1. A reaction in a way other than that which is usual.
2. Sensitiveness to certain foods, animals, pollen, etc.

"After being in the room with the cat for five minutes, her allergy began to act up and she started to sneeze."

1. A misguided reaction to foreign substances by the immune system.
2. The body system of defense against foreign invaders, particularly pathogens (the agents of infection).

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 who are prone to allergies are said to be allergic or atopic.