What are the complications of allergic contact dermatitis?

Usually nothing: Allergic contact dermatitis is a reaction, typically to chemicals which come in contact with the skin. A classic example is poison ivy. If you don't take care of the rash, it will itch, you scratch it and run the risk of spreading the rash and causing a secondary infection. That can result in permanent scarring.

None if treated: Once the trigger for acd is identified, it is important to avoid contact with it. In cases in which the allergen is not avoided (e.g., a machinist who reacts to cutting fluid, but can't find another job), the constant inflammation in the skin leads to worsening skin breakdown, infections of the skin, permanent scarring, and side effects of the meds needed to try to control it.