MPO and infection: Neutrophils are the predominant cell found in the inflammatory infiltrate of the bronchoalveolar lavage (BAL) fluid of infants ventilated for severe RSV bronchiolitis.5 They are recruited to the lungs early in the course of infection where they are known to release cytokines12 and show delayed apoptosis.15, 16 As the clinical condition improves, lower neutrophil concentrations are found in the BAL fluid.4, 5 Neutrophil activation can be studied by looking at cell surface markers such as the integrins CD11b or internal markers such as myeloperoxidase (MPO) found in the neutrophil granules.