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Allergies, like autoimmune diseases such as arthritis and lupus, result from aberrant functioning of the human immune system, the body's remarkable defense against dangerous invaders, including viruses, bacteria and parasites. In the case of hay fever, the immune system perceives the fuzzy grain of pollen as a threat. The cause of the confusion, explains botanist Walter Lewis of Washington University in St. Louis, is a chemical message encoded by proteins in the pollen grain's cell wall.
When this message is delivered and read by a stigma in a flower of the same species, the fertilization process begins. But when the grain lodges in the mucous membrane of a person susceptible to allergies, its protein message is heeded by the human immune system, which confuses it with a menacing invader. Alarmed, the system immediately begins churning out legions of IgE (for immunoglobulin E) antibodies, stationing them on "mast cells," which patrol the body's tissues.
The next time similar pollen grains are detected, the antibodies signal the mast cells, which release a flood of chemicals, including histamine, against the harmless intruder. It is histamine that causes swelling, itching and other irritations all too familiar to hay fever sufferers. At the same time, additional IgE antibodies are produced and placed in position on mast cells, so that the next exposure to the pollen may produce a more severe response.
Another type of immune cell that swings into action at the first hint of pollen produces a substance that is toxic to parasitic worms. "Probably the IgE response is there primarily to protect people against parasites," says Dr. Harold Nelson of the National Jewish Center for Immunology and Respiratory Medicine in Denver. Its response to pollen, he says, is simply a mistake.
What causes the error? "The improper choice of parents," says Wasserman. "Probably there is a genetic predisposition to respond with IgE, and if you're unlucky enough to have both the exposure and the predilection, then you're more likely to have allergies."
David Marsh, a specialist in the genetics of allergy at Johns Hopkins, believes his laboratory has found evidence of a recessive gene that is at least partly responsible for susceptibility to allergies. And allergies indeed tend to run in families. If one parent has allergies, the odds are that close to 1 in 4 of the children will also be allergic. If both mother and father are allergic, probably most of their offspring will be too.
While pollen is the No. 1 troublemaker for allergy sufferers, hundreds of other substances can provoke the immune system into an irrational IgE response. Among the more formidable and difficult to avoid are the droppings of the dust mite, a microscopic insect that thrives by the millions wherever dust collects in a house. Living on sloughed-off flecks of human skin (dander) and other unappetizing protein, it leaves droppings that are about the size of pollen grains -- and just as easy to inhale. Mite dung, unfortunately, is an allergen that produces the familiar sneezing, coughing, itching symptoms in half of all people who have allergies.