Allergies are relatively common. They are caused by an over-sensitive immune response. The immune system normally protects the body against harmful substances such as bacteria and viruses. Allergy occurs when the immune system reacts to substances (allergens) that are generally harmless and, in most people, do not cause an immune response.

When a person with allergies breathes in an allergen, histamine and other chemicals are released as part of the immune response. This causes itching and swelling, mucus production, and in serious cases, hives and rashes, as well as other symptoms.

Symptoms vary in severity from person to person. Most environmental allergens contact the skin or eyes, or are inhaled. Therefore, most symptoms affect the skin, eyes, or the breathing passages.

You may develop an allergic reaction to particles in the indoor or outdoor environment that leads to hay fever-like symptoms. Common allergy triggers include:

• Mold -- Mold spores are carried in the air and may be present all year long. Mold is most common indoors in damp locations such as basements, bathrooms, or washrooms. Fabrics, rugs, stuffed animals, books, or wallpaper can contain mold spores if they are frequently in contact with water or kept in a damp place. Outdoors, mold lives in the soil, on compost, and on damp vegetation.
• Animals -- People who are allergic to certain animals are rarely allergic to the animals' fur or feathers. They are actually allergic to the small scales of skin (dander) that the animal sheds. Some people are allergic to the animal's saliva, particularly cats. Cats have saliva that contains a protein known to cause allergy. You can come into contact with animal saliva if the pet licks you, if you touched the pet after it has groomed itself, or if you touch an object that the animal has recently licked or chewed.
• Dust -- House dust contains tiny particles of pollen, mold, fibers from clothing and fabrics, detergents, and microscopic insects (mites). Dust mites, including small fragments of dead mites, are the primary cause of dust allergy and are found in the highest numbers in bedding, mattresses, and box springs.

A few people develop allergy-like symptoms to other irritants in the environment, including smoke, fumes from industries or cleaning products, tobacco, powder, and laundry detergents.