Inclusion Criteria:  Written informed consent Male or female outpatient aged 18 to 82 years inclusive History of bronchial asthma for at least six months as defined by ATS criteria. The patients will be required to have one or more of the following objective physiological criteria: positive results on methacholine or mannitol challenge during the previous year, bronchodilator reversibility to 400 mg of inhaled Salbutamol of FEV1 ≥12% and 200ml or peak flow variability of ≥20% over two weeks. Diagnosis of severe asthma, defined as asthma that requires treatment with high dose inhaled corticosteroids plus a second controller and/or systemic corticosteroids to prevent it from becoming uncontrolled or that remains uncontrolled despite this therapy. All patients will be medicated with at least 880 μg of fluticasone propionate or the equivalent by inhalation per day and at least three months of treatment with an additional controller. Patients will be allowed to continue their anti-asthma therapy throughout the study. Patients should have uncontrolled asthma commencing mepolizumab treatment, based on investigator assessment, including one or both of the following: Poor symptom control (frequent symptoms or reliever use, activity limited by asthma, night waking due to asthma), defined as ACQ consistently ⩾1.5 or ACT<20. Frequent exacerbations (≥2/year) requiring oral corticosteroids, or severe exacerbations (≥1/year) requiring hospitalization or burst of systemic corticosteroids (≥3 days). All patients will have an eosinophil count of at least 150 cells per microliter in the peripheral blood at screening or at least 300 cells per microliter at some time during the previous year.  Exclusion Criteria:  Principal exclusion criteria:  Diseases and health status: clinically relevant abnormal laboratory values suggesting an unknown disease and requiring further clinical evaluation suffering from COPD (i.e., chronic bronchitis or emphysema) and/or other relevant lung diseases causing alternating impairment in lung function Asthmatic patients and healthy controls currently smoking or with a smoking pack history greater than 10 will be excluded.  Common exclusion criteria:  pregnancy intention to become pregnant during the course of the study breastfeeding participation in another study within the 30 days preceding and during the present study known or suspected non-compliance, alcohol or drug abuse inability to follow the procedures of the study, e.g., due to language problems, psychological disorders