This is a randomized, double-blind, double-dummy, placebo-controlled, parallel-group, single center study in subjects with positive histamine skin prick test and a positive RadioAllergoSorbent Test (RAST) (class > 2) to one of the tested standardized allergenic extracts: tree pollen, cat dander, house dust mite, or a mixture of five grass pollens. Subjects will be randomized to desloratadine 5 mg once daily, levocetirizine 5mg once daily, or placebo once daily for 8 days of treatment followed by 11 days of skin testing after discontinuation of the antihistamine treatment phase. The duration of the suppressive effects of desloratadine on cutaneous allergen-induced wheal and flare responses after discontinuation of a one-week treatment will be established.

Inclusion Criteria:

• Subjects must be 18 years of age or older, of either sex.
• Subjects must have at least a history of positive skin testing to the allergens of tree pollen, grass pollen, house dust mite, or cat dander.
• Non symptomatic volunteers allergic of different pollen they must be included out of the specific pollinic season.
• Subjects must demonstrate their willingness to participate in the study and comply with its procedures by signing a written informed consent.
• Subjects must understand and be able to adhere to visit schedules
• Subjects must be in general good health.
• Women of child-bearing potential must have a negative pregnancy test at Day -1 and must use an accepted method of contraception during the entire duration of the study.

Exclusion Criteria:

• Subjects who have persistent asthma.
• Subjects who have chronic urticaria or atopic dermatosis.
• Subjects who have received any treatment listed below more recently than the indicated washout period prior to Randomization, or who must continue to receive treatment as listed below.

Medications Prohibited During the Trial and Washout Period Prior to Visit 1

• Corticosteroids

  • Intramuscular or intra-articular, 1 month
  • Oral, inhaled, intravenous, rectal, intranasal, or ocular, 7 days
  • High-potency dermatological, 7 days

• Cromolyn/Lodoxamide/Nedocromil

  • Intranasal, ocular, inhaled, or oral, 2 days

• Antihistamines

  • Long-acting prescription (eg, levocetirizine, fexofenadine, and desloratadine), 15 days
  • Short-acting (eg, chlorpheniramine, brompheniramine including over-the-counter [OTC] forms, hydroxyzine), 15 days
  • Ocular (eg, levocabastine), 15 days

• Leukotriene inhibitors (eg, montelukast), 7 days

• Systemic antibiotics, 14 days (the washout refers to antibiotics used to treat an upper or lower respiratory tract infection)

• Immunotherapy (desensitization), 1 year

• Decongestants

  • oral, 2 days
  • local, 2 days

• Ophthalmic non-steroidal anti-inflammatory drugs (NSAIDs; eg, Toradol), 5 days

• Investigational medications, 30 days

• Tricyclic antidepressants (eg, amitriptyline, clomipramine, doxepin, imipramine, trimipramine, amoxapine, desipramine, nortriptyline, protriptyline), 30 days

• Tetracyclic antidepressants (eg, maprotiline, mirtazapine), 30 days

  • Subjects with a history of hypersensitivity to desloratadine, to levocetirizine, or any of their excipients.
  • Women who are breast-feeding, pregnant, or intend to become pregnant.
  • Subjects with any clinically significant condition or situation, other than the condition being studied, that, in the opinion of the investigator, would interfere with the study evaluations or optimal participation in the study.
  • Subjects who have used any investigational drugs within 30 days of randomization.
  • Subjects working between 11 PM and 8 AM (night shift).
  • Subjects who have skin/color pigmentation incompatible with accurate measurements of flare reaction.
  • Subjects with cutaneous hyperactivity: negative prick test control >3 mm.
  • Subjects who are participating in any other clinical study.
  • Subjects who are part of the staff personnel directly involved with this study.