An allergy is a physiologic reaction caused when the immune system mistakenly identifies a normally harmless substance as a substance that is damaging to the body.
Typically, the human immune system is utilized to defend itself against harmful substances such as viruses or bacteria, but sometimes it aggressively attacks normally harmless substances that the body is exposed to and considers them “foreign”. These immune/allergic reactions occur in two varieties: Delayed and Immediate reactions. Delayed reactions or allergic contact dermatitis is the most common allergic presentations on the skin and occurs when various substances get on the skin and a persistent itchy rash develops a day or two later.
A poison ivy rash is typical for a delayed allergy, but a similar type of reaction can occur after contact with shampoos, cosmetics, creams, earrings, and many other chemicals. Immediate immune reactions are those in which substances such as mold, dust, pollen, or animal dander cause swelling of the nose or sinuses, asthma, or itchiness of the eyes within minutes of exposure. This is the reaction with typical seasonal allergies.