An allergy is an overreaction of the immune system to certain otherwise harmless substances from the environment.
Substances that trigger an allergic reaction are called allergens.
After an initial contact with an allergen (also called antigen), it may happen that under certain circumstances or in genetically predisposed individuals, the immune system begins to produce antibodies of type IgE. This process is called sensitization. In sensitized persons a renewed contact with the same allergen leads to an allergic reaction.