Allergen: a term commonly used for describing anything that causes allergy. Using it with reference to chemicals causing contact allergy is questionable from the immunology point of view, as the sensitizing small-molecular weight chemicals (correctly referred to as haptens) in fact are not allergens. Examples of allergens are grass pollen, house dust mite allergens, fruit allergens or cat dander (cause mainly respiratory allergy, urticaria, and sometimes also eczema). 

Allergic Contact Dermatitis (ACD): a disease of the skin caused by inflammatory immune reaction to specific haptens (see also: Contact dermatitis).

Contact allergy: an altered reactivity of the body to substances that in similar doses are well-tolerated by most people. The type and intensity of an immune reaction reflects mainly the type and intensity of immune aberration in an individual, and is not dependent on physiologic effects of the hapten on the organisms. Contact allergy is a hapten-specific phenomenon, which means that an individual becomes sensitized to a limited list of defined haptens (exception: see under "Cross-reactivity").

Contact dermatitis: an inflammatory skin disease caused by skin contact with external substances. There are two major forms of contact dermatitis: allergic contact dermatitis (ACD), when the inflammation is due to a hypersensitivity reaction to a specific hapten a person is sensitized to, and irritant contact dermatitis (ICD) when the inflammation is caused by irritating/toxic properties of external substances.

Cross-reactivity: a situation, in which a patient’s immune system reacts to a previously unknown hapten, due to it’s structural similarity with another hapten the person is indeed sensitized to. Cross-reactivity occurs, when haptens are "too similar to be distinguished from each other" by the immune system. 

Hapten: a chemical with molecule weight below 500 Dalton that cannot itself initiate an allergic reaction, but is capable of reacting with body’s own proteins in such way that they change their spatial conformation (shape) and since then are recognized by the immune system in the same way as foreign allergens.

This text was originally published on the patchtest.org website. Copyright by Radoslaw Spiewak 2009-2014, used with permission.