Therapeutic neutralizing antibodies can be used to target several viral pathogens, but just a few bacterial pathogens. Pseudomonas aeruginosa is a Gram-negative bacterium that causes acute nosocomial infections such as pneumonia and sepsis as well as chronic infections, particularly in patients with cystic fibrosis. P. aeruginosa expresses the type III secretion system (T3SS), a multiprotein structure that is involved in bacterial persistence and virulence. A major protein component of T3SS is the immunogenic PcrV needle tip protein that elicits the generation of PcrV-specific antibodies. In this study, Simonis et al. show that patients with cystic fibrosis infected with P. aeruginosa develop a polyclonal B cell response to PcrV that involves the generation of T3SS-neutralizing monoclonal antibodies (mAbs). Using a combination of single-cell analysis, cryoelectron microscopy and in vitro antibody assays, the authors show that these neutralizing mAbs targeted a surface-exposed C-terminal PcrV epitope and had broad activity against drug-resistant P. aeruginosa clinical strains. In vivo, patient-derived anti-PcrV mAbs effectively reduced bacterial counts in acute neutropenic lung and pneumonia mouse models. The antibodies also exhibited prophylactic activity, with an effect comparable to that of the antibiotic levofloxacin.