When you have your check-up, do you ever have to pant like a puppy while your doctor looks in your mouth? He isn't trying to find out how well you can imitate a dog; he's checking the tissues that line your mouth and throat. Your doctor may also use a tongue depressor -- a medical tool that looks like a popsicle stick -- to push your tongue a little farther out of the way.

When you pant, your tongue flattens, and your doctor gets a clear view of the back of your throat -- including your tonsils. The same thing happens when you open your mouth wide and say "Ahh." Try both methods with a friend, and you'll get a glimpse of what your doctor sees when she looks in your throat. Don't try to use a popsicle stick, though. You could make your friend gag, which isn't much fun.

You have one tonsil on each side of the back of your throat, behind your tongue. Each one is about an inch long, and half an inch wide, and is shaped like an almond. Tonsils are made of a special substance called lymphoid tissue. This tissue produces substances to fight infection. It's part of your immune system -- the part of your body which keeps you from catching every sickness that comes along.

You have two more clumps of the same kind of tissue at the back of your nose. Those two clumps are called your adenoids. Both your adenoids and your tonsils are there for a reason: they act as filters to trap germs that enter your body in the air you breathe.

Have you ever had a very sore throat, one that hurt so much that your parents took you to the doctor? Many different things could have caused that achy, burning feeling that made you hate the idea of swallowing. One of them is an illness called tonsillitis.

When tiny germs called bacteria get into your body through your nose or mouth, your tonsils and adenoids trap them. Then they fight them. They manufacture lots of white blood cells called lymphocytes to do the job. Usually, the white cells win the war against bacteria. But sometimes the fight gets pretty fierce.

While it's raging, your tonsils may swell up, and become hot and red. It hurts to swallow. All you can think about is how sore your throat feels. Your adenoids may become infected too, blocking your nasal pasages and making it hard to breathe through your nose.

Sometimes your body can recover from the battle on its own. Sometimes it needs help in the form of powerful medicines called antibiotics. Sometimes the tonsils get so infected and swollen that your doctor decides it's time to take them out.

Doctors call the procedure for taking your tonsils out a tonsillectomy. Not so long ago -- probably when your grandparents or even your parents were young -- kids often had their tonsils out even if they weren't infected and swollen. Many children had their tonsils removed before they entered the first grade.

Today, most doctors think that the tonsils and adenoids should be left in unless they're causing pretty serious trouble. They think the special tissues are a useful part of your immune system -- the system that fights off disease -- and should stay. Infected tonsils often can be cured with antibiotics, and go back to their duty as guards in your throat.

Even though doctors are doing fewer tonsillectomies now, a lot of people -- including many kids -- still have them. In 1983, 424,000 people had their tonsils removed in U.S. hospitals. And tonsillectomies are still one of the leading causes for kids being admitted to the hospital in the United States.

Doctors may decide to remove tonsils and adenoids when:

* the tissues are so swollen that they get in the way of free breathing and swallowing

* the patient has had three or more bouts of bad tonsillitis

* infection has spread into the inner ear and interferes with normal hearing.

If you get tonsillitis, your doctor will examine you very carefully, and think hard before deciding to take your tonsils out. If you are scheduled for a tonsillectomy, you can be sure that your doctor is doing the operation for a very good reason, and that it will improve your health. Tips for Parents

If you had your tonsils out when you were a child, you proably spent a few days on a diet of ice cream and Jello after the operation. But today, if you come into your child's hospital room after a tonsil operation and find him chewing on a hamburger, don't be alarmed.

"I tell my patients that they're going to be sore for four or five days, but that's important to eat a normal diet," says Dr. Allan Seid. Seid, secretary of the American Academy of Pediatrics Ear Nose and Throat Task Force, practices at Children's Hospital in San Diego, Calif. "After a tonsillectomy, I get them eating as soon as possible. In fact, I won't let them go home unless they've had a regular breakfast -- toast and all. Eating normally helps keep raw areas in the throat clear of any infection, and promotes healing. We've found that patients who eat normally have less postoperative bleeding."