This is a condition where you develop patches of inflammation on your skin (called dermatitis) when your skin reacts against a specific substance. In this type of contact dermatitis, the patches of skin are itchy, red, and scaly. They may also blister. The substance you react to is called an allergen. You are not born with this type of allergy - you must have previously come into contact with the allergen which has 'turned on' your immune system. Your skin then comes out with a rash when it comes into further contact. See the separate leaflet called Contact Dermatitis.
This is a companion discussion topic for the original article at https://patient.info/skin-conditions/contact-dermatitis/patch-testing-for-contact-dermatitis