You probably don’t think much about itching until you have an itchy spot that’s just out of reach. Then, it’s all you can think about until you’re able to get rid of the “not-quite-painful-but-almost” feeling with a satisfying scratch. What exactly is itching? And what purpose does it serve?

A review published in Physiological Reviews explains that there are many different kinds of itch. Some start with the skin, while others originate with the nervous system. Medical conditions and diseases can make you itch. You can have a sudden (acute) itch that goes away quickly. If you’re in the 15% of the population that has chronic itch, the constant or frequent discomfort—which can include burning and stinging—can make your life miserable. Environmental factors such as animal dander, poisonous plants and chemicals in household cleaners can cause itching. And then there’s the “social itch” that happens when you see someone scratch and it makes you itchy. Scientists don’t know why people react this way.

Researchers used to think the purpose of scratching an itch was simply to remove bugs, prickly plants and anything else that’s not supposed to be on your skin. More recently, scientists think scratching serves as an intentional form of damage to the skin’s surface, which then prompts a chain of physiological responses between the nervous and immune systems and the skin. The pleasure you get from scratching makes you scratch more because it feels good. But in a sneaky way, the pleasure of a good scratch may really be your body’s way of making sure it keeps the lines of communication open among all its systems.

When you’re allergic to something—an animal, plant or food, for example—your body produces a chemical called histamine that regulates the action of immune cells and causes itching. You might want to rub your skin instead of scratch when you have an allergic reaction, but researchers aren’t sure what drives this urge. Histamine-induced itch is the most well-studied type of itch.

The brain is the “final station” for processing itch. The pleasure of scratching is processed in the reward center of the brain, the same area that makes you feel good when you eat your favorite meal or feel the “high” from taking addictive drugs. And that’s what keeps you coming back for more when you sense the tingling or pricking feeling of an itch.

— Erica Roth