You wake up in the morning congested and sniffling. Your eyes itch. The roof of your mouth burns. You sneeze.
It's allergy season, and pollen is in the air. Pollen is the archenemy of an estimated 20 million allergy sufferers in the United States alone, according to the Columbia University College of Physicians and Surgeons.
Why is your body so upset? How is it that your neighbor can mow the lawn without batting an eye, while the very sight of cut grass sends you running for the medicine cabinet?
"There's no simple answer," says Dr. Andrew Saxon, chief of the UCLA School of Medicine's Division of Clinical Immunology and Allergy.
On one level, allergies remain a medical mystery. There's nothing truly dangerous about pollen, dust, fur or most other common allergens, so it's uncertain why some people's bodies treat them as invaders. An allergen is anything that triggers an exaggerated, and usually unnecessary, immune response.
But over the past several decades, Saxon and other researchers have made significant progress toward explaining why allergies exist and why they have become so common.
Hard numbers are not available, but Saxon estimates that about 35 percent of the population of the Western hemisphere suffer some form of allergy -- up from about 25 percent in the 1970s. In the early 19th century, hay fever was so rare that it took one early researcher, Jonathan Bostock, nine years to find enough subjects to publish a study.
At that rate of increase, it seems clear that factors other than heredity -- long identified as a crucial determinant of whether one is allergic -- are involved.
Pollution may be one important culprit. According to Saxon's research, breathing irritants such as diesel exhaust sets the immune system on edge, increasing the likelihood that it will overreact to potential allergens. "What diesel and other irritants do is kick the immune system in the side, pushing it toward an attack," Saxon says.
His experiments, which began about 12 years ago, involved two groups of subjects. One group was exposed only to an allergen. The other group was exposed to the allergen and a whiff of diesel exhaust.
Those in the first group had only a "protective" immune response: Their bodies tended not to identify the allergen as a potential threat. Those who also breathed exhaust, on the other hand, were more likely to attack the allergen. Their bodies produced more antibodies and they responded to a smaller amount of the allergen in the future.
Air quality has improved markedly in the United States since 1970. However, Saxon has found that inhaling even small amounts of industrial pollution increases the likelihood of an immune reaction.
More indoor allergens
Civilization may also be heightening people's sensitivity to so-called perennial allergens -- indoor particles such as dust, roaches, mold and dander (bits of animal skin). They have become a widespread problem only in the last century or so.
"We think it may be related to the way homes are being built nowadays," says Dr. Jacqueline Pongracic, acting manager of the division of allergies at Children's Memorial Hospital in Chicago and a professor at Northwestern University.
"Homes are tightly sealed, with better systems of heating and cooling and less outside ventilation. That increases the number of indoor allergens," she says.
"Also, modern society tends to have cats and dogs as indoor pets, whereas in older times, animals were more outdoors, especially in rural areas."
Paradoxically, allergies may also be on the rise because health care for children is improving.
There are two main patterns of immune response -- allergic and infectious. In children who face many viral and bacterial infections, their immune systems may "learn" to operate more in infectious mode than in allergic mode.
That can be beneficial because the infectious response is more of a destroyer than the allergic response. Faced with an infection, the body identifies viruses and bacteria, tries to kill them outright, and girds the body to eliminate invaders when they re-enter the body. That's why people usually don't suffer the same infection, such as chicken pox, twice.
The allergic response is more wishy-washy. The body identifies allergens as threats, but doesn't have any means of protecting itself in the future. So the same allergen usually plagues people for most of their lifetime.
All of which begs a question: Why does the allergic response exist in the first place?
That remains a mystery. But it might have to do with worms.
"The hypothesis is that a long time ago, before civilization, people evolved this hypersensitive response as a way of dealing with parasites," Pongracic says. "But today we don't have much exposure to parasites, so the response shifted to allergens."
People sometimes report that they outgrow their allergies in adulthood, but that is usually an illusion, doctors say. Once the body is sensitized to an allergen, it's programmed for life.
Symptoms taper off
"The reason a lot of people perceive that they are outgrowing allergies is that they're changing their lifestyles and habits," says Dr. Stanley Goldstein, an allergist in Rockville Centre, N.Y. and director of Allergy and Asthma Care of Long Island.
"Whereas they used to be out in the fields playing sports in high school, once they get to college they're inside studying. Or they move out of their parents' house, where they had once been allergic to the family pet."
But symptoms often taper off in old age, due to the general weakening of the immune system. The body isn't strong enough to react to allergens with the same gusto it did in childhood and adulthood.
Still, according to Saxon, allergies are a small price to pay for improved pediatric health and better home insulation.