A broadly neutralizing human antibody, CR6261, recognizes a highly conserved epitope on the hemagglutinin (HA) protein of influenza viruses, offering potential for universal prevention and therapy. The antibody neutralizes influenza viruses by blocking conformational changes necessary for membrane fusion. Crystal structures of CR6261 Fab in complex with HA from the 1918 H1N1 pandemic virus and a lethal H5N1 avian influenza virus reveal that CR6261 binds to a conserved helical region in the membrane-proximal stem of HA1/HA2. This epitope is well-ordered in both complexes, with minimal escape mutations possible due to its conservation and the presence of a glycan that may interfere with antibody binding. CR6261 neutralizes multiple influenza subtypes, including H1, H2, H5, H6, H8, and H9, and protects mice from lethal challenge with H1N1 and H5N1 viruses. The antibody's mechanism of action involves stabilizing the pre-fusion state of HA and preventing pH-induced conformational changes that facilitate membrane fusion. CR6261's epitope is located in the stalk region of HA, interacting primarily with the HA2 A-helix and some residues in the HA1 stem region. The antibody's heavy chain is primarily responsible for binding, and its interaction with the HA is critical for neutralization. The epitope's conservation suggests a critical role in membrane fusion, as it is involved in interactions that occur during the late stages of the process. CR6261's ability to neutralize multiple influenza subtypes highlights its potential for broad-spectrum vaccine design and therapy. The study also identifies the importance of glycosylation at Asn38 in HA1 for CR6261 binding, as this site is subject to N-linked glycosylation and may mask the epitope. The findings suggest that a vaccine stimulating production of CR6261-like antibodies could provide broad protection against influenza viruses. The study underscores the importance of rational vaccine design using broadly neutralizing antibodies to combat the antigenic diversity of influenza viruses. The results highlight the potential of CR6261 as a target for the development of a universal influenza vaccine.A broadly neutralizing human antibody, CR6261, recognizes a highly conserved epitope on the hemagglutinin (HA) protein of influenza viruses, offering potential for universal prevention and therapy. The antibody neutralizes influenza viruses by blocking conformational changes necessary for membrane fusion. Crystal structures of CR6261 Fab in complex with HA from the 1918 H1N1 pandemic virus and a lethal H5N1 avian influenza virus reveal that CR6261 binds to a conserved helical region in the membrane-proximal stem of HA1/HA2. This epitope is well-ordered in both complexes, with minimal escape mutations possible due to its conservation and the presence of a glycan that may interfere with antibody binding. CR6261 neutralizes multiple influenza subtypes, including H1, H2, H5, H6, H8, and H9, and protects mice from lethal challenge with H1N1 and H5N1 viruses. The antibody's mechanism of action involves stabilizing the pre-fusion state of HA and preventing pH-induced conformational changes that facilitate membrane fusion. CR6261's epitope is located in the stalk region of HA, interacting primarily with the HA2 A-helix and some residues in the HA1 stem region. The antibody's heavy chain is primarily responsible for binding, and its interaction with the HA is critical for neutralization. The epitope's conservation suggests a critical role in membrane fusion, as it is involved in interactions that occur during the late stages of the process. CR6261's ability to neutralize multiple influenza subtypes highlights its potential for broad-spectrum vaccine design and therapy. The study also identifies the importance of glycosylation at Asn38 in HA1 for CR6261 binding, as this site is subject to N-linked glycosylation and may mask the epitope. The findings suggest that a vaccine stimulating production of CR6261-like antibodies could provide broad protection against influenza viruses. The study underscores the importance of rational vaccine design using broadly neutralizing antibodies to combat the antigenic diversity of influenza viruses. The results highlight the potential of CR6261 as a target for the development of a universal influenza vaccine.