CAT and pulmonary fibrosis: It is reasonable, as in the normal airway mucosa, anti oxidant defense systems, such as superoxide dismutase (SOD), glutathione peroxidase, and catalase exist to protect the mucosa from various oxidant stimuli and in fact this enzyme has applied for the treatment of PF in some studies (27,28) and they showed inhibitory effects of SOD on bleomycine –induced pulmonary fibrosis.