Is IgE or eosinophils the key player in allergic asthma pathogenesis? Are we asking the right question?

Bronchial asthma (BA) is a chronic inflammatory disease with a marked heterogeneity in pathophysiology and etiology. The heterogeneity of BA may be related to the inducing mechanism(s) (allergic vs non-allergic), the histopathological background (eosinophilic vs non-eosinophilic), and the clinical manifestations, particularly in terms of severity and frequency of exacerbations. Asthma can be divided into at least two different endotypes based on the degree of Th2 inflammation (T2 'high' and T2 'low'). For patients with severe uncontrolled asthma, monoclonal antibodies (mAbs) against immunoglobulin E (IgE) or interleukin (IL)-5 are now available as add-on treatments. Treatment decisions for individual patients should consider the biological background in terms of the "driving mechanisms" of inflammation as this should predict the patients' likely responses to treatment. The question is not whether an anti-IgE or an anti-eosinophilic strategy is more effective, but rather what the mechanism is at the origin of the airway. While IgE is involved early in the inflammatory cascade and can be considered as a cause of allergic asthma, eosinophilia can be considered a consequence of the whole process. This article discusses the different roles of the IgE and IL-5/eosinophil pathways in the pathogenic mechanisms of airway inflammation occurring in allergic asthma, and the possible reasons to choose an anti-IgE mAb or anti-IL-5 treatment.