The Cytochrome-P-450 enzymes (CYP) are among the most important xenobiotic-metabolizing enzymes, which produce reactive oxygen species (ROS) as the result of metabolizing xenobiotics. ROS are believed to play important roles in the pathophysiology of autoimmune diseases. ROS can alter the structure of cellular antigens to produce a "neo-antigen" which could mount an autoimmune response against the original antigen through molecular mimicry. ROS are involved in apoptosis, activation of antigen presenting cells and initiation or amplification of diverse immunologic reactions. Taking all these facts together, it could be speculated that CYP may be involved in the initiation and/or amplification of autoimmune phenomena.