Allergies are caused by heightened sensitivity to relatively harmless substances. A body's immune system is designed to fight off any foreign cell or object identified as harmful to the body. These foreign substances are commonly called antigens in relation to the immune system. Although antigens can be any kind of virus or bacteria, they can also be any number of respiratory allergens, such as pollen, dust, or animal dander. The body usually identifies allergens as harmful as soon as they enter the sinuses. The immune system responds by producing antibodies to fight off the allergens.

With initial exposure, the body’s immune response is mild, but over time with repeated exposure, the body responds by releasing large amounts of what is known as histamine. This causes traditional allergic symptoms such as a runny nose, coughing, itchy eyes, and sneezing. These reactions are designed to expel allergens from the body. To control pollen effects and symptoms, allergy sufferers take antihistamine. Such drugs block the effects of histamines in the body, preventing the symptoms.