Protein digestion primarily occurs in the stomach, where the secretion of pepsinogen, an inactive enzyme, is crucial. Pepsinogen is activated to pepsin in the acidic environment of the stomach, allowing for protein breakdown. The stomach does not digest itself due to protective mechanisms, including a layer of mucus that lines the stomach walls. This mucus serves as a barrier, preventing the digestive enzymes from damaging the stomach tissue. Additionally, the cells of the stomach are continuously renewed, which further protects against self-digestion.