The term Islet in the medical field would oftentimes refer to the islet of Langerhans. The islet of Langerhans is actually a portion of the pancreas in which endocrine cells can be found. These endocrine cells are the types of cells which produce certain hormones. The islet of Langerhans was discovered in 1869 by Paul Langerhans, a German pathologist.

There are actually about a million islets of Langerhans scattered all throughout the pancreas. Each islet has a size of around 0.2 millimeter in diameter. When all of the islets are weighed altogether, this would have a mass of around 1 to 1.5 grams. These islets account for about 1 to 2 % of the total mass of the pancreas.

Different endocrine or hormone-producing cells comprise the islet of langerhans. Each type of cell produces a specific hormone. For instance, the alpha cells produce glucagon, beta cells for amylin and insulin, and the delta cells for somatostatin. Other hormone-producing cells of the islet of Langerhans are PP cells and epsilon cells which also produce specific types of hormones. Studies have also found out that different species tend to have distinct pancreatic islets especially in terms of its cycloarchitecture. The different endocrine cells also have specific function in the paracrine system. The hormone insulin which is produced by the beta cells of the islet of Langerhans helps activate the beta cells but acts oppositely to the alpha cells. Glucagon on the other hand activates alpha cells which consequently activates both beta and delta cells. Somatostatin on the other hand inhibits both alpha and beta cells.