In our study, cellular and humoral cross-reactive immunity seemed to target antigens other than HA. Infl uenza (H5N1) cases occur mainly in young people (32) . This fi nding may be explained by hypothesizing that older people, although not previously exposed to H5N1 subtype, may have gained protective immunity by previous infections sustained by circulating infl uenza virus strains. It has also been shown that immunity to the N1 NA from the human infl uenza virus cross-reacts with the avian N1 NA virus and that this cross-reactivity protects mice against infection with the avian infl uenza virus (H5N1) (26) . All these fi ndings may be explained by hypothesizing that cross-reactive immunity is targeting the N1 NA antigen. However, whether cross-reactive antibodies to NA and CD4 T cells would be protective against illness and death, especially from infl uenza (H5N1) infection is not known. Further studies will be necessary to elucidate this point.