1. A substance that is foreign to the body and can cause negative reactions in certain people: Examples of allergens include, pollen, dust mite excretion, dander or house dust, mold, and ragweed pollen which can cause hay fever.

When seeking medical care, the physician needs to know whether the symptoms vary according to the time of day, or the season of the year, and whether there are any pets or other possible sources of allergens in the home.

2. Etymology: from Greek allos, "different, strange" + ergon, "action".