Used primarily as an immunosuppressive agent in organ transplantation to prevent rejection by suppressing the body's immune response. Also widely prescribed in autoimmune disorders such as rheumatoid arthritis, systemic lupus erythematosus, and inflammatory bowel diseases like Crohn's disease and ulcerative colitis. It works by inhibiting the synthesis of DNA, RNA, and proteins in rapidly dividing immune cells, thereby reducing inflammation and disease activity. Often used when other treatments fail or as a steroid-sparing agent to reduce long-term corticosteroid use. Requires careful monitoring due to potential bone marrow suppression and increased risk of infections.