The skin and mucous membranes provide a nonspecific first line of defense against invaders entering through the skin or through openings into the body. The first line of defense features the following mechanisms:
- Skin is a physical and hostile barrier covered with oily and acidic (pH from 3 to 5) secretions from sebaceous and sweat glands, respectively.
- Antimicrobial proteins (such as lysozyme, which breaks down the cell walls of bacteria) are contained in saliva, tears, and other secretions found on mucous membranes.
- Cilia that line the respiratory tubes serve to sweep invaders away from the lungs.
- Gastric juices of the stomach, by the action of hydrochloric acid or enzymes, kill most microbes.
- Symbiotic bacteria found in the digestive tract and the vagina outcompete many other organisms that could cause damage.