What are some of the mechanical and chemical barriers to infection?

Mechanical barriers include skin and mucous membranes. The mucous membranes in the respiratory passageways are lined with ciliated epithelium. The cilia continuously move particles trapped in the mucus in a direction away from the lungs. This epithelium is eventually destroyed in smokers, causing them to be susceptible to respiratory diseases.

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.