stimulus or agent that causes an adverse reaction
Allergy is a term that is often misused to describe all kinds of adverse reactions. There are two main types of adverse reactions that may be experienced: irritant reactions and allergic reactions. In fact theres a big difference between being irritated by a substance and being allergic to it.
Irritant reactions are the most common adverse reaction and should not be confused with allergic reactions. An irritant type of reaction occurs rapidly following use of a product. Redness of the skin, sometimes with an itch, is characteristic of an irritant effect. This clears up rapidly after ceasing to use the product that is causing the problem.