Allergens.

Allergens are characterized by their ability to invoke an overreaction from your immune system that causes inflammation. In dermatitis caused by an allergy, the inflammation is present within the layers of the skin. The offending allergen could be any one of or several substances found in the environment, originating from animals, foods, plants, or manmade substances. There have been thousands of substances identified as potential allergens, but fewer than 30 are responsible for the majority of allergic and contact dermatitis cases.
12 common allergens:
Poison ivy, poison oak, poison sumac
Nickel (found in hairpins, earrings, and zippers)
Latex (rubber) products
Pet dander
Fish
Peanuts
Wheat
Eggs
Milk
Soy products
Mango
Dermatitis that is caused by an allergen or irritant is called contact dermatitis. Approximately 20% of contact dermatitis cases are the result of allergic reactions; the other 80% are caused by irritants such as chemicals in the workplace. It can be difficult to differentiate an allergenic dermatitis from an irritant dermatitis because the rashes are very similar in appearance, and some chemicals can be allergens, too. An irritant-caused dermatitis will generally show up on the skin within minutes of coming into contact with the irritant, and the rash is usually more painful than itchy. A dermatitis caused by an allergic reaction can take 24–48 hours to appear, but sometimes will take even longer to develop, with repeated contact with the allergen required before sensitization takes place and a rash appears.