About Rhinitis: Causes
Hay fever is an atopic disease like asthma, meaning that the body tends to overreact to certain types of outside substances. One way it overreacts is by releasing histamine, a chemical present in almost every cell. Histamine widens the blood vessels, slowing blood flow and reducing blood pressure. It can help in guarding against infections by keeping them from spreading throughout the body. When it's released in the nasal passages or the airways, however, it narrows them, allowing less air to enter. It can also cause muscle spasms that show up as sneezing and coughing.
Hay fever, like asthma, can be inherited, but you probably don't inherit particular allergies, such as to cat dander or ragweed. Instead, you just inherit the tendency to be allergic. Whatever you're most exposed to will probably become the allergen that bothers you.