CD40LG and bacterial infectious disease: According to European Society for Immunodeficiencies (ESID) [2], male patient with less than 2% CD19+ B cells could be diagnosed with probable agammaglobulinemia if all of the following are positive including (1) recurrent bacterial infections in the first 5 years of life;(2) serum IgG, IgM, and IgA more than 2 SD below;(3) absent isohemagglutinins and/or poor response to vaccines;(4) other causes of hypogammaglobulinemia have been excluded.