inflammation
If your skin or some other part of your body becomes red, swollen, hot or sore, we say it is inflamed. It means that your body is trying to protect you from germs, from something in your body tissues that can hurt you (like a thorn or sliver), or from things that cause allergies (allergens). Inflammation is part of the way the body heals an infection or injury.

allergen
If you have an allergy to something, your body overreacts when you're around it. The thing you are allergic to is called an allergen. Most allergens are harmless to most people. But if you're allergic to something, your body's system for fighting infection (your immune system) is too sensitive to that allergen. It triggers changes that we call allergic reactions. For example, pollen is an allergen for many people. If you're allergic to pollen, you'll sneeze and have runny eyes when pollen is in the air.

allergic reaction
You have an allergic reaction when your immune system reacts to something outside your body that is normally harmless. You can be allergic to particles in the air, like pollen, which causes hay fever, or to chemicals, like detergents, which can cause a rash. People can also have an allergic reaction to drugs, like penicillin.