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CELLS TALK TO each other constantly. They chat about what we eat and drink, about where bacteria have crept in, about whether we are stressed or afraid. Should we start an inflammation here? Should these blood vessels expand? Is the heart beating fast enough? Are we breaking down enough fat? Billions of conversations are taking place without making a sound.

The language of cells is written in molecules. They communicate by sending and receiving chemical messages, often different proteins. Some of them resemble loud cries, and race through blood from one end to another. If you’ve just eaten, your pancreas screams out a protein: INSULIN! And once the liver cells receive this protein message, they begin to assemble blood sugar into long chains which they save for later. It would be terribly confusing and tiring for the liver if the pancreas didn’t keep it updated about your daily meals. Ask a rocking horse about how much food it eats a day! Your liver takes care of your blood sugar and has to alternate between storing your energy for later and using it now. If you skip breakfast or suddenly eat a piece of cake before dinner, you can be sure it will be discussed by your cells immediately. The cells can also have more intimate conversations with their neighbours by releasing tiny quantities of a certain substance into the surrounding fluid. In addition, it’s not unusual for them to talk to themselves a little. An immune cell that has detected an infection will give itself a kind of pep talk before it is ready to attack.

All cells are surrounded by a thin film called the cell membrane, and only a few molecules manage to sneak through it into the cell without permission. Instead, most of the messages are delivered indirectly by hooking on to a type of molecule called a receptor, which sits on the cell’s surface. The messages fit the receptors like a key fits a keyhole. For example, on the surface of a liver cell there is a receptor for insulin. When the insulin molecule hooks on to the receptor, it triggers a chain reaction within the cell, and the liver cells begin to store nutrition.

Many diseases occur due to communication failure between cells. In type 1 diabetes, the pancreas struggles to be heard – it cannot produce enough insulin. For unknown reasons, the body’s immune system begins to attack the cells that make insulin, so that the usual Oi! to the liver comes out more like a polite Ahem. That means it’s up to the patient to supply insulin messages to the body using a syringe. In type 2 diabetes, the pancreas attempts to report that the person has eaten, but the cells don’t hear it. The insulin passes through the blood, but the receptors on the cell’s surface struggle to pick it up. The danger with diabetes is that the cells are convinced they are starving, no matter how heartily the person eats. The liver, oblivious, keeps breaking down its energy reserves and the blood sugar level becomes dangerously high. Unable to use the sugar, the body has to get rid of it through the urine. In the early days of medicine, it was actually common practice to diagnose diabetes patients by tasting their urine. This didn’t seem to bother the English doctor Thomas Willis, who in 1674 wrote that the urine he had sampled was ‘wonderfully sweet as if it was imbued with honey or sugar’. He suggested adding the Latin suffix mellitus to the disease’s name, which means ‘honeydew’. The term diabetes mellitus is still used today.