An allergy develops when the body’s immune system reacts to an allergen as though it is harmful, like it would an infection.

It produces a type of antibody (protein that fights off viruses and infections) called immunoglobulin E (IgE), to fight off the allergen.

When the body comes into contact with the allergen again, IgE antibodies are released, causing chemicals to be produced. Together, these cause the symptoms of an allergic reaction.

One of the chemicals involved in an allergic reaction is histamine, which causes:

• tightening of your muscles, including those in the walls of your airways
• more mucus to be produced in your nose lining, causing local itching and burning

Common allergens

An allergen is any substance that causes your body’s immune system to overreact and produce antibodies against it.

There are thousands of allergens that can trigger allergies, but some of the most common include:

• grass and tree pollens (hay fever)
• dust mites
• animal dander (tiny flakes of skin or hair)
• food allergy (particularly fruits, shellfish and nuts)

Who is at risk?

Some people are more likely to develop an allergy because it runs in their family. Those who this applies to are said to be atopic, or to have atopy. People who are atopic are more likely to develop allergies, because their body produces more IgE antibodies than is normal.

Environmental factors also play a part in the development of allergic disorders. The exact role of the environment is unknown, but studies have shown that several factors seem to increase the chance of a child developing atopy, such as:

• growing up in a house with smokers
• exposure to house dust mites
• exposure to pets
• using antibiotics

Boys are more likely to develop atopy than girls, as are babies who have a low birth weight. The reasons for this are unclear.