Both antacids and antiallergic drugs are antihistamines but they cannot replace each other. Explain why?

(D) Antacids and antiallergic drugs cannot replace each other because they act on different receptors in the body. Antihistamines (like $Ranitidine$) are used to prevent the interaction of histamine with the receptors present in the blood vessel walls of the stomach,thereby controlling the secretion of $HCl$ (acidity). On the other hand,antiallergic drugs (like $Brompheniramine$) are used to block the histamine receptors that cause allergic reactions. Since the receptors involved in acidity and allergy are distinct,these drugs are not interchangeable.