Your body is equipped with a nonspecific first line of defense against diseases and infectious invaders trying to enter it. This first line features the following. 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 lungs serve to sweep invaders out of the lungs. Gastric juice of the stomach, by the action of hydrochloric acid or enzymes, kills most microbes. Symbiotic bacteria found in the digestive tract and the vagina out compete many other organisms that could cause damage.