In the absence of a hematological malignancy, the Th2-type cytokine interleukin-5 (IL-5) is critical for the development of a peripheral eosinophilia [24] whilst B lymphocyte class-switching to IgE synthesis is dependent on the Th2-type cytokines IL-4/IL-13 [25]. The gene discussed is IGHE; the disease is hematologic disorder.