Lecture Notes: Immune System Defense Mechanisms

First Line of Defense

• Lysozyme
  • Found in tears
  • Breaks down peptidoglycan in bacteria
• Sebum
  • Waxy substance in body secretions
  • Prevents bacterial entry
• Lactoferrin
  • Binds free iron to prevent bacterial survival
• Antimicrobial Peptides
  • Includes defensins, germicides
  • Can create holes in bacterial membranes

Second Line of Defense

• Inflammatory Mediators

  • Histamine
    • Released by mast cells
    • Causes vasodilation and increased vascular permeability
    • Can cause bronchial smooth muscle contraction leading to asthma
  • Bradykinin
    • Increases vascular permeability
  • Leukotrienes and Prostaglandins
    • Produced in response to injury
    • Prostaglandins can cause fever

• Cytokines

  • Include interleukins and chemokines
  • Facilitates communication and recruitment of immune cells
  • Interferons
    • Important in viral infection defense

Complement System

• Basics

  • 30+ proteins synthesized by the liver
  • Always circulating in the bloodstream
  • Part of innate immune system, can interact with adaptive immune systems

• Activation Mechanisms

  • Can be triggered by antibody binding or other pathways
  • C3 Protein
    • Splits into C3A and C3B
    • C3B coats bacteria stimulating phagocytosis
  • Opsonization
    • Makes bacteria more susceptible to phagocytosis

• Complement Cascade

  • C5 breaks into C5A and C5B
  • C3A and C5A bind to mast cells, releasing histamine
  • C5B forms membrane attack complex (MAC), causing bacterial lysis

Interferons

• Role in Antiviral Activity
  • Produced by infected cells
  • Activates immune cells, triggers apoptosis, signals uninfected cells
  • Induces antiviral protein production in uninfected cells

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