Allergen common is a term used to describe substances that can cause an allergic reaction in some people. The three most common allergen common substances are DANDER (6 letters), POLLEN (6 letters) and DUST (4 letters). DANDER is a type of skin cell shed by animals, such as cats and dogs, which can cause an allergic reaction in some people. POLLEN is a fine powder released by plants, which can cause hay fever and other allergic reactions in some people. DUST is a combination of small particles, such as dirt, DUST mites, and mold spores, which can also cause allergic reactions in some people.