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.