Any substance that can bring about an allergic response in an organism. Hay fever and asthma are two common allergic responses. The allergens that evoke these responses include pollen, fungi, and dust. Allergens can be described as host-specific agents in that a particular allergen may affect some individuals, but not others. A number of air pollutants are known to be allergens. Formaldehyde, thiocyanates, and epoxy resins are examples. People who are allergic to natural allergens, such as pollen, are more inclined to be sensitive also to synthetic allergens, such as formaldehyde.