TRUE STUFF

Thudd wanted to tell you more about big-eater cells and bones, but he was busy keeping Andrew and Judy from being munched by a parasite and lost in Uncle Al’s brain. Here’s what he wanted to say:

• There are many things outside of our bodies—bacteria, viruses, poisons, and tiny animals—that could hurt us if they got inside. We have immune systems made up of cells and molecules that patrol our bodies like armies to destroy dangerous things.

• All the cells in our bodies are marked with special molecules. This molecule code tells our immune system that these cells are part of us and should not be attacked.

  Blood cells have markers that say they belong to a group called a blood type. Your blood cells have markers for one of these blood types. If you got into a bad accident and needed blood, you would have to get it from someone with your blood type. If you received a different type of blood, your immune system would attack and destroy the strange cells. The attack would not only destroy the new blood cells, it could kill you, too.

• Macrophages have such big appetites, they actually eat until they die! After you scrape your knee or get a cut, do you ever see gooey yellow or white stuff around it? This is called pus. Pus is actually made up of dead white blood cells!

• Blood cells are red for the same reason that rusty iron is red. Iron turns red when it combines with the oxygen in the air. Blood cells have iron. Iron is what carries oxygen from our lungs to our bodies. When the iron in blood combines with oxygen, blood looks red.

• If you look at your wrists, you will probably see veins where your blood looks blue. Inside your veins, this blood is really deep red or purple. But it looks blue because you’re seeing it through your skin.

• Are you allergic to cats or pollen or bee stings? Poison ivy or peanuts? None of these things are dangerous all by themselves. Many people are not bothered by them at all.

  An allergy means that your immune system is confused about its enemies. It mistakes harmless stuff for dangerous stuff—and attacks. The problem is that the weapons of the immune system can cause lots of damage to your body, too—resulting in sneezing, itching, swelling, fevers, and worse.

• When your body is attacked by bacteria or viruses, the temperature of your body may get hotter. You have a fever.

  Fevers are one of the ways your body fights infections. For example, higher temperatures help to kill some bacteria. When you have a fever, your body hides some of the food that bacteria need to survive. Your immune system also makes more of the cells needed to attack the invaders.

  So making a fever is one of the ways your immune system helps you to get well. Fevers aren’t dangerous unless they get too high.

  Aspirin and aspirin-like medicines are often used to lower fevers. But if you’re under nineteen years old, you should never take aspirin or aspirin-like medicines to lower a fever unless a doctor tells you to. Aspirin-like medicines could cause you to develop Reye’s syndrome, a problem worse than a fever or a cold.

• Our immune system can turn against our own cells and attack them. When this happens, we actually become allergic to ourselves. Such attacks by our immune system can lead to diseases like asthma and intestinal problems.

  Most people think that parasites, such as hookworms, are disgusting and unhealthy. However, scientists have discovered that people who have hookworms aren’t attacked by their own immune systems. These parasites make stuff that controls the immune system and keeps it from attacking.

  For some serious diseases, doctors are actually giving people parasites to eat!

• When you get a cold or the flu, it takes a while for your immune system to make the special army needed to defeat the “bad guys.” That’s why it takes about ten days for you to get better. However, once your immune-system army has met and defeated the bad guys, it will be instantly ready to wipe them out if the two forces meet again—before you get sick.

  That’s how vaccines (vack-SEENZ) work. For example, a shot of flu vaccine prepares your body to recognize and destroy the enemy as soon as they meet. And you don’t get sick!

• It’s easy to think of bones as solid and rocky and not really alive. But bones are changing all the time.

  If bones didn’t change, you couldn’t grow taller and bigger. And your bones couldn’t heal if they broke. As you get older, the shape of your face will change as the bones in your skull change.

  Bones change because they’re made up of two kinds of cells that have opposite jobs. Osteoclasts [AHS-tee-uh-klasts] are constantly breaking down old bone. Osteoblasts [AHS-tee-uh-blasts] are always busy making new bone, mainly from the calcium in your milk and other food.