Initially discovered on mast cells and basophils whose function is to mediate cellular degranulation and the release of various mediators such as histamine [13], FcεRI has been later detected on many inflammatory cells including human cutaneous Langerhans cells, dendritic cells, monocytes of patients with a number of allergic disorders, on eosinophils from subjects with hypereosinophilic syndrome or asthma, on platelets, and bronchial epithelial cells [14], [15], [16], [17], [18]. This evidence concerns the gene FCER1A and hypereosinophilic syndrome.