Deslor (desloratadine) is a long-acting antihistamine. Histamine is a substance that is released when an allergic reaction takes place. The body produces histamine when the immune system tries to fight off allergens, such as pollen, dust, molds, pet dander, etc. Histamine is responsible for causing inflammation of the mucus membranes of the upper respiratory tract and the skin. Desloratadine suppresses the secretion of histamine in order to bring down inflammation and provide relief from allergy symptoms.