No insulin action. Type 1 diabetes occurs when the body has no way of having Insulin to help the body use sugar as a fuel for its activities. It is the same as trying to run a car without gasoline. The treatment is to supply the Insulin by injection.
See below. Only 5% of people with diabetes in the us have type 1 diabetes. This is caused by the inability of your body to produce insulin, a hormone that is needed to convert starches, sugar and other food into energy for you to live. Type 2 diabetes is much more common and is more complicated. In this form, your Insulin is either not working well or at all or you don't make enough insulin.
Quirky antibodies. T1dm is usually the endpoint to a quirky reaction to a viral illness. The body makes antibodies to the virus which chases them off, but the antibodies linger & attack & destroy the Insulin producing cells in your body. The antibodies eventually end your ability to make Insulin & you must use artificial Insulin to replace it. Without insulin your blood sugar cannot enter the cells, gets too high.