CD40LG and immune system disorder: The diagnostic criteria for CVID were established by the European and Panamerican Immunodeficiencies Societies (ESID/PAGID) in 1999, and still in force so far, include: marked decrease (at least 2 standard deviations below the mean for age) of IgG, IgA and/or IgM in serum; to be older than 4 yrs, to be negative for isohemagglutinins and/or to have poor responses to vaccines; besides, other causes of hypogammaglobulinemia must have been excluded.