Pulmonary aberrant immune response is considered a significant feature of ARDS induced by 2009 H1N1 virus [19, 32] . In the present mouse model, the number of leukocytes observed in the BALF of virus-infected mice significantly increased compared with the control mice on day 8 p.i. Different counts in BALF showed that the proportion of neutrophils dramatically increased. These innate immune cells were capable of reducing the virus load in the lung [33] ; however, they could cause lung injury through direct or indirect mechanisms. Neutrophil oxidants and proteases can cause direct injury of cells in the alveolar-capillary membrane [34] . Neutrophils and macrophages can secrete copious amounts of chemokines and cytokines that can recruit more immune cells into lung tissues, and produce a ''cytokine storm'', one of the most important factors in the production of ARDS [35] .