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. Now, let's discuss the mechanism by which an allergic reaction takes place and let's focus on some of the or one of the most common types of allergic reactions known as hay fever. Hay fever is not an actual fever in the sense that it doesn't actually increase the core temperature. Instead, hay fever is simply the name for the allergic reaction that takes place when grass pollen actually enters the air passageways of our lungs.