Glucagon is secreted by the alpha (α) cells of the islets of Langerhans in the pancreas. It is released when blood glucose falls below normal (hypoglycaemia) and acts primarily on the liver to: (1) stimulate glycogenolysis (glycogen → glucose), (2) stimulate gluconeogenesis (amino acids/lactate → glucose), and (3) inhibit glycolysis. Glucagon is the primary counter-regulatory hormone to insulin, maintaining glucose homeostasis.