Two factors come into play with all allergic reactions. These are the allergen and the person who is sensitive to it. An allergen is any substance that causes an allergic reaction in an individual. To understand some of the common causes of allergic reactions, a little simple science is all that is needed.

An allergy is the result of an overreaction by the body's immune system in the presence of an allergen. However, how does that work, exactly? Knowing how the immune system responds to allergies will help you to  understand what is happening when you suffer that runny nose, itchy rash or stomach upset.