When Kelly goes to school, she carries a book bag just like all the other kids do. But if you had X-ray vision and could look inside her bag, you'd see that she carries something besides her books and her lunch: her asthma medicine.

Kelly is one of millions of children in the United States who have asthma, a condition that interferes with normal breathing. Experts believe that as many as 11 million people in this country are asthmatic; half of these people are kids. You probably have friends with asthma, or you may have it yourself.

Kelly had her first asthma attack when she was a little girl, before she even started school. One night she woke up feeling like she couldn't get enough air into her lungs. It seemed that the harder she breathed, the worse it felt. She was scared, and so were her parents.

The next morning, Kelly's parents took her to the doctor. Kelly told the doctors her symptoms -- the things that happened to her when she felt ill. He listened to her lungs, and said, "I think you might have asthma. We have to try to figure out what's causing it. And we need to start you on some medicine to control the attacks of wheezing."

It turned out that Kelly's asthma was triggered by an allergy to dust mites, tiny creatures found in common house dust. In some people, allergies cause sneezing, runny noses and itchy eyes. Other people, like Kelly, have especially sensitive lungs. Their bodies react to allergens -- irritating things in the air they breathe or the food they eat -- quite violently.

Here's what happens during an asthma attack: The muscles that control the airways -- two pipes called the bronchial tubes that reach down into the lungs -- contract. That means they squeeze up, causing the airways to narrow and tighten.

Imagine what would happen if you squeezed a garden hose very tightly between your hands. The hose would narrow, and water would have more trouble getting through.

A similar thing happens when the muscles squeeze the bronchial tubes. It's harder for air to pass through the narrowed tube. The wheezing sound the lungs make during an asthma attack shows that air is not flowing freely into the lungs. Also, the wet lining of the airways swells, and gives off more sticky mucus than normal. This adds to the blockage in the tubes, and makes breathing even more difficult.

Because dust triggered Kelly's asthma attacks, her doctor recommended ways to reduce dust in her house -- especially the bedroom. First, her parents bought a plastic sack to put around her mattress. Then they got rid of her feather pillows and her down comforter and bought bedding made for people with allergies. They took the carpeting out of her room, and removed the curtains.

All of those changes were okay with Kelly. The hardest thing she had to do was give up sleeping with her stuffed animals. They're still in her room, sitting on a shelf. But she can no longer snuggle up to them when she goes to sleep at night. Even the dust in her teddy bear's fur is enough to start a reaction in Kelly's sensitive lungs.

Other things besides allergens can cause asthma. Some kids have asthma attacks when they get viruses like colds or the flu. Others get asthma when they exercise, especially if they do something strenuous in cold, dry air. Doctors often recommend that kids who have exercise-related asthma take up swimming in an indoor pool, where the air is warm and moist, as an exercise routine. They also suggest that wearing a mask or a scarf over the mouth and nose during exercise can help, because it keeps the airways moist.

During the last 10 years, many asthmatic patients have started using a medicine called cromolyn sodium. Inhaling cromolyn on a regular basis can prevent asthma attacks from happening. Theophylline, another kind of medicine for asthma, stops attacks after they start by dilating, or opening, narrowed airways.

For Kelly, asthma is a fact of life. She has gotten used to it in the years since her doctor first told her she had the condition. She doesn't mind carrying her inhaler around with her. It means that she can do all the things other kids do without having to worry about wheezing. Tips for Parents

*The Greater Southeast Community Hospital at 1310 Southern Ave. SE, has a free asthma support group for children every Tuesday afternoon. Kids see a movie about Cataline, a puppet who has asthma and has had to give away a beloved dog, and can talk over their fears and frustrations about asthma. Call 574-6776 for more information.

*The M.A. Report, published by Mothers of Asthmatics Inc., provides practical advice. Subscriptions can be ordered for $8 a year from M.A. Report, c/o Nancy Sander, 5316 Summit Dr., Fairfax, Va., 22030.

Asthma Update, a newsletter for people with asthma, is $8 annually. Write to Asthma Update, 123 Monticello Ave., Annapolis, Md. 21401.

The Asthma and Allergy Foundation of America, 1835 K St. NW, Washington, D.C. 20036, furnishes a variety of information, including an Exercise and Asthma booklet.