Study Overview: This study aims to check how well a medicine called teplizumab works compared to a placebo (a fake treatment) in kids and young adults aged 1 to 25 who have recently been diagnosed with Stage 3 Type 1 Diabetes (T1D). This study will take about 18 months for each participant. Teplizumab and placebo are given through an IV, which means it's put directly into a vein.

Type 1 Diabetes (T1D) is a condition where the body can't produce insulin, a hormone that helps control blood sugar. Stage 3 means the disease is more advanced.

• The study will last 18 months for each person.
• Participants must be between ages 1 and 25.
• Involves regular IV infusions and check-ups.

Eligibility: Participants must have a recent Stage 3 T1D diagnosis and test positive for T1D-related antibodies. Women must use effective birth control if they can have children. People with other types of diabetes, certain infections, or recent vaccinations can't join.

Before joining, it's important to understand the study's requirements and risks. Always talk to a doctor for more details.