The pancreatic islets or, historically, the Islets of Langerhans (named after German pathologist Paul Langerhans who first described them in 1869) are the cells that produce pancreatic hormones, the best known and most important of which is insulin. They make up about 5% of the volume of the pancreas, but receive about 10-15% of its blood flow.
The islets are the subject of much research to establish the etiology for diabetes mellitus. However, despite this research, it is still not known if the islets of diabetes patients have been damaged in some way (possibly by an autoimmune response) or they just become non-functional. Many diabetic patients have what appear to be perfectly healthy islets.