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:

• Bacteria, viruses, and animal parasites
• Chemicals
• Dust
• Drugs (such as antibiotics, serums)
• Foods (such as milk, chocolate, strawberries, wheat)
• Perfumes
• Plants
• Pollen
• Smoke

See also:

• Allergies
• Allergic reaction
• Food allergy
• Allergy to drugs
• Antigen
• Antibody