Allergic asthma and rhinitis are characterized by IgE-mediated inflammation consequent to the exposure to a specific allergen. Therefore, IgE production may be considered the hallmark of allergy. Actually, allergen immunotherapy (AIT) is the unique causal treatment for respiratory allergy. Indeed, successful AIT can induce a physiologic immune tolerance toward the causal allergen. There is evidence to show that AIT is safe and effective; its indication has been rigorously stated by robust position papers. AIT efficacy assessment is mainly based on symptom reduction and drug saving. AIT responder quote may range between 60 and 90% of AIT prescription. As AIT is expensive and long-lasting, the availability of a marker able to predict AIT responder is a crucial relevance. This issue has been the topic of several studies. Very recently, it has been preliminarly reported that a cut-off of serum specific IgE levels could define AIT-responder. However, further rigorous studies will confirm this view.