible.