Histamine is a chemical substance that stimula.es the secretion of pepsin and hydrochloric acid in the stomach. Antacids are designed to prevent the interaction of histamine with the receptors present in the stomach wall. As a result, less HCI is released in the stomach.

Histamine contracts the smooth muscles in the bronchi and gut and relaxes other muscles in walls of fine blood vessels. As a result, histamine causes allergy. Antihistamines interfere with the natural action of histamine by competing with histamine lor binding sites of receptors where histamine exerts its effect.

Since antiallergic ard antacids rugs work on different receptors, therefore, antihistamines remove allergy while antacids remove acidity.