Medical experts have been baffled by what causes asthma. Most of them favor the idea that it stems from “helper” cells that have gone awry. But researchers at Harvard Medical School have come up with convincing evidence that the answer lies in a special type of natural “killer” cell.

We were very, very surprised,” admits Dale Umetsu, a professor of pediatrics at the Medical School and at Harvard-affiliated Children’s Hospital in Boston. “People have been confused about which cells in the lungs are responsible for all these years. Now, we have to rethink the results of so many studies. Our new findings were totally unexpected.”

An estimated 17 million-20 million people in the United States suffer from asthma, and cases of it have been increasing since the early 1980s, according to the Asthma and Allergy Foundation of America. Every day, in this country, 30,000 people suffer an asthma attack, and 14 people die from the disease.

So, knowing exactly which cells should be targeted for treatment is a vital part of relieving a lot of misery for lots of people.

Asthma occurs when the body’s natural system of defense against bacteria, viruses, and other harmful microbes becomes overprotective. It misidentifies relatively harmless pollens, dust, and dander, setting up a reaction that narrows and inflames small airways in the lungs. From that comes breathlessness, wheezing, tissue damage, and, in the worst cases, death.

In the standard textbook version of what happens, so-called T helper cells respond to ragweed, dust, and other irritants by secreting proteins that attack the irritants as if they are bits of disease-causing bacteria or viruses. Umetsu and his colleagues decided to take a look at this process from a new perspective. They checked the lung cells of 25 patients, 14 of whom were nonsmokers with moderate to severe asthma. To their surprise, they found that most of the trouble-raising cells in the lungs of asthmatics aren’t helper cells but a little-known group of natural killer cells. In general, killer cells enjoy the reputation of destroying disease-causing invaders, but this special group wages war on otherwise normal lungs.

The finding means that physicians may not be treating asthma sufferers with the right kinds of drugs. For example, natural killer T cells seem to be resistant to the corticosteroids in widely used inhalers.

The details of the experiments are reported in the March 16, 2006 issue of The New England Journal of Medicine.