We analyzed: age at diagnosis; prevalence of women; 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; serum levels of IgG, IgG subclasses, IgA, and IgM; blood lymphocyte subpopulations; human leukocyte antigen (HLA)-A and -B types and haplotypes; and responses to 23-valent pneumococcal polysaccharide vaccination (PPSV23; Pneumovax®23, pneumococcal vaccine polyvalent (Merck and Co., Inc., Kenilworth, NJ)). Here, CD40LG is linked to Respiratory tract infection.