A substance that is foreign to the body and can cause an allergic reaction in certain people. For examples, pollen, dander, mold. An allergen is a substance that's capable of producing an allergic reaction. In some people with asthma, allergens that are breathed in can trigger symptoms such as coughing, wheezing, and shortness of breath. Allergens that commonly cause asthma symptoms include animal dander, dust mites, mold, and pollen.