Insulin is a hormone which is produced in the pancreas, by the cells called the islets of Langerhans.

What does insulin do?

Insulin controls the use of glucose, fats, and lipids by the body.
An excess of insulin, sometimes caused by a pancreatic tumor, causes a low level of sugar in the blood (hypoglycemia); and a lack of insulin produces an abnormally high level of sugar in the blood (hyperglycemia), which is a symptom of diabetes mellitus.
Since severe deficiency of insulin can be life-threatening, the natural insulin level in diabetic patients is augmented by insulin injections.

Insulin Production

Insulin is obtained from the pancreases of cattle or it is produced synthetically, using recombinant DNA which does not stimulate the formation of antibodies against insulin, or sensitization, as the animal insulin may do.
Insulin is prepared in various ways to make it act quickly (soluble insulin), or slowly, in combination with zinc and other substances, so that only one or two injections a day are necessary.
The strength of insulin is expressed in units of activity; the dosage is measured in syringes marked off in units.