When insulin attaches itself to something else. This can occur in two ways. First, when a cell needs energy, insulin can bind with the outer part of the cell. The cell then can bring glucose (sugar) inside and use it for energy. With the help of insulin, the cell can do its work very well and very quickly. But sometimes the body acts against itself. In this second case, the insulin binds with antibodies. If the insulin is an injected form of insulin and not made by the body, the body sees the insulin as an outside or "foreign" substance. When the injected insulin binds with the antibodies, it does not work as well as when it binds directly to the cell.