Patient description: The patient is a 15-year-old boy with asthma diagnosed a year ago. He presents with shortness of breath, chest tightening and cough. According to his mother, he didn't respond to the usual corticosteroid inhaler. He was admitted to the emergency department with diagnosis of severe asthma exacerbation. He is a candidate for general corticosteroid therapy. Spirometry revealed a forced expiratory volume in the first second (FEV1) of 60% of the predicted. His past medical history is non-significant. His family history is significant for asthma in his mother and his uncle. He used to be treated with combination of inhaled corticosteroids and Zafirlukast.

Clinical trial eligibility criteria: eligible ages (years): 18.0-50.0,  Mild Asthma Allergy A history of episodic wheezing, chest tightness, or shortness of breath consistent with asthma, or physician diagnosed asthma. 2. Specific allergy to house dust mite Dermatophagoides farinae confirmed by positive immediate skin test response. 3. Provocative concentration of methacholine of 10 mg/ml or less producing a 20% fall in FEV1 (PC20 methacholine) by the method used in a separate screening protocol (98-CEMLB-293) that is already approved by the UNC IRB. 4. FEV1 of at least 80% of predicted and FEV1/FVC ratio of at least .70 (without use of bronchodilating medications for 12 hours), consistent with lung function of persons with mild episodic or mild persistent asthma  Any chronic medical condition considered by the PI as a contraindication to the exposure study including significant cardiovascular disease, diabetes requiring medication, chronic renal disease, or chronic thyroid disease. 2. Subjects with a history of immunologic disease, or undergoing therapeutic immune suppression for cancer or other diseases. 3. Physician directed emergency treatment for an asthma exacerbation within the preceding 12 months. 4. Use of systemic steroid therapy within the preceding 12 months for asthma or the following asthma symptoms; cough, wheeze, shortness of breath. 5. Use of inhaled steroids, cromolyn or leukotriene inhibitors (Montelukast or Zafirlukast) within the past month (except for use of cromolyn exclusively prior to exercise). 6. Use of daily theophylline within the past month. 7. Use of medications that might alter the response to methacholine or antigen challenge including anti-inflammatory and anti-histamine agents within one week of challenge. 8. Subjects using tricyclic antidepressants such as doxepin and MAO inhibitors. 9. Subjects using beta-adrenergic blockers or any other medications known to interfere with the treatment of anaphylaxis. 10. Inability to withhold inhaled or oral bronchodilating medications for 12 hours prior to allergen challenge. 11. Pregnancy (positive urine pregnancy test at the baseline visit) or nursing a baby. Pregnant women are excluded due to the risk of fetal exposure to radiation. 12. Women of child bearing potential who are not using dependable contraception (such as birth control pill, IUD, estrogen patches) or who are not completely abstinent. 13. Cigarette smoking >0.5 packs per week within the past 12 months. 14. Nighttime symptoms of cough or wheeze greater than 1x/week at baseline (not during a clearly recognized viral induced asthma exacerbation) which would be characteristic of a person of moderate or severe persistent asthma as outlined in the current NHLBI guidelines for diagnosis and management of asthma. 15. Exacerbation of asthma more than 2x/week which would be characteristic of a person of moderate or severe persistent asthma as outlined in the current NHLBI guidelines for diagnosis and management of asthma. 16. Daily requirement for albuterol due to asthma symptoms (cough, wheeze, chest tightness) which would be characteristic of a person of moderate or severe persistent asthma as outlined in the current NHLBI guidelines for diagnosis and management of asthma. (Not to prophylactic use of albuterol prior to exercise). 17. Viral upper respiratory tract infection within 4 weeks of challenge. 18. Any acute infection requiring antibiotics within 2 weeks of challenge (or 4 weeks in the case of azithromycin due to the prolonged half-life). 19. Participating in any study utilizing an investigational agent within 4 weeks of this challenge