In addition to patients with selective IgE deficiency (IgE < 2.5 kU/mL, but normal IgG, IgA and IgM levels), ultra-low IgE levels may also be found in patients with common variable immunodeficiency (CVID) [38], hyper-IgM syndrome and XLA agammaglobulinemia, due to defects in B cell lineage and IgE-isotype class switching. Here, IGHE is linked to agammaglobulinemia.