The duration of cutaneous inflammation preceding sensitization influences the resulting allergic response; the innate immune system instructs the adaptive immune response. Potent allergens that function as their own irritant cause classic T helper cell type 1 skewed dermatitis. Examples include poison ivy, epoxy resin, and methylchloroisothiazolinone. Less potent allergens, such as food proteins and propylene glycol, sensitize skin affected by chronic dermatitis resulting in a T helper cell type 2 skewed response, sometimes with associated systemic contact dermatitis. Systemic contact dermatitis should therefore be suspected in patients with positive patch tests to ingested allergens in the setting of chronic dermatitis.