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

Explanation

Histamine is a chemical substance that stimulates the secreation of pepsin and hydrochloric acid in stomach. Antacids are designeed to prevent the interaction of histamine with the receptors present in the stomach wall. As a result, less HCl is released in stomach.
Histamine contracts the smooth muscles in the bronchi and gut and relaxes other muscles in walls of fine blood vessels. As a result. As a result, histaine causes allergy. Antihistarnines interfere with the natural action of histamine by competing with histamine for binding sites of receptor where histamine exerts its effect.
Since, antiallergic and antacids drugs works on different receptors, therefore, antihistamines remove allergy while antacids remove acidity.