Hyper-immunoglobulin M syndrome (HIGM) is a rare primary immunodeficiency, in which defective B cell isotype switching leads to a phenotype characterized by low or absent serum levels of IgG, IgA and IgE, whereas the IgM concentration is either normal or increased [1–4]. The gene discussed is CD40LG; the disease is hyper-IgM syndrome type 1.