Pepsin is first produced as a pepsinogen, which is a zymogen whose primary structure has an additional 44 amino acids. The hormone gastrin and the vagus nerve trigger the release of both pepsinogen and Hydrochloric acid from the stomach lining when food is ingested. Then this pepsinogen is activated by the hydrochloric acid, which is released from parietal cells in the stomach lining. Hydrochloric acid creates an acidic environment which allows pepsinogen to unfold and cleave itself in an autocatalytic fashion, thereby generating pepsin.
Pepsin was discovered in 1836 by Theodor Schwann who also coined this enzyme's name from the Greek word pepsis, meaning digestion (peptein: to digest). It was the first animal enzyme to be discovered, and in 1929 it became one of the first enzymes to be crystallized, by John H. Northrop. Pepsin is a digestive protease.