Most of the cystic fibrosis (CF) patients are or have been pulmonary colonized with bacteria such as Pseudomonas aeruginosa or Staphylococcus aureus. Aim of this study is to detect virulence factor neutralizing antibodies in the sera of the study population followed by B cell repertoire analyses to design B cell-derived neutralizing monoclonal antibodies. The functionality of neutralizing antibodies rests on inhibition of virulence factors by binding of crucial epitopes rather than merely the induction of opsonization. Focusing on patients with bacterial colonization/chronic infections or a history of an acute infection in the past, will increase the likelihood for identification of serum with neutralizing activity as in vivo antigen contact is a prerequisite for antibody development and maturation. Since virulence factors are essential for infection, dissemination and tissue damage, inhibition of these factors by developed neutralizing antibodies might contribute to a favorable outcome of life-threatening infections.