An abnormality of the islet of Langerhans, i.e., of the regions of the pancreas that contain its endocrine cells. These are the alpha cells, which produce glucagon, the beta cells, which produce insulin and amylin, the delta cells, which produce somatostatin, the PP cells, which produce pancreatic polypeptide, and the epsilon cells, which produce ghrelin. The phenotype is Abnormality of the pancreatic islet cells (HP_0006476, a Human Phenotype Ontology id).