Luminal eosinophilia in asthma arise as a result of (i) the recruitment of mature eosinophils from the periphery in response to locally elaborated chemo-attractants such as eotaxin and/or (ii) the localized maturation of eosinophil lineage-committed progenitors, termed “in situ differentiation” in the presence of locally elaborated cytokines such as IL-5 [48, 49]. This evidence concerns the gene IL5 and asthma.