The mechanism of this side-effect, probably caused by interferon, remains unclear and several pathophysiological mechanisms have been proposed, centering on the known immunomodulatory activity of interferon [28] . Interferon has direct antiviral and immunomodulatory effect, including cytokine reduction, increased natural killer cell function, and enhanced cellular expression of major histocompatibility class 1 antigens [28] [29] [30] . It is plausible that interferon triggers a lung-specific immune-mediated response resulting in interstitial pneumonitis, similar to other autoimmune diseases.