And that causing agent that caused that allergic reaction in the first place is known as an allergen. Now, different people are allergic to different things. So allergens differ from one individual to another. People are generally allergic to certain foods or drugs or certain things found in our environment. For example, people can be allergic to pollen found in grass or flowers. They can be allergic to certain foods such as peanuts, or certain drugs such as penicillin.