1. A negative reaction to foreign substances by the immune system and the body system of defense against foreign invaders, particularly pathogens (the agents of infection): When Mary was in the room with a cat for a few minutes, her allergy became apparent as she started to sneeze over and over again.

The substances that trigger allergies include pollens, dust mite, molds, danders, and even certain foods.

Although allergies can develop at any age, the risk of developing them is often genetic or is related to one's family history.

Hay fever is the most common of the allergies and refers to seasonal nasal symptoms that are caused by pollens and year round or perennial rhinitis is usually due to indoor elements; such as, dust mites or molds.

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