This study is for people with B-cell Acute Lymphoblastic Leukemia (B-ALL) that has come back after treatment. Researchers are comparing two treatments: blinatumomab (a special protein that helps the body find and kill cancer cells) alone, or combined with nivolumab (a medicine that boosts the immune system).

Key Points:

• The study is for patients aged 1 to 31 years, including those with Down syndrome.
• Participants will be randomly assigned to different groups and receive different combinations of treatments over about 2 years.
• Regular check-ups, including blood and other tests, are necessary throughout the study.

Participants must meet specific health requirements and cannot have certain other illnesses. Women who can have children must use birth control, and breastfeeding is not allowed. After treatment, follow-up visits are required every 3 months for a year.