researchers determined the crystal structure of the fi6 antibody when it was bound to h1 and h3 ha proteins. sitting atop the ha spike is a globular head domain that binds to cellular receptors during viral entry and contains the major antigenic sites targeted by the immune system. because of this selective pressure, the sequence in the head domain drifts enough to require an updated seasonal vaccine most years. a stalk domain connects the head to the viral membrane and is responsible for fusing viral and host membranes so that the pathogen can invade human cells. the immune system usually does not have a strong response to the partially hidden stalk domain, so portions of the stalk remain highly conserved across all influenza subtypes. the fi6 antibody makes extensive contacts with conserved parts of the stalk, thereby blocking ha from harpooning a sticky fusion peptide into the host membrane during viral entry.