To examine the relative potency, onset of action and duration of action of fexofenadine HCl 180 mg (ALLEGRA) and desloratadine 5 mg (CLARINEX) as compared to placebo on skin wheals and flares induced by histamine.

Inclusion Criteria:

• Male and female subjects, 12 to 55 years of age, may participate
• Positive histamine skin prick tests (or duplicate histamine skin prick test) with a summation flare (ΣF) greater than or equal to 20 mm larger than diluent control, and summation wheal (ΣW) greater than or equal to 6 mm larger than diluent control at the screening visit 1
• All female subjects must have a negative urine pregnancy test at the screening visit
• Female subjects who are sexually active will be expected to use one of the following birth control methods throughout the study (see Section 4.4)
• Subjects must be within 15% of normal body weight for height or a BMI less than 29.9 (based on NHLBI guidelines)
• Subjects willing and able to adhere to visit schedules and all study requirements
• All female subjects must have a negative urine pregnancy test at each treatment visit (Visit 2, 4, and 6).
• Continues to meet all inclusion and exclusion criteria

Exclusion Criteria:

• Asthma that requires treatment with medication other than an inhaled, short-acting beta agonist
• Signs and symptoms of currently active allergic disease (seasonal allergic rhinitis, perennial allergic rhinitis, episodic allergic rhinitis)
• Upper respiratory tract infection, sinusitis, asthma or flu-like symptoms within 2 weeks prior to visit 1
• Subjects who have dermatographism or other skin conditions which might interfere with the interpretation of the skin test results
• Subjects who are receiving immunotherapy
• Any excessive amounts of alcohol (no more than two drinks/day on average)
• Any excessive use of caffeine (more than six cups of coffee per day or equivalent)
• Any use of tobacco/nicotine products within 90 days of visit 1
• Any disease state or surgery known to affect the gastrointestinal absorption of drugs
• Known hypersensitivity to the investigational product or to drugs with similar chemical properties
• Subjects who will be visiting a tanning salon during the study
• Subjects who will need to use artificial tanning products during the study
• Night or variable shift workers during the study
• Pregnancy
• Breast-feeding
• History of hypersensitivity to the study medications or to drugs with similar chemical structures
• Treatment with other H1-receptor antagonists in the last year before study entry
• Likelihood of requiring treatment during the study period with drugs not permitted by the clinical study protocol (see Section 6.2)
• Treatment with any investigational product in the last 30 days before study entry
• No person or child of a person directly associated with the administration of the study may participate as a study subject
• Clinically relevant cardiovascular, hepatic, neurologic, endocrine, or other major systemic disease making implementation of the protocol or interpretation of the study results difficult
• Mental condition rendering the subject unable to understand the nature, scope, and possible consequences of the study
• Subject unlikely to comply with protocol, e.g., uncooperative attitude, inability to return for follow-up visits, and unlikelihood of completing the study
• Use of any of the following drugs within the time indicated prior to the first dosing visit: