We performed a backward stepwise variable regression on age at onset of frequent or severe respiratory tract infection using these independent variables: sex; age at diagnosis; diabetes; autoimmune condition(s); atopy; allergy; corticosteroid use; body mass index; serum Ig isotype levels; blood lymphocyte subsets; three HLA-A and -B haplotypes; and referring physician specialties. Here, HLA-A is linked to allergic disease.