The scope of potential allergens is essentially limitless and encompasses virtually anything that can be eaten, inhaled, or touched.
Common allergens are pollen, foods, dust, molds, venoms, fur, and medications.
In many cases, the sufferer will know exactly what they are allergic to. In other cases, careful questioning by a doctor will help narrow down the list and even pinpoint the allergen involved. A person's entire lifestyle and history must be considered, from their medical past to allergies among close relatives, hobbies, food reactions, emotional health, and work environment. This treatment is based, as might be guessed by the name, on the theory that a person will become less sensitive to a substance when they are repeatedly exposed to it. The most effective way to treat an allergy is to avoid having a reaction in the first place.