Insulin is a protein that is secreted by islet cells within the pancreas (i.e. An organ that sits behind the stomach) in response to detected increase in blood glucose (blood sugar). This helps a cell to absorb sugar (glucose) from the bloodstream into the inside of the cell. If you are lack of insulin or your body cant produce insulin (e.g. Type 1 diabetes), you can be ill and constant hungry without insulin shot.

In general, commercial Insulin can be divided into three groups: Animal Insulin, Human Synthetic Insulin and GM (Genetically Modified) Synthetic Analogues. The earliest animal insulin came from cows. You can find more about insulin here.