A complex of influenza hemagglutinin with a neutralizing antibody that binds outside the virus receptor binding site

The structure of a complex of influenza hemagglutinin (HA) with a neutraliz ing antibody shows that the antibody binds to HA at a distance from the vir us receptor binding site. Comparison of the properties of this antibody and its Fab with those of an antibody that recognizes an epitope overlapping t he receptor binding site leads to two main conclusions. first, inhibition o f receptor binding is an important component of neutralization. Second, the efficiency of neutralization by the antibodies ranks in the same order as their avidities for HA, and their large size makes these antibodies highly efficient at neutralization, regardless of the location of their epitope in relation to the virus receptor binding site. These observations provide ra tionales for the range of antibody specificities that are detected in immun e sera and for the distribution of sequence changes on the membrane-distal surface of influenza HAs that occur during 'antigenic drift.'