Allergic rhinitis is caused by an oversensitive immune system (the body's natural defence system against infection) reacting to allergens, such as pollen, as if they are harmful. The immune system reacts by producing antibodies to fight off the allergen. Antibodies are special proteins contained in blood that are normally made by the immune system to fight off viruses and infections.
Allergic reactions do not happen the first time you come into contact with an allergen. The immune system has to recognise and memorise the allergen before making antibodies against it (a process known as sensitisation). The time taken to become sensitised to an allergen varies from days to years.
Once you have developed sensitivity to an allergen, whenever particles of the allergen come into contact with the inside of your nose and throat, antibodies called immunoglobulin E (IgE) are released. These cause blood cells to release chemicals (including histamine), which together cause the symptoms of allergic rhinitis.
The chemical histamine causes most of the symptoms that occur in allergic rhinitis, such as:
- swelling of the mucus membrane (the inside layer of your nose), which blocks the airway and causes congestion
- the production of excess mucus, which occurs as a result of the swelling and causes a runny nose and sneezing
Common allergens
Allergic rhinitis is triggered by breathing in tiny particles of allergens. The most common airborne allergens that cause rhinitis are:
- house dust mites (tiny insects that feed on the dead flakes of human skin). They can be found in mattresses, carpets, soft furniture, pillows and beds. Rhinitis is not caused by the dust mites themselves, but by a chemical found in their excrement. They are present all year round, though their numbers tend to peak during spring and autumn
- tree and grass pollen. Tiny particles of pollen produced by trees and grasses can cause allergic rhinitis. Most trees pollinate between early to mid spring. Grasses pollinate at the end of spring and beginning of summer
- animals. Many people are allergic to animals and pets, such as cats and dogs, but animal fur does not cause the allergic reaction. It is caused by the flakes of dead animal skin, their 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
- work-related allergens. Some people are affected by allergens that are present in their work environment, such as wood dust, flour dust or latex
Who is at risk?
It is not fully understood why some people develop oversensitivity to allergens.
However, some people are more likely to develop an allergy because it runs in their family. If this is the case, you 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 normal.
Environmental factors also play a part in the development of allergic disorders such as rhinitis. The exact role of the environment is unknown, but studies have shown that a number of factors seem to increase the chance of a child developing atopy, such as:
- growing up in a house with smokers
- exposure to dust mites
- exposure to pets
- using antibiotics
