An allergy develops similarly as an immunity against an infectious disease develops. A typical allergic reaction happens when a person is exposed, overtime, to pollen, dust mites, molds and other things. They can be exposed by swallowing as with food and medicine, or injected as with a bee sting. The bodies of most people simply ignore these things, but others treat them as foreign invaders and cause a reaction. When this happens, a substance in the blood known as an antigen becomes active.
Antigens cause the formation of antibodies whose job it is to fight off the invading substance.
Upon further exposure, your body begins to release histamine-like substances which cause your allergy symptoms of watery eyes, sneezing, congestion, runny nose and rashes. With more severe allergies, histamines can provoke an asthma attack or induce cramps and pain in the stomach. Many who have allergies belong to families in which parents and other close relatives also have allergies. Specific allergies, such as those to pollen, are not inherited. A person becomes sensitive, or allergic, to pollen over time.