Concept:
The pancreas has both endocrine and exocrine functions. The endocrine part of the pancreas is made up of the islets of Langerhans.
Step 1: The islets of Langerhans contain different types of cells.
Step 2: Alpha cells secrete glucagon.
Step 3: Beta cells secrete insulin.
Step 4: Delta cells secrete somatostatin.
Step 5: Insulin lowers blood glucose level by helping glucose enter body cells.
\[
\boxed{\text{Beta cells}}
\]