The most prevalent polygenic antibody-deficiency IEI is common variable immunodeficiency (CVID), defined by IgG levels two standard deviations below the age-appropriate reference range, combined with either low IgA or IgM, poor antibody responses to vaccination, age greater than 4 years, and exclusion of secondary causes of hypogammaglobulinemia [71–73]. Here, CD40LG is linked to agammaglobulinemia.