Influenza A virus is unique among human viruses in its capacity to alter the antigenic phenotype with relative ease and evade neutralizing antibodies. This property is ascribed to the accumulation of a series of amino acid changes in the antigenic regions of hemagglutinin (HA) molecule. Neutralizing antibodies against HA prevent the early stage of infection, whereas neuraminidase (NA) antibodies mediate the antiviral effect by restricting spread of viruses in the host cells after infection. Thus the understanding of antigenic structure on those protective antigen is significant. Sequence analysis of natural variants and escape mutants selected by monoclonal antibodies allowed us to assign each antigenic sites and epitopes to a particular region on the three dimensional structure of HA and NA molecules.