Histamine is a chemical substance that stimulates the secretion of pepsin and hydrochloric acid in stomach. Antacids are designed to prevent the interaction of histamine with the receptors present in the stomach wall.
As a result, less $\mathrm{HCl}$ 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, histamine causes allergy. Antihistamines interfers 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 work on different receptors, therefore, antihistamines remove allergy while antacids remove acidity.