Humoral Factors of Innate Immune System

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Lysozyme

  • Lysozyme is one of the major bactericidal agents in secretions
  • Helps to protect vulnerable sites such as the eyes and nasal passages
  • Exerts bactericidal effects by digesting bacterial cell walls
    • Gram-positive bacteria are more sensitive to lysozyme action than gram-negative bacteria
    • The outer membrane of gram-negative bacteria helps to protect them

Complement

  • The Complement system is a group of about 30 proteins within the body fluids of all vertebrates and some invertebrates
  • Complement promotes phagocytosis or causes lysis of an invading organism
  • Complement acts as a cascade, like the blood clotting system
    • The early enzymes in the cascade are bound to invading bacteria and fungi
      • They have an affinity for components of microbial cell membranes
    • This binding initiates a cascade so that the binding of one molecule will eventually lead to the fixation of millions of later molecules
  • The early components act as targets for phagocytes
  • The later components punch holes in bacteria, causing their lysis

Interferons

  • Lysozyme and complement have only marginal effects on virus infections because these are intracellular
    • The body has evolved non-specific mechanisms to protect against viruses
      • The most notable of these is the interferons
  • Interferons are small polypeptides produced mainly by virus-infected cells
    • Interact with uninfected cells and render them resistant to infection
      • This resistance is mainly due to the production of enzymes that digest viral nucleic acids