Which of the following is classified as an allergen?

Animal dander is classified as an allergen because it is a substance that can trigger an allergic reaction in sensitive individuals. Allergens are typically proteins that provoke an immune response, leading to conditions like asthma, allergic rhinitis, and other hypersensitivity reactions. In the case of animal dander, tiny flakes of skin, urine, or saliva from animals, particularly cats and dogs, can become airborne and provoke respiratory issues in susceptible individuals.

In contrast, while perfumes, cleaning supplies, and chemicals in air pollution can cause irritation and even trigger asthma symptoms, they are not traditionally categorized as allergens. Perfumes may cause allergic reactions in some individuals, but they do so through irritants rather than through an immune-mediated response typical of allergens. Cleaning supplies can contain irritants that exacerbate asthma symptoms but are not specifically allergens. Chemicals in air pollution can worsen asthma and trigger symptoms by causing inflammation in the airways rather than acting as allergens. Thus, animal dander stands out as a clear example of an allergen among the options provided.