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What exactly is an itch, and why do we get itchy?

The short answer

An itch is a sensation triggered by specialized nerve endings in the skin. Itchiness is thought to have evolved to protect us from allergens and parasites. But there are a multitude of other reasons we get itchy including dry skin, disease, psychiatric issues, and nerve damage.

The long answer

What is an itch?

An itch is a sensation caused by a stimulation of specific nerve endings called pruriceptors that are only there to produce an itchy feeling in the skin. Itchiness is a signal your brain sends to let you know that something that is irritating the skin.

Why do we get itchy?

Itchiness is an evolutionary advantage to help us protect our skin against toxins and critters, but there are other reasons we get itchy.

Generally there are five main causes of itchiness:

1. Contact with an allergen or parasite: Your body is on alert at all times for intruders and toxins from the outside world. So when your body senses an allergen (like mosquito bite) or some other toxic chemical (like poison ivy), it triggers the release of histamines. Histamines boost blood flow to the affected area which creates inflammation and delivers chemicals from your immune system. The inflammation stimulates the pruriceptors, resulting in an itchy feeling.
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2. Dry skin: When skin gets dry, it becomes more rigid and this increased friction results in microfractures. To heal the fractures, your body increases blood flow which results in inflammation. This inflammation again stimulates the pruriceptors, resulting in an itchy feeling.
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3. Certain diseases: Diseases like liver disease, kidney disease, anemia, diabetes, thyroid problems and certain cancers have been known to cause itchiness. The reasons vary but are primarily due to either a build up of toxins in the skin or a chemical imbalance that causes dry skin.​
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4. Psychiatric issues: More frequent itchiness is associated with people who have heightened stress, anxiety, obsessive-compulsive disorder, and other mood disorders. The "psychogenic itch" is not well understood at this time, but it's thought to be related to increased stress levels making it difficult for the brain discriminate between itch-worthy and non-itch-worthy stimuli, along with the potential for a feedback loop of pleasure that results from scratching an itch.
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5. Nerve damage: Nerve damage can cause itching for a number of reasons. It can affect the way that the brain processes sensations from the skin, leading to sensations of itching even when there is no stimulus present. In other cases, nerve damage can cause changes in the way that the skin responds to irritants, leading to itching as a result. Additionally, nerve damage can also lead to decreased blood flow in the affected area, which can cause itching due to dry skin.

Bonus brain points

Why does scratching feel so good?

When you go to scratch that itch, you're rubbing not only pruriceptors but also other pain and touch nerve endings. This sends mixed signals to your brain which results in temporary relief. But as soon as you stop scratching, the itch will return. It's tempting to scratch for the momentary respite from the itch, but it's best to keep your hands off as scratching may cause the skin to become even more irritated.

​Sources​

Book of the Week

​The Invisible Kingdom: Reimagining Chronic Illness by Meghan O'Rourke​

I just finished reading this book and found it to be an incredibly illuminating and, at times, heartbreaking account of what it's like to live with chronic illness and autoimmune issues. The book spans personal reflections, the latest medical breakthroughs (and wide gaps), and history of how our culture views disease and health. With a dizzying array of symptoms that are nearly impossible to diagnose in certain cases, people with chronic illness face marginalization and neglect from the medical community. Through its blend of lyricism, empathy, and critical analysis, "The Invisible Kingdom" offers hope, solace, and a call for a radical shift in our approach to disease.

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