A marker that indicates the inflammatory state of the lung would be useful to identify infective/inflammatory exacerbations as opposed to worsening due to pulmonary vascular disease or simply upper airway infection. This marker might help to guide therapy for intensity and duration. Evidence in mice suggest that lymphocytes may be a driving force for inflammation in the CF lung, particularly during exacerbations, and also that human CF lymphocytes have dysfunctional production of cytokines.