An allergic reaction is an exaggerated response to a foreign substance (allergen) by the body's immune system. The immune system is the body’s organized defense mechanism. Normally, it ignores harmless substances or rids the body of them uneventfully, and reacts only if harmful foreign invaders (viruses, bacterial infections, parasites, etc.) attack the body.

In people with allergies, this same defense reaction is triggered by harmless, everyday substances – such as pollen. Instead of ignoring the allergen, it responds by producing immunoglobulin E (the allergy antibody).

All of us have the IgE antibody in small amounts. Allergic persons, however, produce IgE in large quantities. The reaction of the allergen with IgE leads to the release of histamine. This chemical, in turn, causes inflammation and all sorts of other symptoms to occur, ranging from mild to severe to life-threatening. Histamine also stimulates pain receptors, causing the itchy, scratchy nose, eyes, and throat common in allergic rhinitis.