Any substance that acts in the manner of an antigen on coming into contact with body tissues by inhalation, ingestion, or skin adsorption. The allergen causes a specific reagin to be formed in the bloodstream. The ability to produce reagins in response to a given allergen is an inherited characteristic that differentiates an allergic from a nonallergic person. A reagin is actually an antibody. The specificity of the allergen-reagin reaction and its dependence on molecular configuration are similar to those of an antigen-antibody reaction.The allergen molecule (often a protein such as pollen or wool) may be regarded as a key that precisely fits the corresponding structural shape of the reagin molecule. Allergies in the form of contact dermatitis can result from exposure to a wide range of plant products, some metals, and a few organic chemicals. Though they are alike in some ways, antigen-antibody reactions protect the individual, whereas allergen-reagin reactions are harmful.See Antigen-antibody.
