Selective Immunoglobulin M deficiency (sIgMD) is characterized by isolated low to absent levels of serum IgM and defective IgM antibody response following vaccination, infection or natural exposure, while the number of peripheral blood B lymphocytes, immunoglobulin isotype class switch, and serum levels of other immunoglobulins and IgG antibody responses are intact in the majority of patients (15–17). This evidence concerns the gene CD40LG and selective IgM deficiency.