In conclusion, the human anti-plague antibodies reported here represent perhaps the ones that are closest to practical clinical use. They may be safer and more efficient in the human system due to their fully human nature, and a likely longer half-life in humans. Also, intravenous application of the antibodies in humans may be a rapid delivery system that may augment antibiotics treatment in plague-exposed individuals. Finally, the affinity of all three antibodies can be further increased using readily available techniques, may reduce the dose required for efficient protection. The successful development of these three human anti-plague Figure 9 . Post-challenge administration of the human anti-F1 m252 mAb conferred better protection. The human anti-F1 m252 mAb was administered before or after Y. pestis challenge, and mice were monitored for 21 days after challenge. The mouse anti-V 7.3m mAb (100 mg) was used as a positive control mAb. Normal human serum (NHS, 500 mg), which was used as a negative control, had only 5 mice per group. The numbers behind each antibody represent the time in days in which the antibody was administered (500 mg) to mice relative to the day of challenge (day 0). The two numbers after the human anti-F1 m252 (21 and +5) represent two different days when the mAb (500 mg) was added to the same group of mice relative to the day of challenge. doi:10.1371/journal.pone.0013047.g009 antibodies in this model suggests that new and more potent anti-V antibodies can be potentially developed using the same approach but with restricted V-antigen subunit fragments containing the critical neutralizing epitopes, and perhaps other virulent factors.