Chemical barriers against infection

Chemical secretions are produced against invading pathogens. Secretions include:

  • stomach acid
  • tears
  • saliva
  • mucus

Stomach acid

Stomach acid is a chemical barrier against infection. It is hydrochloric acid and is strong enough to kill any pathogens that have been caught in mucus in the airways or consumed in food or water.

Tears and saliva

Both tears and saliva have enzymes that destroy bacterial cells by breaking down their cell walls. These enzymes are called lysozymes. Like stomach acid, they are a form of chemical defence against infection.

A picture of an eye affected by conjuctivitis. The membrane around the eye is red sore and inflamed.

Conjunctivitis can occur if pathogens are not killed by lysozymes


Mucus is a sticky substance secreted by goblet cells in the trachea. Foreign particles and pathogens adhere to the surface and cilia sweep the mucus away from the lungs.