This study is for patients with a type of blood cancer called B-cell acute lymphoblastic leukemia (B-ALL) that has returned after treatment. The study aims to find out if a combination of two treatments, **blinatumomab** and **nivolumab**, works better than blinatumomab alone. Blinatumomab is a special protein that helps the body identify and attack cancer cells. Nivolumab is a medicine that can strengthen the immune system to fight cancer. **Key Points to Consider:** - **Participation**: The study is open to patients aged 1 to under 31 with certain types of B-ALL relapse. - **Commitment**: Patients will receive treatments over several cycles, with some receiving additional medications. There are follow-ups every 3 months for a year after treatment. - **Risks and Safety**: The study will check how safe and tolerable the drug combination is. Patients with Down syndrome are included, and their safety will be closely observed. Participants will need to undergo regular tests, including blood and bone marrow checks. It's important to discuss with a doctor to understand if joining this study is a good fit for you.