Increased levels of cytokines and high-mobility group box 1 are associated with the development of severe pneumonia, but not acute encephalopathy, in 2009 H1N1 influenza-infected children

► The 2009 H1N1 influenza virus has caused severe pneumonia and acute encephalopathy. ► Elevated serum levels of Th2 cytokines appear to be unique to 2009 H1N1 influenza. ► HMGB1 could play an important role in the pathogenesis of severe pneumonia. ► There appear to be different pathologic processes for encephalopathy and pneumonia.