A person may have one or both types of hay fever. Symptoms of seasonal hay fever are worst after being outdoors. Symptoms of perennial hay fever are worst after spending time indoors. Both forms of hay fever can develop at any age. In most cases, they first appear during childhood. They may become either worse or better over time.
Causes
Hay fever is a kind of immune reaction. The immune system consists of cells, tissues, and molecules whose job it is to fight off foreign invaders, such as bacteria and viruses. When a foreign substance enters the body, the immune system releases antibodies. Antibodies are chemicals with the ability to destroy the foreign substances.
In the case of hay fever, the immune system becomes confused. It treats dust, pollen, and other harmless substances as if they were dangerous invaders. Substances that cause this kind of reaction are known as allergens. The immune system releases antibodies against allergens the way it does against bacteria, viruses, and other dangerous substances.
The combination of antibody and allergen sets off a series of reactions designed to protect the body. These reactions cause cells and blood vessels to leak fluids.
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