Fig. 3. (a) Hemagglutinin (HA) is a trimeric protein that binds to sialic acid-terminated glycans in a reversible manner. (b) Neuraminidase (NA) is a tetrameric protein that binds and cleaves sialic acid-terminated glycans. (c) HA and NA are present on the surface of influenza in high copy numbers, approximately in a 6 : 1 ratio.
