There is a distinct interest in developing monoclonal antibodies for use in a therapeutic setting. We selected three representative antibodies of the set we have identified for detailed functional analysis both in vitro (Fig. 3 C) and in vivo ( Fig. 5 and Fig. 6) , including: EM-4C04, 1009-3B06, and 70-1F02. The antibodies EM-4C04 and 1009-3B06 are specific for the active site of the HA molecule, whereas 70-1F02 binds to the stalk region. Furthermore, EM-4C04 is highly specific for pandemic H1N1, whereas 1009-3B06 and 70-F02 display the idea that a vaccine might be developed that preferentially targets the HA stalk, thus providing broad protection against many influenza strains.