Final answer:
Stomach cells are protected by a thick mucus layer (option D) that neutralizes acid, and a robust mucosal barrier that prevents self-digestion and bacterial infection, which when compromised, can lead to ulcers caused by H. pylori.
Step-by-step explanation:
The stomach cells are protected from acid and disease-causing bacteria primarily by a thick layer of mucus. This mucus is secreted by the surface mucus cells of the stomach's epithelial lining, providing a protective coat that is alkaline due to the presence of bicarbonate ions. The mucus forms a physical barrier that neutralizes acid and also prohibits gastric juices from reaching the underlying tissues. Furthermore, epithelial cells are tightly joined together, preventing penetration of gastric juice. In addition to this, stem cells rapidly replace damaged cells on the mucosal surface, which completely renews every 3 to 6 days, ensuring ongoing protection against self-digestion and bacteria.
Mucosal barrier imbalances, like when the mucus lining is ruptured without proper regeneration, can lead to ulcers. These ulcers are typically caused by the bacteria Helicobacter pylori. Therefore, maintaining the integrity of the mucus barrier is essential for stomach health and preventing ulcers.
Moreover, the stomach acid itself helps in killing off potential invaders that are ingested, while other defense mechanisms like lysozyme in saliva contribute to the body's overall protection against harmful microorganisms.