Chapter 9: Macrophages: The Cleaners Who Live in the Lungs

Someone Is Living in Your Lungs

Right now, deep inside your chest, something is moving. It is not your heart, and it is not your breathing. It is something much smaller. In the tiny air sacs of your lungs, living cells are crawling slowly over the walls, stretching out, turning around, and searching. They are looking for anything that does not belong there. When they find something, they gobble it up.

These busy cells are called macrophages, said like "MAK-roh-fay-jez." One of them is called a macrophage, said like "MAK-roh-fayj." The name comes from very old words that mean "big eater," and it is a good name. Eating is what macrophages do best. They eat dust, they eat germs, and they eat worn-out bits of your own body that need to be cleared away.

In the last chapter, you learned that the alveoli, the hundreds of millions of tiny balloons at the ends of your airways, have very thin walls. They have no carpet of cilia and no thick blanket of mucus, because those would get in the way of oxygen. So the alveoli need a different kind of guard. The macrophages are those guards. They live right inside the alveoli, and they keep the deepest part of your lungs clean, day after day.

You can think of a macrophage as a little cleaning machine that roams around a house all by itself. It moves from room to room, it bumps into things, and it picks up crumbs and dust along the way. But a macrophage is much cleverer than any machine. It is alive. It can tell the difference between a harmless speck of dust and a dangerous germ, and it can call for help when it needs to.

People first learned about cells like these more than a hundred years ago. A scientist was studying a young sea star, so small and clear that he could see right through it under a microscope. He gently poked a few tiny thorns into its body and waited. The next morning, he saw that many moving cells had gathered around the thorns and were trying to swallow them. He realized that bodies have cells whose job is to eat things that do not belong, and he gave them the name that means "big eater."

A Family of Defenders

Macrophages belong to the big family of defenders called white blood cells, which you first met in the tonsils in Chapter 4. Your blood is full of red blood cells, which carry oxygen and give blood its red color. But mixed in among them are white blood cells, which defend your body against germs. There are far fewer white blood cells than red ones, roughly one white cell for every several hundred red cells. Yet the white blood cells are your body's army, and they are very good at their jobs.

White blood cells are made in the soft, spongy center of some of your bones, a part called the bone marrow. Your bone marrow makes new blood cells all the time, both red and white. Some white blood cells live for only a day or two, and others live for many years. Because new ones are always being made, your body always has fresh defenders ready to go.

There are several kinds of white blood cells, and each kind has its own special talents. Some are fast responders that rush to the scene whenever germs are found. These are called neutrophils, said like "NEW-truh-filz." Others are the planners and rememberers. They learn to recognize particular germs, and some of them make antibodies, special proteins that stick to germs and mark them for destruction. These are called lymphocytes, said like "LIMF-oh-sites."

Then there are the big eaters, the macrophages. Macrophages live in many parts of your body, not just in the lungs. There are macrophages in your skin, in your liver, in your gut, and in many other places. Each group is suited to the place where it lives. The macrophages that live in the alveoli are especially good at their job in the lungs: crawling over the thin, wet walls of the air sacs and keeping them clear.

Where do the lung macrophages come from? Many of them settled in the lungs very early in life, even before you were born, and they make more of themselves as needed, a bit like a family that has lived in the same neighborhood for generations. When extra help is needed, other white blood cells called monocytes, said like "MON-oh-sites," can travel from the blood into the lungs. Once they arrive, monocytes can grow into macrophages too.

A macrophage is small, but compared with a germ, it is a giant. A single macrophage is about ten or more times wider than a typical bacterium, and many, many times wider than a virus. Still, you would need a microscope to see a macrophage. About four or five of them side by side would be as wide as a single hair on your head.

How a Big Eater Eats

A macrophage does not have a mouth or teeth. So how does it eat? It uses its whole body. A macrophage is soft and can change its shape. When it finds something to eat, it reaches out with parts of itself, a little like stretching out two arms. These soft arms flow around the speck of dust or the germ until they meet on the other side.

Imagine holding a soft blanket and wrapping it around a ball until the ball is completely covered. The macrophage does something similar. Once its arms have wrapped all the way around the germ, the germ ends up inside the macrophage, sealed in a little bubble. Scientists call this kind of swallowing engulfing. The germ is now trapped, and it cannot escape.

Next, the macrophage begins to break the germ down. Inside the macrophage, there are other small bubbles filled with strong chemicals. These bubbles join with the bubble that holds the germ, and the strong chemicals pour in. The chemicals break the germ apart into tiny pieces, much as the juices in your stomach break down your food. In a way, each macrophage has its own little stomach inside it.

