This causes sensitization and when exposure to the same allergen at a later time at the same or different skin site leads to a secondary immune response at the point of contact. The mechanisms by which this reaction occurs are complex, with many levels of fine control. Their immunology centres on the interaction of immunoregulatory cytokines and discrete subpopulations of T lymphocytes. Allergens include nickel, gold, Balsam of Peru (Myroxylon pereirae), chromium, and the oily coating from plants of the genus Toxicodendron, such as poison ivy, poison oak, and poison sumac. Acrylates, rubber chemicals, emulsifiers and dyes, epoxy resin chemicals are just several of the substances that might induce Allergic Contact Dermatitis. Much of the allergic contact dermatitis that arises is caused by occupational exposure. Non-occupational exposure to allergens in medicaments, clothing, cosmetics, and plants are also a significant cause of allergic contact dermatitis. Photocontact dermatitis
Sometimes termed "photoaggravated", and divided into two categories, phototoxic and photoallergic, PCD is the eczematous condition which is triggered by an interaction between an otherwise unharmful or less harmful substance on the skin and ultraviolet light (320–400 nm UVA) (ESCD 2006), therefore manifesting itself only in regions where the affected person has been exposed to such rays. Without the presence of these rays, the photosensitiser is not harmful. For this reason, this form of contact dermatitis is usually associated only with areas of skin that are left uncovered by clothing, and it can be soundly defeated by avoiding exposure to sunlight.
It may be applied to the skin as a cream or ointment. If the reaction covers a relatively large portion of the skin or is severe, a corticosteroid in pill or injection form may be prescribed.In severe cases, a stronger medicine like halobetasol may be prescribed by a dermatologist. Antihistamines. Prescription antihistamines may be given if non-prescription strengths are inadequate. See also
Urushiol-induced contact dermatitis
Nickel allergy
Eczema

References
External links

eMedicine Health article on contact dermatitis