Abstract Specific antibody responses to the annual influenza vaccine can protect against infection if the vaccine and the infective strain are antigenically well-matched. However, due to the variability of the influenza genome, emergent strains arise for which the vaccine proves to be ineffective, and moreover, necessitate its yearly reformulation. Efforts have been made into identifying epitopes within more conserved elements of the influenza virus as an alternative attack on a wider range of strains. Through our platform of examining monoclonal antibodies (MAbs) expressed from plasmacytes derived from memory B cells from normal healthy human donors, we have found a previously unknown conformational antigenic determinant within the relatively invariable ectodomain of M2 (M2e) of influenza A. The MAbs bind to virus and cell-surface expressed M2, but not to the corresponding M2e peptide. The M2e region recognized is in almost all strains isolated to date. Furthermore, in a mouse model of infection, the MAbs can protect against lethal challenge. These results suggest that naturally expressed M2 can elicit a broadly cross-reactive and protective antibody response. And, the human recombinant anti-M2e MAbs might serve as therapeutic agents against infection.