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What is an allergy?

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Quick answer

An allergy is when your immune system overreacts to a normally harmless substance, like pollen or peanuts, treating it as a threat. The defensive response — sneezing, itching, swelling — is the body fighting something that isn't actually dangerous. The immune system mistakes a harmless substance for a threat.

Key things to understand

  1. The immune system mistakes a harmless substance for a threat.
  2. It releases chemicals like histamine, causing symptoms.
  3. Triggers ('allergens') include pollen, foods, dust, and pet dander.
  4. Reactions range from sneezing to life-threatening anaphylaxis.
  5. It's an immune misfire, not a sign of a weak immune system.

People also ask

What causes an allergic reaction?
The immune system wrongly flags a harmless substance as dangerous and releases chemicals like histamine, producing the symptoms.
Why do some people have allergies and others don't?
Genetics, early-life exposures, and environment all influence whether the immune system overreacts to particular substances.
What is anaphylaxis?
A severe, rapid, whole-body allergic reaction that can be life-threatening and needs emergency adrenaline (an EpiPen).