Have you ever tried to talk your mother into letting you do something by being really nice to her? Maybe she said, "Don't try that on me. I'm immune to your charm."

Being immune to something means being able to resist it, or fight it. Your body comes equipped with an immune system to help you fight off germs and disease. It is a very complicated and delicate system -- and most of the time it works very well.

You aren't aware of it, but your body is constantly on the alert to protect you from getting sick. Most of the diseases human beings can catch are carried by tiny germs. Germs are also called micro-organisms. Micro means tiny. Micro-organisms are so small they can only be seen through a microscope. Some kinds of micro-organisms are called bacteria. They cause infections and sicknesses like tonsillitis.

Another type of micro-organism called a virus can cause sicknesses like colds, the flu and polio.

A virus is a strange thing. Outside the body, it doesn't do much. But if viruses get inside and attach themselves to your cells, more of them begin to grow. This process can make you sick.

Luckily, doctors have found ways to prevent us from catching many of the worst illnesses viruses can cause. For example, when your grandmother was a little girl, polio was a very serious disease that made many people very sick. Then a vaccine was invented which prevents people from getting polio.

Vaccines are used to prevent illnesses. They teach the immune system how to fight certain viruses. Do you remember eating special sugar cubes your doctor gave you? Those cubes contained a vaccine to help you fight the polio germ. Today, people who take the polio vaccine don't have to worry that the virus will infect them.

When micro-organisms get into the body, the immune system goes into action. Special white cells in your blood rush to defend you against invading germs.

Your immune system depends on different kinds of white blood cells. One kind is a fighting cell called a lymphocyte. Lymphocytes are always on the lookout for invading germs. When they find one, they make substances called antibodies to destroy the poison made by the germ. Antibodies attach themselves to the germ and start to fight.

Sometimes antibodies just grab onto the invader, and that's enough to make it harmless. At other times, special cells called macrophages are called in to take over the clean-up job. These cells come along and gobble the invaders up. They recognize the unwelcome micro-organisms because of the antibodies coating them.

Because each germ makes a different kind of poison, lymphocytes must produce a different kind of antibody to fight each one. And the antibodies can be used over and over again.

Let's say you catch chicken pox. While you're itching, your body is fighting a battle against the chicken pox virus. Once you get well, the antibodies your immune system made to fight the illness stay inside you. If another chicken pox virus should get inside you later on, the old antibodies would rush to it and overpower it before it got a chance to make you sick. This ability to resist an invading germ is called immunity.

Sometimes the immune system can break down. Recently, you've probably heard about a disease called AIDS.

AIDS stands for "acquired immune deficiency syndrome." The immune systems of AIDS victims become deficient, or weak, and stop fighting off disease.

AIDS makes people sick by attacking a kind of lymphocyte called a T-cell. For some reason which doctors don't understand yet, lymphoctyes cannot fight the AIDS virus. Instead, the AIDS virus attacks the lymphocytes, takes over the cells and begins reproducing. This interrupts the immune system, and it breaks down.

It's extremely rare, though, for children to catch AIDS.

Researchers around the world are working hard to find a way to fight the disease. The United States government is spending millions of dollars to do medical research about AIDS, and everyone is hoping for a breakthrough soon. Tips for Parents

According to a Washington Post poll, 97 percent of people in the area know what AIDS is. So don't be surprised if your children come to you with anxieties and questions. If they do, says Dr. Colleen Conley, a psychiatrist at Children's Hospital, give simple, straightforward answers.

"With a disease like AIDS, it's hard to make a blanket recommendation about how to handle kids' fears or questions," says Conley. "In general, I'd acknowledge their concern, be reassuring, and tell them that only certain people get it, and that almost none of those are children." For most school-age children, that should be enough. But if you children continue to probe, Conley says, use your own judgment about what to tell them. "Most parents," she says, "will sense how much information their children can handle."