Allergic asthma is marked by symptoms like difficulty breathing, coughing and wheezing, triggered by inhalant allergens (e.g. tree pollen, grass pollen, house dust mite, animal hair or mold). The diagnosis is based, amongst others, on measures of lung function. In asthma, spirometry shows small airway narrowing, which will normalize after inhalation with bronchodilators. In addition, there is often an elevated level of nitric oxide (eNO) in the exhaled air as a marker of eosinophilic airway inflammation. As with allergic rhinoconjunctivits, allergen-specific immunotherapy is the only treatment approach that directly targets the cause of allergic asthma.