Ustekinumab is a medication used to treat autoimmune conditions such as psoriasis, psoriatic arthritis, and Crohn's disease. It works by targeting specific proteins in the immune system, which helps reduce inflammation and the overactive immune response associated with these diseases. Administered via injection, ustekinumab is typically given initially and then maintained with regular doses. It is important for patients to discuss potential side effects and monitor their health with a healthcare provider while using this treatment.