Neonatal respiratory infections remain a leading cause of mortality and morbidity worldwide, yet there is still little information about the role of B cell populations in the respiratory tract. CD19 + B220 lo are B cells that are found in neonatal lungs, and antibiotic treatment of pregnant females and their offspring reduced the number of these cells and modified their VH immunoglobulin repertoire. Activation in vitro by the TLR2 (PAM3 and FSL1) and TLR4 (LPS) ligands induced proliferation and