• The body’s first line of defense consists of different types of barriers that keep most pathogens out of the body.
  • Pathogens are disease-causing agents, such as bacteria and viruses.
  • The skin is the most important mechanical barrier.
  • Chemical barriers destroy pathogens on the outer body surface, at body openings, and on inner body linings.
  • Sweat, mucus, tears, and saliva all contain enzymes that kill pathogens.
  • Urine is too acidic for many pathogens, and semen contains zinc, which most pathogens cannot tolerate.
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