What happens if i were to have an alergic reaction, when would i?

Need More Info: There are many different types of allergic reactions. Because of this more information is needed to answer your question. Did you mean a reaction to a food, drug, or cosmetic? How about a hay fever type reaction? Or, did you have a reaction and are wondering if the reaction is allergic? You could also have allergic reactions to insect stings. Please provide more information.