The complement system is a biochemical cascade of the immune system that helps, or “complements”, the ability of antibodies to clear pathogens or mark them for destruction by other cells. The cascade is composed of many plasma proteins, synthesized in the liver, primarily by hepatocytes. The proteins work together to:

  • trigger the recruitment of inflammatory cells
  • “tag” pathogens for destruction by other cells by opsonizing, or coating, the surface of the pathogen
  • form holes in the plasma membrane of the pathogen, resulting in cytolysis of the pathogen cell, causing the death of the pathogen
  • rid the body of neutralised antigen-antibody complexes.

There are three different complement systems: Classical, alternative, Lectin

  • Classical: starts when antibody bind to bacteria
  • Alternative: starts “spontaneously”
  • Lectin: starts when lectins bind to mannose on bacteria

Elements of the complement cascade can be found in many non-mammalian species including plants, birds, fish, and some species of invertebrates.