FTY720 (S)-Phosphate is primarily used in the treatment of multiple sclerosis (MS). It functions by modulating sphingosine-1-phosphate receptors, which helps to reduce the migration of lymphocytes to the central nervous system. This action decreases inflammation and prevents the immune system from attacking the myelin sheath, thereby slowing the progression of the disease. Additionally, it has shown potential in other autoimmune conditions and organ transplant scenarios by suppressing immune responses. Its ability to regulate immune cell trafficking makes it a valuable therapeutic agent in managing diseases characterized by immune system dysregulation.