Oversensitive immune system
If you have allergic rhinitis, your natural defence against infection and illness (your immune system) will react to an allergen as if it were harmful.
If your immune system is oversensitive, it'll 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 do not happen the first time you come into contact with an allergen.
The immune system has to recognise and "memorise" it before producing antibodies to fight it. This process is known as sensitisation.
After you develop sensitivity to an allergen, it'll be detected by antibodies called immunoglobulin E (IgE) whenever it 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 cause the inside layer of your nose (the mucous membrane) to become inflamed and produce too much mucus.
This is what causes the typical symptoms of sneezing and a blocked or runny nose.
Allergic rhinitis is triggered by breathing in tiny particles of allergens. The most common airborne allergens that cause rhinitis are dust mites, pollen and spores, and 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 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.
Dust mites are 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 pollinate from 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.
Many people are allergic to animals, such as cats and dogs. The allergic reaction is not caused by animal fur, but flakes of dead animal skin and their urine and saliva.
Dogs and cats are the most common animals to cause allergies, although some people are affected by horses, cattle, rabbits and rodents, such as guinea pigs and hamsters.
But 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.
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's not fully understood why some people become oversensitive to allergens, although 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. Studies have shown certain things may increase the chance of a child developing allergies, such as growing up in a house where people smoke and being exposed to dust mites at a young age.