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Most children with asthma get symptoms when their immune system overreacts to things in the air.¹ These things are called allergens.

The most common allergens are:

• House dust mite droppings
• Pollen
• Flakes of skin or hair from animals.

To read more about house dust mites, see Trying to get rid of dust mites from your home.

When your child breathes in an allergen, their immune system overreacts in case the allergen is dangerous. It pumps lots of chemicals into the blood. These chemicals make your child's airways swollen and inflamed. The muscles in the airways also tighten, the air passages get narrower and your child has trouble breathing. It all happens very quickly.

About 30 percent to 50 percent of people with asthma get these symptoms again six to 10 hours after they breathe in an allergen. This is called a late reaction.¹

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.