Allergic reaction. Rash on a patient's back that spells out 'IgE?'. IgE is an antibody in the blood that is involved in allergic reactions. This rash was produced by rubbing the skin. This is a very sensitive type of allergy, and it is known as dermographism (skin writing). A similar response would be seen in an allergy test where a suspected allergen is applied to the skin. Any skin reaction indicates an immune response, and the patient may be allergic to that particular substance. Skin allergy tests help identify allergens that cause conditions like asthma or allergic rhinitis (hay fever). Knowing which substances can provoke a reaction allows the substances to be avoided.