 The study will determine whether the anti-CD3 monoclonal antibody, teplizumab, can help to prevent or delay the onset of type 1 diabetes T1D in relatives determined to be at very high risk for developing the disease. Teplizumab has been studied in new onset type 1 diabetes for testing of efficacy and safety in previous studies; other studies are currently in progress. The results of previous studies indicate that teplizumab reduces the loss of insulin production during the first year after diagnosis in individuals with type 1 diabetes. The purpose of this study is to determine if teplizumab can interdict the immune process that causes the destruction of insulin secreting beta cells in the pancreas during the pre-diabetic state and thereby prevent or delay the onset of type 1 diabetes. 