An allergy is a disorder in which the body becomes hypersensitive to a particular, normally harmless allergen. A specific reaction is then provoked whenever the allergen is inhaled, ingested or otherwise comes into contact with the person affected. Allergic rhinitis, or hayfever, is caused by an allergy to pollens from grasses, flowers, weeds or trees. This form of rhinitis causes sudden, frequent attacks of sneezing, nasal discharge and congestion.
These episodes may also be accompanied by eye symptoms, such as stinging and watering eyes. Symptoms of hayfever occur because the allergen, in this case pollens, stimulates the production of antibodies, which in turn causes cells of the immune system to release inflammatory substances. Once the body has developed antibodies to a particular foreign antigen, the same reaction will occur the next time it comes across it and the cycle of allergy symptoms continues.
For more information, ASCIA website.