Spring is here. Even on the cool days, you know it. You know it because it stays light later every evening. You know it because there are softball practices on every playground, and people are wearing brighter colors. The trees are turning green, and flowers are beginning to pop up in gardens. And if you get hay fever, your nose knows it's spring, too. There's one sure sign of that: the sneeze.

In this part of the country, allergy season opens in March as the trees start giving off pollen. Pollen is that dusty stuff that floats in the air at this time of year. Bees love it, and trees depend on it to reproduce. But if you get hay fever, your nose doesn't need it or like it at all.

When you call the weather number on the telephone at this time of year, you'll hear a report of the pollen count. This count gives people an idea of how much pollen is in the air. If the count is high -- say around 100 -- allergy sufferers will start sneezing, and sneezing, and sneezing some more when they go outdoors. If it's on the low side -- around 15 or 20 -- only very sensitive noses will suffer. Experts count pollen by putting a glass slide covered with a sticky gel out in the air on a high building. Pollen sticks to the gel. After several hours, someone looks at the slide through a microscope, and estimates the number of pollen grains on the slide. That rough count is the pollen count.

Right now, the pollens on the slide come from trees. Later this year, grasses will produce pollen, too. At the end of the summer, a plant called ragweed will start causing trouble.

When pollen gets in your breathing passages, the grains irritate the delicate skin in there. Many people develop allergies to the irritating grains. Allergies happen when the immune system -- the part of the human body that keeps out germs and other invading substances -- becomes over-sensitive to dust, cat or dog fur, pollens and other irritants.

Your immune system makes substances called antibodies to fight off the particles that enter your body in the air you breathe. These antibodies attach themselves to special cells called mast cells in the lining of your nose and throat. Let's say you're allergic to tree pollen. When the grains get in your nose, the antibodies trigger your mast cells to make chemicals called histamines. The effect is kind of like setting off tiny bombs inside your nose. The bombs send out chemicals that produce the swelling, itching, runny nose and sneezing we call hay fever.

Hay fever isn't a fever, and it's not caused by hay. But it can make you feel miserable. Its medical name is allergic rhinitis. The words mean that the suffrerer has a swollen nose, but the name sounds like it has something to do with a rhinoceros. When you have hay fever, your nose may get so swollen and itchy and that you feel like a rhinoceros. But at least you won't be the only person feeling that way. Some 15 million people in the U.S. suffer from hay fever each year.

One of the most common symptoms of hay fever is sneezing. When you're riding the bus or sitting in study hall this spring, you'll probably notice that there's a lot of sneezing going on. Everybody seems to have a slightly different sneezing style. Some sound like explosions. Others are quieter. Some people sneeze just once. Others may sneeze more than 10 times during each "fit."

When you have a cold, draining mucus -- the sticky stuff in your nose -- tickles your sneeze reflex. When you have an allergy, the histamines your body produces have the same effect. Because a sneeze is a reflex, you do it without thinking. You can't control it, either. You may have tried to stifle sneezes, and you probably found out that you felt like you were going to explode.

Here's what happens when you sneeze:

The muscles that help you breathe draw in a huge gulp of air. Then the muscles contract, or squeeze, and force the air back out. It's an explosive and forceful movement. It pushes lots of air and drops of moisture out of your nose. And they travel fast. How fast? Scientists have clocked sneezes at 103 mph when they leave your nose and mouth.

Anything irritating can cause a sneeze: cigarette smoke, a change in air temperature, even bright light. The purpose of the loud AH-CHOO is to carry irritating things away. But when the problem is pollen, you just take more back in with your next breath. And so the cycle starts again. At this time of year, it's a good idea to carry some tissue everywhere you go. You never know when your sneeze reflex will get its next tickle. Tips for Parents

Because various over-the-counter allergy remedies have such side effects as drowsiness or insomnia, Dr. Kenneth Grundfast, chairman of the department of otolaryncology at Children's Hospital National Medical Center, does not recommend their use for children. If your child's runny noses, itchy eyes and sneezes persist for more than a few weeks, he suggests consulting your pediatrician or an allergist for treatment. For more information on allergies, contact the Asthma and Allergy Foundation of America, 1302 18th St. NW, Suite 303, Washington, D.C. 20036 (293-2950), or your local chapter of the American Lung Association.