Humoral Factors of Innate Immune System

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

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

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