The pancreas has both exocrine and endocrine functions. The endocrine portion consists of small clusters of cells called islets of Langerhans (named after Paul Langerhans, who discovered them in 1869), scattered throughout the exocrine pancreatic tissue. There are approximately 1-2 million islets in a healthy human pancreas, making up only about 1-2% of total pancreatic mass, yet they are critical for whole-body glucose homeostasis. Each islet contains several distinct cell types arranged in a characteristic pattern, with beta cells typically clustered in the centre/core and alpha, delta, and other cell types distributed around the periphery, though this arrangement varies between species (human islets have a more intermingled architecture than rodent islets).

Alpha (A) cells make up roughly 15-20% of the cells in pancreatic islets. They synthesise and secrete the hormone glucagon, a 29-amino-acid peptide hormone. Glucagon release is stimulated by low blood glucose (hypoglycaemia), amino acids (especially after a protein-rich meal), exercise, and sympathetic nervous system activation. Glucagon release is inhibited by high blood glucose, insulin, and somatostatin. Once released, glucagon travels to the liver and binds glucagon receptors, activating a G-protein coupled signalling cascade (via cAMP and protein kinase A) that triggers glycogenolysis (the breakdown of stored glycogen into glucose) and gluconeogenesis (the synthesis of new glucose from amino acids, lactate, and glycerol). The net effect is a rise in blood glucose concentration, making glucagon the primary counter-regulatory hormone to insulin.

Beta (B) cells are the most abundant islet cell type, comprising 65-80% of all islet cells. They produce insulin, a 51-amino-acid hormone consisting of two polypeptide chains (A and B chains) linked by disulfide bonds. Insulin is initially synthesised as preproinsulin, processed to proinsulin, and then cleaved by prohormone convertases into mature insulin plus C-peptide (a useful clinical marker of endogenous insulin production). Insulin secretion is triggered when blood glucose rises: glucose enters beta cells via GLUT2 transporters, is metabolised through glycolysis, raising the ATP/ADP ratio, which closes ATP-sensitive potassium channels, depolarises the cell membrane, opens voltage-gated calcium channels, and triggers calcium-dependent exocytosis of insulin-containing secretory granules. Insulin lowers blood glucose by promoting glucose uptake into skeletal muscle and adipose tissue (via GLUT4 transporter translocation), stimulating glycogen synthesis in liver and muscle, promoting fat storage, and suppressing hepatic glucose production.

Delta (D) cells (5-10% of islet cells): secrete somatostatin, a hormone that acts locally within the islet to inhibit both insulin and glucagon secretion, functioning as a paracrine brake on islet hormone output. Somatostatin is also produced elsewhere in the body, including the hypothalamus (where it inhibits growth hormone release) and the gastrointestinal tract (where it inhibits various digestive secretions). PP cells (F cells, less than 5%): secrete pancreatic polypeptide, which inhibits pancreatic exocrine secretion and gallbladder contraction, and may play a role in appetite regulation. Epsilon cells (very rare, less than 1%): secrete ghrelin, the "hunger hormone" better known for being produced by the stomach, which stimulates appetite and growth hormone release.

Blood glucose regulation depends on a dynamic balance between insulin (the only hormone that lowers blood glucose) and several counter-regulatory hormones that raise it (glucagon, adrenaline/epinephrine, cortisol, and growth hormone). After a meal, rising blood glucose triggers insulin release from beta cells, which promotes glucose uptake and storage, bringing levels back down. During fasting or between meals, falling blood glucose triggers glucagon release from alpha cells, which mobilises stored glucose to maintain adequate blood levels for vital organs like the brain (which depends almost entirely on glucose for energy under normal conditions). This reciprocal relationship between insulin and glucagon - normally insulin is high and glucagon is low after eating, and vice versa during fasting - is the foundation of normal glucose homeostasis, keeping blood glucose within a tight range (approximately 70-100 mg/dL fasting) despite highly variable food intake.

Type 1 diabetes mellitus: an autoimmune condition in which the immune system destroys insulin-producing beta cells, leading to absolute insulin deficiency. Typically diagnosed in children and young adults. Patients require lifelong exogenous insulin therapy since they produce little to no endogenous insulin. Type 2 diabetes mellitus: characterised by insulin resistance (target tissues respond poorly to insulin) combined with progressive beta cell dysfunction over time. Strongly associated with obesity, sedentary lifestyle, and genetic predisposition. Often managed initially with lifestyle changes and oral medications, though many patients eventually require insulin as beta cell function declines. Diagnostic criteria for diabetes include fasting blood glucose ≥126 mg/dL, random blood glucose ≥200 mg/dL with symptoms, or HbA1c ≥6.5%. Chronic hyperglycaemia in untreated or poorly controlled diabetes damages blood vessels and nerves over time, leading to complications including retinopathy (vision loss), nephropathy (kidney damage), neuropathy (nerve damage), and increased cardiovascular disease risk.

C-peptide test: since C-peptide is released in equal amounts to insulin during processing of proinsulin, measuring C-peptide indicates how much insulin the body is producing on its own - useful for distinguishing Type 1 diabetes (low C-peptide, little endogenous insulin) from Type 2 diabetes (often normal or even elevated C-peptide due to insulin resistance) and for assessing beta cell function in long-standing diabetes. Oral glucose tolerance test (OGTT): measures how the body handles a glucose load over 2 hours, used to diagnose diabetes and gestational diabetes. HbA1c (glycated haemoglobin): reflects average blood glucose over the preceding 2-3 months, providing a longer-term picture of glucose control than a single blood glucose measurement. Glucagon stimulation test: used in some clinical contexts to assess pancreatic beta cell reserve by measuring the insulin/C-peptide response after glucagon administration.

Questions distinguishing alpha cell and beta cell hormone secretion are extremely common in physiology and biology examinations because the two are easily confused, yet the distinction is fundamental. A reliable memory aid: "Alpha cells - Always raises (glucagon raises blood sugar)" while "Beta cells - Brings down Blood sugar (insulin Brings down/lowers blood sugar)." Both alphabetically and functionally, remembering that alpha comes first and glucagon acts first during fasting (raising glucose) while beta and insulin work after eating (lowering glucose) can help cement the relationship. It is also worth noting the numerical predominance of beta cells (the majority islet cell type) reflects the fact that insulin secretion needs to be tightly and rapidly regulated since hypoglycaemia is acutely dangerous, whereas the body has multiple backup mechanisms (glucagon, adrenaline, cortisol, growth hormone) for raising blood glucose if needed.