Contact dermatitis is the acute or chronic inflammatory reaction to certain substances that come in contact with the skin.
Irritant (nonallergic) contact dermatitis may occur in normal skin or cause an exacerbation of preexisting dermatitis as a result of exposure to a chemical irritant.
Contact dermatitis - plant
Allergic contact dermatitis is a classic, delayed cell-mediated hypersensitivity reaction. Exposure to a strong sensitizer such as poison ivy resin results in sensitization within a week, whereas a weak allergen may take months to years to induce sensitization.
Contact dermatitis - nickel