inflammations resulting from exposure of the skin to nonspecific irritants (mechanical, chemical, physical, or biological) or from the action of specific allergens.
Allergic skin reactions develop in previously sensitized persons when their skin comes into contact with an allergen. They can also be a sign of a systemic allergic reaction to an intramuscular or intravenous injection of an allergen or to its introduction into the alimentary canal.
In clinical practice skin reactions are widely used to diagnose allergies. An allergen is injected intradermally or applied to the skin by pricking or scratching (scarifying) it. In some cases the Prausnitz-Kiistner reaction, which involves the passive transfer of skin sensitivity, is used. A healthy recipient is given an intradermal injection of blood serum from an allergic person. Later, an allergen is applied to the same part of the skin. This procedure induces in the healthy subject the same skin reaction as in a sensitive person to whom only the allergen is administered.