FIG. 3.

Antigenic structure of influenza B virus HAs. (a) Antigenic structure of B/HK HA. Mutations in four overlapping regions, the 120 loop (cyan), the 150 loop (green), the 160 loop (blue), and the 190 helix (red), have been found to cause antigenicity variation. Together they form a single large antigenic site with overlapping epitopes. The RBS is labeled. (b) Sequence alignment of influenza B virus HAs in the region of residues 161 to 166, highlighting the insertion or deletion in this region. A dot indicates a deletion in the sequence. By using the sequence of B/HK HA as a reference, the extra residues between HA₁162 and HA₁163 present in other strains are treated as insertions and numbered accordingly. For example, B/VI has a 3-residue insertion numbered Asn162′, Asp162′′, and Asn162′′′. Thus, all of the sequences mentioned in the text are based on B/HK HA numbering. B/MD/59, B/Maryland/59; B/SI/64, B/Singapore/64; B/SI/79, B/Singapore/222/79; B/OR/80, B/Oregon/5/80; B/ID/86, B/Idaho/1/86; B/GA/86, B/Georgia/1/86.