The cohort of patients included in the study will consist of severe asthmatic patients receiving Mepolizumab as part of routine medical care, i.e. eligible for Mepolizumab treatment according to the Italian Medicine Agency (AIFA; Agenzia Italiana del Farmaco). The patients will receive Mepolizumab treatment irrespective of the enrollment in the study but because of the clinical indication.  Inclusion criteria:  severe asthmatic patients according to GINA document (Step 4 and 5 GINA) blood eosinophilic count above 300 cells/mcl at least once in the previous year and ≥150 cells/mcl in the absence of systemic corticosteroid treatment before biological treatment initiation. In addition, one of the following conditions will be required: 1) ≥2 exacerbations in the previous year despite appropriate inhaled treatments or 2) the need for systemic corticosteroids on top of inhaled treatment for at least 6 months in the previous year.  Exclusion criteria:  COPD Active smoking Chronic interstitial lung diseases Systemic corticosteroid/immunosuppressive treatments for concomitant conditions other than asthma