What is the mechanism of action of H2 receptor antagonists in reducing gastric acidity?

• A. They neutralize excess hydrochloric acid (HCl) that has been secreted into the stomach.
• B. They compete with histamine for binding sites on the surface of parietal cells and block hydrogen ion (H+) secretion.
• C. They inhibit the secretion of the digestive enzyme pepsin into the stomach.
• D. They combine with, and convert, HCl in the stomach to a salt and water.

Answer: (B)

Blocking histamine from activating the parietal cells on the stomach lining lowers acid production. Histamine normally binds to H2 receptors on these cells and triggers a signaling cascade that increases proton pump activity, pushing hydrogen ions into the stomach and creating gastric acid. By competitively blocking those H2 receptors, the signal for acid secretion is reduced, so less acid is produced. This is why these agents are effective at lowering gastric acidity, especially when acid secretion would be stimulated by histamine, gastrin, or acetylcholine. They don’t neutralize existing acid in the stomach, don’t inhibit pepsin secretion, and don’t convert HCl to something else—that’s why the mechanism is best described as blocking histamine’s action at the parietal cell receptors.