CD40LG and common variable immunodeficiency: The diagnosis of CVID is established after the 4th year of age (but symptoms may be present earlier) and based on the following criteria (2): marked decrease of immunoglobulin (Ig)G, IgA and/or IgM as compared with age-related standard, impaired or absent antibody production, exclusion of secondary causes of hypogammaglobulinemia, and no evidence of profound T-cell deficiency.