1. A misguided reaction to foreign substances by the immune system, the body system of defense against foreign invaders, particularly pathogens (the agents of infection): "When the woman was in the room with the cat for a few minutes, her allergy became apparent as she started to sneeze over and over again."
2. Etymology: from about 1911, from German Allergie, coined in 1906 by Clemens E. von Pirquet (1874-1929), Austrian pediatrician, which came from Greek allos, "other, different, strange" + ergon, "activity".

The allergic reaction is misguided in that these foreign substances are usually harmless. The substances that trigger allergy are called allergen. Examples include pollens, dust mite, molds, danders, and certain foods. People prone to allergies are said to be allergic or atopic.

Although allergies can develop at any age, the risk of developing allergies is genetic. It is related to one's family history of allergy. If neither parent is allergic, the chance for allergies is about 15 percent. If one parent is allergic, the risk increases to 30 percent and if both are allergic, the risk is greater than 60 percent.

Hay fever (allergic rhinitis) is the most common of the allergic diseases and refers to seasonal nasal symptoms that are due to pollens. Year round or perennial allergic rhinitis is usually due to indoor allergens, such as dust mites or molds.

Symptoms result from the inflammation of the tissues that line the inside of the nose (mucus lining or membranes) after allergens are inhaled. Adjacent areas, such as the ears, sinuses, and throat can also be involved.