Cross-reactive neutralizing antibodies against influenza A viruses have received attention for their potentials for prophylactic and therapeutic. These antibodies usually bind to relatively conserved stem domains of influenza hemagglutinin, one of surface glycoproteins responsible for viral binding to sialic acid-tagged cellular receptors and for membrane fusion to initiate a release process of viral genomes inside cells. Recently, a similar approach extended to influenza B viruses, which causing annual epidemics only in the human population, and some of human monoclonal antibodies exhibited promising efficacies against two antigenically diverged lineages of influenza B viruses. Moreover, one of these broadly neutralizing antibodies protected mice against both of influenza A and B challenges. Appropriate immunization may selectively enhance the efficacy of these antibodies, and this strategy may lead individuals to be prepared with broad immune responses against various influenza viruses.