(1) Serum IgG levels exceeding two standard deviations below the age‐adjusted reference range, confirmed on a minimum of two occasions separated by more than 3 weeks unless levels are severely diminished (< 100–300 mg/dL, contingent on age); (2) a concurrent reduction in either IgA or IgM; (3) compromised antibody responses to vaccination; (4) age exceeding 4 years; and (5) exclusion of secondary aetiologies of hypogammaglobulinemia [90]. Here, CD40LG is linked to agammaglobulinemia.