Here’s a quick crash course on what you need to know about allergies by Dr. Miggy Villanueva! What is it? Allergies are a reaction of an overactive immune system. When a person with an allergy is exposed to the allergen, their immune system mistakenly identifies it as a dangerous foreign invader, and produces antibodies which trigger a chain of reactions. Causes? Allergies can come in many forms: atopic dermatitis, allergic rhinitis, asthma, and urticaria to name just a few. Common allergens inc