An allergy is when your body’s immune system reacts to normally harmless substances that it sees as harmful. The allergy-causing substances are called allergens.

Your body’s immune system is a natural defence against infection and other foreign material. If you are allergic to something, your immune system will react every time you come into contact with that allergen.

Causes of allergies

Many substances can cause an allergic reaction, but some of the most common are:

  • pollen
  • mould
  • animal hair
  • dust and dust mites
  • latex
  • medicines
  • insect stings
  • foods.

Children and allergies

Allergies are common in childhood and may begin with the development of food allergies in children under 12 months old. Milk (dairy), eggs and peanuts are the most common food allergies in this age group, but fish, shellfish, tree-nuts, sesame, kiwifruit, wheat and soy can also be a problem.

Many children grow out of their food allergies in late childhood, although allergies to peanuts, tree-nuts, fish and shellfish tend to remain.

Eczema, which affects up to a third of children under 12 months of age, is related to the development of food allergies but may also be triggered by non-food allergens (things that trigger allergies) such as house dust mites. Most children with eczema do not have food allergies, but most children who have food allergies have eczema.

Allergies often run in families, but not every family member may be allergic to the same thing. Children living in homes with smokers are more likely to develop asthma.

Kinds of allergic reaction

Different allergens will cause different kinds of allergic reactions. The most common allergic conditions are hay fever, asthma and skin problems (eg, eczema, rashes, hives).

  • Airborne allergens such as mould, dust, pollen, grasses and weeds can cause hay fever
  • Pollens, moulds and house dust can trigger asthma attacks
  • Skin reactions can be caused by contact with an allergen (like latex or certain metals), by insect bites or stings, or by eating food or taking medicine you are allergic to.

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