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 infection.