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

Antihistamines are the drugs which control the allergy effects produced by histamines. Antacids are the substances which neutralize gastric acidity. Antihistamine does not affect the secretion of acid in the stomach and the reason is that both antiallergic and antacid drugs work on different receptors. Therefore, antihistamines remove allergy while antacids remove acidity.