The stomach is lined with a special type of cell called epithial cells. Epithial cells produce mucus and the mucus forms a barrier between the contents in the stomach and the stomach lining. Certain enzymes produced by glands which aid digestion are not chemically activated until the enzymes pass through the mucus lining and mix with other digestive secretions.
source: https://www.theguardian.com/notesandqueries/query/0,5753,-1364,00.html