The importance of TNFRSF13B and BLIMP-1 for development of plasma cells and production of much of the Ig in blood was suggested by investigation of genetic basis of hypogammaglobulinemia, i.e., IgG-deficiency, IgM-deficiency, and IgA-deficiency observed in common variable immunodeficiency (CVID) and in selective IgA deficiency (13, 14). Here, TNFRSF13B is linked to selective IgA deficiency disease.