Final answer:
Histamine2-receptor antagonists work by competing with histamine for binding sites on parietal cells in the stomach, reducing the secretion of acid and indirectly raising stomach pH; they are not used for allergy symptom relief.
Step-by-step explanation:
The action of histamine2-receptor antagonists (H2 blockers) is to compete with histamine for binding sites on the parietal cells in the stomach. When these receptors are blocked by an antagonist, the secretion of stomach acid (HCl) is reduced, which can help treat conditions like peptic ulcers, gastroesophageal reflux disease (GERD), and syndromes with excess stomach acid production. It is important to note that these medications do not address the symptoms of allergies related to histamine release, as that is the role of H1-antihistamines. Common examples of H2 blockers include ranitidine (Zantac), famotidine (Pepcid AC), and cimetidine (Tagamet). They do not irreversibly bind to H+/K+ ATPase, as proton pump inhibitors do, nor do they decrease stomach pH directly. Instead, by reducing histamine stimulation on parietal cells, they cause a decrease in acid secretion and indirectly raise the stomach's pH over time.