**4. Role of Th17 in autoimmunity**

In some cases, the immune system attacks our own tissues, causing autoimmunity. IL-17-producing cells play important roles in the development of different autoimmune diseases including rheumatoid arthritis (RA), an inflammation disorder which attacks the synovial joints and multiple sclerosis (MS), characterized by inflammation of the myelin sheath, resulting in de-myelination. It was reported that IL-17-knockout mice were protected against these autoimmune diseases [39]. In contrast, a high level of IL-17 was detected in the serum of patients with MS, RA, and systemic lupus erythematous (SLE). This suggests that Th17 cells expressing high levels of ROR-γt and IL-23R could be one of the causes of these diseases [40]. In addition, it was also reported that the Th17 subset increases the severity of EAE, diabetes, and RA [41, 42].
