An allergen is a substance that can cause an allergic reaction . In some people, the immune system thinks allergens are foreign or dangerous. This is what leads to allergy symptoms.
Common allergens include:
Wasserman SI. Approach to the patient with allergic or immunologic disease. In: Goldman L, Schafer AI, eds. . 24th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 257.
Positive reaction to allergen