Chemical barriers:
Lysozyme—A chemical found in tears, saliva, and blood plasma that breaks down bacterial cell walls.
Pepsin—An enzyme in the stomach that lyses (disintegrates) many microorganisms.
Hydrochloric acid—Secreted by the parietal cells in the stomach, it creates a low pH that is lethal to many pathogens.
Complement—A series of enzymatic proteins that are activated by both specific and nonspecific
mechanisms.
Interferon—Any of a group of proteins that are produced by virus-infected cells and some immune system cells, inhibiting viral growth.