Causes - Allergic rhinitis

Allergic rhinitis is caused by an allergic reaction to an allergen. Allergens include pollen, dust, or certain animals.

Oversensitive immune system

The immune system is your body's natural defence against infection and illness. If you have allergic rhinitis, your immune system reacts to an allergen as if it's harmful.

If your immune system is oversensitive, it produces antibodies to fight off the allergens.

Antibodies are special proteins in the blood produced to fight viruses and infections.

Allergic reactions do not happen the first time you come into contact with an allergen.

Your immune system has to recognise and 'memorise' an allergen before producing antibodies to fight it. This process is known as sensitisation.

After you develop sensitivity to an allergen, it's detected by antibodies called immunoglobulin E (IgE). This happens when the allergen comes into contact with the inside of your nose and throat.

These antibodies cause cells to release some chemicals, including histamine. The chemicals can cause the mucous membrane (the inside layer of your nose) to become swollen and produce too much mucus.

This is what causes the symptoms of sneezing and a blocked or runny nose.

Common allergens

Breathing in tiny particles of allergens can trigger allergic rhinitis. Common airborne allergens that cause rhinitis include:

• dust mites
• pollen and spores
• animal skin, urine and saliva

House dust mites

House dust mites are tiny insects that feed on the dead flakes of human skin. They can live in mattresses, carpets, soft furniture, pillows and beds.

Rhinitis is not caused by the dust mites themselves. But by a chemical found in their poo.

Dust mites are around all year round, but their numbers are higher in the winter.

Pollen and spores

Tiny particles of pollen produced by trees and grasses can sometimes cause allergic rhinitis.

Most trees pollinate from early to mid-spring. Grasses pollinate at the end of spring and the beginning of summer.

Spores produced by mould and fungi can also cause rhinitis.

Animals

Many people are allergic to animals, such as cats and dogs. Animal fur does not cause an allergic reaction. It's the flakes of dead animal skin and their urine and saliva.

Dogs and cats are the most common animals to cause allergies. But some people are affected by horses, cattle, rabbits and rodents, such as guinea pigs and hamsters.

Being around dogs from an early age can help protect against allergies. There's also some evidence that this might be the case with cats.

Allergens in the workplace

Some people are affected by allergens found in the workplace. These can include wood dust, flour dust or latex.

Who's most at risk

It's not clear why some people become oversensitive to allergens. But you're more likely to develop an allergy if there's a history of allergies in your family.

If this is the case, you're said to be 'atopic', or to have 'atopy'. People who are atopic have a genetic tendency to develop allergic conditions.

Their increased immune response to allergens results in increased production of IgE antibodies.

Environmental factors may also play a part. Certain things may increase the chance of a child developing allergies. These can include growing up in a house where people smoke and being exposed to dust mites at a young age.