Complement system

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1. Pathogen enters the body

  • A bacterium or virus invades.
  • The body recognizes it as “foreign” through immune surveillance.

2. Complement system activates

There are three pathways: classical, lectin, and alternative.
All converge to activate C3, which splits into:

  • C3a → small fragment, acts as signal.
  • C3b → binds to pathogen surface (opsonization, marks it for phagocytosis).

Later, C5 is cleaved into C5a and C5b:

  • C5b → starts formation of MAC (Membrane Attack Complex), which can punch holes in pathogen.
  • C5a → one of the most powerful chemotactic signals.

 3. Chemotactic signal is released

  • C3a and C5a diffuse around the infection site.
  • These fragments act like a “chemical smell” for immune cells.
  • They bind to receptors on neutrophils, macrophages, and other phagocytes.

 4. Immune cells move toward the signal

  • Neutrophils detect higher concentration of C5a and move up the gradient → this is positive chemotaxis.
  • They essentially “follow the trail” to the site of infection.
  • This movement is guided by the chemical gradient: more C5a → closer to infection.

5. Immune cells act at infection site

  • Neutrophils and macrophages reach the pathogen.
  • They engulf it (phagocytosis) and release enzymes to destroy it.
  • Complement fragments like C3b help by sticking to the pathogen (opsonization).
  • Meanwhile, C5b completes MAC formation → pathogen lysis.

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