Figure 1. Antibodies can target the head or stalk of the hemagglutinin protein to provide different mechanisms of action.
(I) Anti-influenza A virus hemagglutinin (HA) antibodies that bind to the HA globular head inhibit virus binding to host cells through the sialic acid receptor, thus preventing virus entry.

(II) Anti-HA antibodies that bind to the HA stalk region prevent the HA conformational changes needed to allow fusion of the viral envelop with the host membrane, impeding progression of the virus life cycle.