Humoral Factors of Innate Immune System

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