How does a macrophage know where to go? It follows signals. Germs and damaged cells give off tiny traces of chemicals, and the macrophage can sense them. It moves toward the place where the signal is strongest, the way you might follow the smell of warm bread to find the kitchen. Macrophages also have special spots on their surface that can recognize the outer coats of many common germs. When these spots touch a germ, the macrophage knows it has found something to eat.

A macrophage also needs to know what not to eat. It would be a disaster if it started eating the healthy cells of your own lungs. Luckily, your healthy cells carry special markers on their surface that work like a sign saying, "I belong here. Please do not eat me." The macrophage checks for these signs and leaves healthy cells alone. Worn-out or damaged cells lose their signs and show different ones instead, which tell the macrophage that it is time to clear them away.

A single macrophage can eat many germs, one after another, and it can keep working for a long time. Some lung macrophages stay on duty for months or even longer. They patrol the same area over and over, so the alveoli are checked again and again. Most of the time, they find only a few harmless specks to tidy up, and they do it so quietly that you never feel a thing.

Macrophages have one more everyday cleaning job in the alveoli. Do you remember surfactant, the soap-like liquid that keeps the tiny air sacs from sticking shut? Your lungs make it all the time, and old surfactant needs to be cleared away so that fresh surfactant can take its place. The macrophages help by eating up the used surfactant. In this way, they keep the lining of the alveoli fresh and working well.

Dust, Smoke, and Other Specks

Not everything a macrophage eats is a germ. Every day, you breathe in thousands of liters of air, and that air carries tiny specks of dust, soot, and pollen. The nose, the windpipe, and the bronchi catch most of these specks. But a few of the very smallest specks drift all the way down to the alveoli. The macrophages eat these specks too, so that they do not pile up inside your lungs.

Pollen is another speck that macrophages often meet. For most people, a few bits of pollen are no trouble at all, and the macrophages simply tidy them away. Some people, though, have allergies. In their bodies, other parts of the defense system treat harmless pollen as if it were a dangerous germ. That is why some people sneeze, have itchy eyes, or get a runny nose at times of year when plants release lots of pollen.

Some specks are easy for a macrophage to break down, but others are not. A tiny bit of soot or mineral dust cannot be digested the way a germ can. So the macrophage simply holds on to it, keeping it wrapped up and out of the way. Then the macrophage has a few different ways to take the speck out of the alveoli.

One way is to crawl. A macrophage carrying a load of dust can crawl from the alveolus up to the smallest airways. There, it reaches the bottom of the moving staircase of mucus and cilia that you learned about in the windpipe chapter. The macrophage climbs aboard, and the escalator carries it, along with its load, all the way up to the throat. Then it is swallowed and leaves the lungs for good.

Another way is to travel into the tiny drainage channels of the lymph system. These channels carry fluid away from the lungs to the lymph nodes, the small bean-shaped guard stations you met in your neck and near the bronchi. Some macrophages take their load of dust there. In grown-ups who have breathed a lot of dusty or smoky air over many years, these lymph nodes can slowly turn darker, because of all the specks that have been stored in them.

Macrophages are hard workers, but they can be overloaded. When a person breathes in smoke every day, such as tobacco smoke, the macrophages in the lungs have to eat huge amounts of tiny smoke particles. They become stuffed with dark specks, and they can become tired and less able to fight germs. This is one of the reasons why people who smoke tend to catch more chest infections. Keeping the air around you clean makes the work of your macrophages much easier.

Ringing the Alarm

Most days, the macrophages in your lungs work alone. They eat a speck here and a germ there, and that is enough. But sometimes, many germs arrive at once. Perhaps someone nearby sneezed, and you breathed in a cloud of germs from a cold. When a macrophage finds more germs than it can handle by itself, it does not just keep eating. It calls for help.

A macrophage calls for help by releasing message chemicals. These chemicals spread out from the macrophage like the sound of an alarm bell spreading through a building. The message reaches the nearby blood vessels, and it reaches other white blood cells. It says, in a way, "Germs are here. Come quickly."

The fast responders, the neutrophils, are among the first to answer. They squeeze out of the nearby blood vessels and rush to the place where the alarm is ringing. There, they join the macrophages in eating germs. The blood vessels nearby also change a little. They widen and become slightly leaky, so that more defenders and more helpful fluid can reach the area where the fight is going on.

