Direct evidence for a role of epitope masking in IgG-mediated suppression is difficult to obtain, and this hypothesis will probably remain a “diagnosis by exclusion.” Epitope masking by IgM antibodies was, however, suggested to play an important role in selection of B cells in germinal centers (25), and to be the most important explanation for poor secondary antibody responses to the stem of the influenza surface hemagglutinin during vaccinations (45). The gene discussed is CD40LG; the disease is influenza.