Causes of allergic rhinitis

Allergic rhinitis is caused by an allergic reaction to an allergen, such as pollen, dust and certain animals.

Oversensitive immune system

If you have allergic rhinitis, your immune system your natural defence against infection and illnesswill react to an allergen as if it were harmful.

If your immune system is oversensitive, it will react to allergens by producing antibodies to fight them off. Antibodies are special proteins in the blood that are usually produced to fight viruses and infections.

Allergic reactions don't occur the first time you come into contact with an allergen. The immune system has to recognise and "memorise" it before producing antibodies to fightit. This process isknown as sensitisation.

After you develop sensitivity to an allergen, it will be detected by antibodies called immunoglobulin E (IgE)wheneverit comes into contact with the inside of your nose and throat.

These antibodies cause cells to release a number of chemicals, including histamine, which can causethe inside layer of your nose (the mucous membrane) to become inflamed and produce excess mucus. This is what causes the typical symptoms of sneezing and a blocked or runny nose.

Common allergens

Allergic rhinitis is triggered by breathing in tiny particles of allergens. The most common airborne allergens that cause rhinitis are described below.

House dust mites

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

Rhinitis isn't caused by the dust mites themselves, but bya chemical found in their excrement. Dust mitesare present all year round, although their numbers tend to peak during the winter.

Pollen and spores

Tiny particles of pollen produced by trees and grasses can sometimes cause allergic rhinitis. Most trees pollinatefrom early to mid-spring, whereas grasses pollinate at the end of spring and beginning of summer.

Rhinitis can also be caused by spores produced by mould and fungi.

Animals

Many people are allergic to animals, such as cats and dogs. The allergic reaction isn't caused by animal fur, but flakes of dead animal skin andtheir urine and saliva.

Dogs and cats are the most common culprits, although some people are affected by horses, cattle, rabbits and rodents, such as guinea pigs and hamsters.

However,being around dogs from an early age can help protect against allergies, and there's some evidence to suggest that this might also be the case with cats.

Work-related allergens

Some people are affected by allergens found in their work environment, such as wood dust, flour dust or latex.

Who's most at risk?

It isn't fully understood why some people become oversensitive to allergens, although you're more likely to develop an allergyif 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 atopichave 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 apart. Studies have shown certain things mayincrease the chance of a child developing allergies, such as growing up in a house where people smoke and being exposed todust mites at a young age.