This analysis indicated: (i) the presence of productive IgH rearrangements starting at 2-month old bronchiolitis infants; (ii) a dominant VH1 usage in the IgM and IgG repertoires and VH3 in IgA sequences; (iii) more diverse CDRs in IgM-repertoires than in IgG; (iv) higher VH mutation and selection strength in IgA-repertoires, and (v) intraclonal switching among VH family sequences in the NLF. The gene discussed is CD40LG; the disease is bronchiolitis.