Allergy is a disorder of the immune system that is characterized by excessive activation of certain white blood cells called mast cells by a type of antibody  resulting in an extreme inflammatory response. Common allergic reactions include eczema, hives, hay fever, asthma, food alleriges, and reactions to the venom of stinging insects such as wasps and bees.

Many allergens such as dust or pollen are airborne particles. In these cases, symptoms arise in areas in contact with air, such as eyes, nose and lungs. For instance, allergic rhinitis, also known as hay fever, causes irritation of the nose, sneezing, and itching and redness of the eyes. Inhaled allergens can also lead to asthmatic symptoms, caused by narrowing of the airways (bronchoconstriction) and increased production of mucus in the lungs, shortness of breath (dyspnea), coughing and wheezing.