Gastritis is a condition in which the stomach lining (known as the mucosa) is inflamed, or swollen. The stomach lining contains glands that produce stomach acid and an enzyme called pepsin. The stomach acid breaks down food and pepsin digests protein. A thick layer of mucus coats the stomach lining and helps prevent the acidic digestive juice from dissolving the stomach tissue. When the stomach lining is inflamed, it produces less acid and fewer enzymes. However, the stomach lining also produces less mucus and other substances that normally protect the stomach lining from acidic digestive juice.