An Allergen is the substance that causes an allergic response. Allergens contain protein and almost anything can be an allergen for someone.
The most common are house dust mites, pollen from trees and grasses, cats, dogs, insects such as wasps and bees, milk, eggs and peanuts. Less common allergens include tree nuts, fruit and latex.
Allergens are in the form of an injection that a doctor gives to build up your immunity to what you are allergic to.