This study is researching a drug called **teplizumab** to see how it helps people with **Stage 3 Type 1 Diabetes (T1D)**, which is a condition where the body cannot produce insulin properly. **Teplizumab** will be compared to a **placebo**, which looks like the drug but has no effect. Both will be given through an **intravenous (IV) infusion**, which means the medicine goes directly into the bloodstream through a vein. Participants will be watched for **84 weeks (about 19 months)** to check the changes in their blood sugar levels and how much they need insulin. **Key Points:** 1. **Study length:** Participants will be involved for **84 weeks**. 2. **Visits and Treatment:** Medication is given by IV infusion, comparing teplizumab to a placebo. 3. **Eligibility:** Must be aged 1-25 years with Stage 3 T1D and meet specific health criteria. This study is important because it might help people with T1D manage their condition better. There are certain rules about who can join this study, so talk to your doctor to see if you qualify.