That's a great question. An itch (which is scientifically known as"pruritus") is caused by irritation of skin cells and the nerves connected to the skin. The itch-causing irritant can be physical (like a feather),an infectious organism (like lice), thermal, or chemical. Itches can be really annoying, but they are also important because an itch (like other skin sensations such as heat and pain) warns you that something is affecting your skin and can help a person to protect themselves from potentially harmful things. Itching is also caused by a number of skin diseases as well as diseases that can affect the entire body (like autoimmune disorders or allergies).

Just like pain, itchiness is the reaction to some external problem that the body wants you to deal with. When something touches your skin and makes it itch, your brain encourages you to do something about it by scratching. For example, if there is a bug on your skin, it makes you itchy which will encourage you to flick off the bug to stop the itch sensation. If you accidentally come into contact with poison ivy, the itch will cause you to avoid touching it in the future which will ultimately make you healthier. So it’s basically a way to tell you how to deal with an immediate problem and to teach you how to take care of your skin in the future.

Itches on our body are explained by an evolutionary argument. When we feel that something is itchy, we want to touch it or scratch it to remove the irritant. This is advantageous to our evolutionary fitness because we can prevent potential injury. For example, a tickle could be from a biting insect landing on your arm or an itch could be from an irritating plant rubbing against your skin that could cause a rash or infection if not removed. Believe it or not, these kinds of injuries could be life threatening, especially for early humans that did not have access to antivenoms for poisonous insects/spiders or antibiotics for infections. As a result, the humans with a stronger itch-reaction to small irritants were more likely to avoid bites and infections and survive to reproduce. That is how evolutionary fitness works and explains so many of the curiosities of our modern-day bodies.