The inflammatory component of allergic contact dermatitis is primarily a cell-mediated, immununologic reaction but it may also contain an IgE component.
Patch testing is used to determine the causative agent in contact eczematous dermatitis.
The usefulness of patch testing to identify allergic reactions other than those originating on cutaneous surfaces has not been determined. Patch tests are generally placed on the upper back, avoiding the paraspinal skin at least 2.5 cm from the midspinal area. A positive patch test requires correlation with likely sources of an exposure to the substance in the patient's environment including hobbies, habits, occupation, cosmetics, lotions, and ointments. Recall antigen skin tests can be used to predict survival of immunocompromised patients, detect disease- related changes in immunity, and follow the outcomes of therapy. Intact delayed type hypersensitivity provides evidence of cell-mediated immunity and predicts the host's abillity ato eliminate obligate intracellular pathogens and parasites.