Abstract
Trimeric hemagglutinin and tetrameric neuraminidase molecules isolated from influenza virus bind an average of 9 and 13 molecules respectively of monovalent antibody fragments prepared from IgG isolated from polyclonal sera. In each case this represents an average of approximately three molecules of antibody binding to each protomer. Although there is compelling evidence for the presence of multiple adjacent and overlapping epitopes covering the surface of these two viral antigens, steric hindrance ensures that even under saturating conditions only three molecules of monovalent antibody fragments can be simultaneously accommodated on each monomer.