House-dust mites, mold, and animals cause these most often. Environmental irritants such as such as cigarette smoke, perfumes and strong odors, aerosol sprays, smoke from wood stoves or fireplaces, and car exhaust can make either type of allergy significantly worse. An allergy is a heightened sensitivity to a substance called an allergen. Allergens are normally harmless substances that most bodies ignore. When you have allergies, though, your immune system mistakes allergens for enemies that must be destroyed, such as viruses or bacteria.
Antibodies leap into action, sticking to the allergens and causing mast cells to release histamine.
The histamine then irritates surrounding nasal tissues and the air passages inside your nose become swollen and inflamed. Your symptoms soon make you feel as if you actually were invaded and made ill by a virus. Allergens can also cause symptoms in other parts of your body, such as your eyes, ears, and lungs. All of that is the bad news. The good news is that we understand allergies better all the time, and have increasingly effective treatments for them.