This whole response is called inflammation, said like "in-fluh-MAY-shun." You have probably seen inflammation on your skin. When you scrape your knee, the area around the scrape can become red, warm, a little swollen, and sore. That is your body sending extra blood and extra defenders to protect the wound. A similar thing can happen in your airways when you have a cold. The swelling and extra mucus are signs that your defenders are busy.

Some of the message chemicals travel even further. They can reach your brain and tell it that the body is fighting germs. The brain may then turn up your body's temperature, which is what a fever is. These messages can also make you feel tired and sleepy, so that you rest and save your energy for the fight. You will learn more about fever and rest in a later chapter.

Macrophages also help to teach other defenders. After a macrophage has broken a germ into pieces, it can show some of those pieces on its outside surface, a bit like holding up a picture of a wanted visitor. Lymphocytes passing by can look at these pieces and learn what the germ looks like. Some lymphocytes then make antibodies that fit that germ exactly, and some remember the germ for a long time. If the same germ ever comes back, your body can recognize it and fight it off faster.

Not every germ gives up easily. Some germs have tricks to hide from macrophages, and a few can even survive for a while after being swallowed. This is why your body does not rely on just one kind of guard. Macrophages, neutrophils, lymphocytes, antibodies, mucus, and cilia all work together, so that if a germ slips past one guard, another is waiting. Many layers of defense make your body much safer than any single guard could.

Cleaning Up and Calming Down

When the fight is over, there is a lot of mess to tidy up. There are broken bits of germs, worn-out neutrophils that have finished their work, and damaged cells. The macrophages take on this job too. They eat up the leftover pieces, and they clear the alveoli so that air can fill them properly again.

The macrophages also help to turn off the alarm. After the germs are gone, they release different message chemicals that tell the body to calm down. The swelling goes down, the extra defenders leave or are cleared away, and the lining of the lungs begins to heal. Turning off the alarm at the right time is just as important as turning it on. An alarm that kept ringing long after the danger had passed would cause trouble of its own.

In fact, lung macrophages are usually calm, steady workers. Every day, they meet many harmless specks of dust and pollen, and they do not ring the alarm for each one. That would make your lungs swollen and sore all the time. Instead, they quietly eat the harmless specks and save the alarm for real danger. This careful balance helps to keep your lungs healthy and comfortable.

You may notice the cleanup work after a cold. The worst of the cold passes, your energy comes back, and you start to feel like yourself again. But a little cough may stay for a while, sometimes for a week or more. That is often because the lining of your airways is still healing, and your cleaners are still tidying up. If a cough goes on for a long time or seems to be getting worse, a grown-up should talk to a doctor.

You do not need to do anything special to help your macrophages. They work on their own, without you having to think about them. But the same healthy habits that help the rest of your body also help these cleaners. Getting enough sleep, eating a mix of good foods, drinking water, and playing actively all help your body's defenders stay strong. Breathing clean air, away from smoke, means your macrophages have less work to do.

The next time you take a deep breath, think about the macrophages in your alveoli. They are crawling, searching, eating, and tidying up, all without making a sound. They have been doing it since before you can remember, and they will keep doing it for the rest of your life. They are some of the most faithful guards your body has.

But macrophages are not the only travelers in your lungs. Remember the monocytes that can come from the blood to become new macrophages, and the neutrophils that rush in when the alarm rings. All of them travel through your body in the blood. The blood also carries the oxygen that your alveoli collect, delivering it to every cell from your head to your toes. In the next chapter, you will ride along with the blood, the red train that never stops moving.

Key Words

Macrophage: A large white blood cell that eats germs, dust, and worn-out cells. Its name means "big eater."

White blood cells: The cells in your blood that defend your body against germs. There are several kinds, each with its own job.

Engulfing: The way a macrophage eats, by wrapping its soft body around a germ or a speck until the germ or speck is sealed inside it.

Neutrophils: Fast-moving white blood cells that rush to places where germs are found and help to destroy them.

Lymphocytes: White blood cells that learn to recognize particular germs. Some of them make antibodies, and some remember germs for a long time.

Monocytes: White blood cells that travel in the blood and can grow into macrophages when they move into the body's tissues.

Antibodies: Special proteins that stick to germs, marking them so that other defenders can find and destroy them.

Inflammation: The body's response to germs or injury, in which extra blood and defenders come to an area, often causing redness, warmth, and swelling.

Allergy: A reaction in which the body's defenders treat something harmless, such as pollen, as if it were a dangerous germ. It can cause sneezing, itchy eyes, or a runny nose.