The following story illustrates what causes allergies to happen, such as seasonal pollen allergy or hay fever:
It is fall. The days have started to grow shorter and cooler, and weeds are producing pollen to make seeds for the next growing season. Over a field of ragweed floats an invisible cloud of pollen that is soon carried by the wind to a nearby town. A child who has never before been exposed to ragweed goes outside and inhales the pollen.
Because of a predisposing genetic factor the child's immune system overreacts. It produces large numbers of IgE antibodies with the specific function to attack ragweed pollen. These antibodies attach to special cells (most importantly mast cells) in the child's nasal passages and upper respiratory tract. These cells contain strong chemicals called mediators. The best-known mediator is histamine. The child has just become sensitized to ragweed pollen.
Second Exposure to Allergen
When the sensitized child is exposed to the same pollen again, the IgE antibodies that are now attached to the surface of the mast cells bind to the pollen. This sets off an explosion, as histamine and other mediators burst from inside the mast cells. Histamine causes blood vessels to leak fluid leading to swelling and redness. The eyes become itchy and watery; the nose becomes congested, mucus production increases; and sneezing begins - all classic symptoms of hay fever and re-exposure to an allergen.
Visiting an Allergist
The child's parents become concerned because her nose is running and her eyes are watering; she is itching and sneezing and looks extremely uncomfortable. They suspect allergies but are unsure because she has never had such severe symptoms before. They schedule an appointment with the family doctor, who then refers her to an allergist (doctor who specializes in allergies). After skin testing, the allergist informs them that the child is allergic to ragweed pollen, as well as pet dander and mold spores.
As treatment, the allergist recommends environmental controls to reduce the child's exposure to ragweed pollen, pet dander and mold spores by keeping windows in the house closed, using an air filtration device, and staying away from cats and dogs. Medications, such as oral antihistamines or nasal sprays, may be prescribed. The allergist also advises the parents that immunotherapy (allergy shots) may be helpful if the child's allergies become more severe or if the medications do not work.