An allergen is a substance that can cause an allergic reaction. Allergens are substances that, in some people, the immune system recognizes as "foreign" or "dangerous" but cause no response for most people.
Common allergens include:
Animal proteins and animal dander
Bacteria and viruses
Drugs (such as antibiotics or medications you put on your skin)
Foods (such as milk, chocolate, strawberries, wheat)
David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.