Desloratadine is the active ingredient in the drug Clarinex.  Clarinex is mainly used in the treatment of allergies or allergy symptoms caused by histamine.  Histamine attaches itself to other cells that have histamine receptors.  This activates the cell, causing it to make chemicals which cause the allergies we are familiar with.  Desloratadine prevents this by blocking one type of histamine receptor, preventing the activation of the cell.
 
 

References:
http://www.medicinenet.com/desloratadine/article.htm
http://www.kuwaitpharmacy.com/newdr/clarinex.html
http://www.edrugclinic.com/clarinex.htm
Page prepared by:  Joshua R. Williams