The influenza virus’s hemagglutinin (HA) surface proteins then bind to the sialic acid receptors on the surface of a human respiratory tract cell. The structure of the influenza virus’s HA surface proteins is designed to fit the sialic acid receptors of the human cell, like a key to a lock. Once the key enters the lock, the influenza virus is then able to enter and infect the cell. This marks the beginning of a flu infection.
See PHIL 15325, for this image with no labels, and PHIL 15327, for both its labels and accompanying text. The cut-out box shows a close-up view of how an influenza virus’s HA surface protein binds to a sialic acid on the surface of a human respiratory tract cell.