Mepolizumab does not alter the blood basophil count in severe asthma.

[First paragraph] To the Editor: Mepolizumab (anti‐IL‐5) depletes blood and airway eosinophils, and, clinically, allows down‐titration of oral corticosteroid and a reduction in the frequency of eosinophil‐dependent exacerbations.1 Basophils also express IL‐5Rα, participate in T2‐mediated inflammatory pathways2 and have been associated with exacerbation frequency.3 Whilst basophil progenitors are unlikely to depend on IL‐5 for development,4 blood basophil counts measured in routine clinical laboratories suggest they decrease following mepolizumab treatment.5-8