We performed a backward stepwise regression on IgG2 levels using these independent variables: age at diagnosis; sex; upper and lower respiratory tract infection; other infection; autoimmune condition(s); atopy; other allergy; reports of frequent or severe respiratory tract infection in first-degree relatives (dichotomous); and IgG, IgG1, IgG3, IgG4, IgA, and IgM levels. Here, CD40LG is linked to Recurrent lower respiratory tract infections.