An allergy is an immune response by the body to certain stimuli in the environment that are normally harmless. The immune system is made up of chemical pathways and cells within the body. When these are activated, an allergic response occurs.
Allergies occur in response to normally harmless triggers known as allergens.
The body of a person with an allergy responds to an allergen by attacking it. The immune response activates certain immune cells called mast cells. Mast cells trigger the release of chemicals. These chemicals include histamine and leukotrienes. They act on tissues in the body and create the allergic response.