Unit 11 · Your Body's Defenses and Choices

Think about the doorknob in a school hallway. Hundreds of hands touch it each day, and it carries many germs. Yet most students do not get sick from touching it. The main reason is your skin. Its outer layer is made of tough, flat, dead cells packed tightly together. Most germs cannot get through this wall. The skin is also slightly acidic and a little dry, which many germs do not like.

Germs can also enter through openings, such as your nose, mouth and eyes. These places are lined with mucus, a slippery, sticky fluid. Mucus traps dust and germs the way flypaper traps flies. In your airways, tiny hairlike parts called cilia sweep the mucus up and out. You then swallow it or cough it out. Tears and saliva also hold substances that break down some bacteria.

These barriers explain simple habits. Washing hands with soap removes germs before they reach your eyes, nose or mouth. Covering a cut with a clean bandage protects a break in the wall. Maya’s scrape was a break in her skin wall. That is why the nurse cleaned it first. Barriers are the body’s first line of defense, and they work all day without you noticing.

When a germ gets past the skin, the body does not wait. Within minutes, certain white blood cells arrive. This fast, general response is called innate immunity. Innate means "you are born with it." These defenders do not need to have met the germ before. They react to signs that something is foreign or that tissue is damaged.

Some of these defenders are phagocytes, a word that means "eating cells." A phagocyte wraps itself around a germ and pulls it inside. Then it breaks the germ apart with chemicals. Neutrophils are the most common kind in the blood and are often first on the scene. Larger cells called macrophages also eat germs and clean up dead cells and debris.

In Maya’s knee, phagocytes moved out of tiny blood vessels and into the scrape. They swallowed bacteria that came in with the dirt. The clear fluid she saw was partly fluid from the blood that carried these helpers. Innate defenses are quick, but they are not specially aimed. For a germ they cannot clear, the body calls in a slower, targeted team.

Maya’s knee turned red, warm and puffy. These are the signs of inflammation. Doctors in ancient Rome described four signs of it: redness, heat, swelling and pain. The Roman writer Celsus listed them about 2,000 years ago. Doctors still use the same list today. Later, a fifth sign was added: the injured part may not work as well for a while.

Here is what causes each sign. Damaged cells and defender cells release chemical signals. The signals tell nearby blood vessels to widen. More blood flows in, which makes the skin red and warm. The vessel walls also become leakier. Fluid and white blood cells slip out into the tissue, which causes swelling. Some signals press on nerve endings, which causes pain.

Inflammation is a helpful part of healing. It brings defenders and repair materials to the right spot. It usually calms down in a few days as the area heals. But inflammation that spreads, grows worse or comes with fever can be a sign of infection. That is the time to tell a trusted adult and see a nurse or doctor. They can decide whether more care is needed.

Suppose a cold virus gets into your nose and throat. The innate defenders fight it, but the virus keeps making copies. After several days, a second team takes over. This is adaptive immunity, the part of the immune system that learns. It is slower to start, but it aims at one exact germ. That is often why a cold is worst for a few days and then gets better.

The adaptive team is made of lymphocytes, a kind of white blood cell. There are two main kinds. B cells make antibodies, which you will study next. T cells do other jobs. Helper T cells send signals that direct the whole response. Killer T cells find body cells that a virus has taken over and destroy them, so the virus cannot keep copying itself.

How do these cells know what to attack? Each germ carries molecules on its surface called antigens. An antigen is like a name tag. Each B or T cell recognizes one shape of name tag. When a cell finds its match, it multiplies into an army of copies, all aimed at that germ. Lymph nodes, the small lumps in your neck, are where many of these meetings happen. That is why they can swell when you are sick.

When a B cell finds its matching antigen, it multiplies. Many of the new cells become antibody factories. An antibody is a Y-shaped protein. The tips of the Y fit one antigen, like a key fits one lock. One of these factory cells can release thousands of antibodies every second into the blood.

Antibodies help in several ways. They can coat a virus so it cannot enter your cells. This is called neutralizing it. They can clump germs together so they are easier to catch. They also act like flags. A germ covered in antibodies is much easier for phagocytes to find and eat. In this way, the adaptive and innate teams work together.

Doctors can test the blood for antibodies. Finding antibodies to a certain germ shows the body has met it, through an infection or a vaccine. For example, before some jobs in hospitals, workers may have a blood test to check for antibodies to measles. That test tells the doctor whether the person is likely protected.

Many people who had chickenpox as children never get it again. Why? After the fight is over, most of the extra B and T cells die off. But some stay behind for years, sometimes for life. These are memory cells. They remember the antigen of the germ they fought.

The first time you meet a germ, the adaptive response can take a week or more to build up. That is the primary response. If the same germ returns, memory cells spring into action at once. They multiply fast and make large amounts of antibodies within days. This faster second round is called the secondary response. Often it stops the germ before you ever feel sick.

Memory has limits. Some germs change their name tags. Flu viruses change a little each year, so old memory cells may not fully match. That is one reason a new flu shot is offered each year. Other germs, like measles, change very little. So protection against measles can last a very long time.

In 1796, the English doctor Edward Jenner tested an idea he had heard among farm workers. Milkmaids who caught cowpox, a mild disease from cows, seemed not to catch smallpox, a deadly disease. Jenner took material from a cowpox sore and put it into a small cut on the arm of a boy named James Phipps. Later he exposed the boy to smallpox material, and the boy did not get sick. Today such a test on a child would not be allowed. But it launched the idea of the vaccine. The word comes from vacca, Latin for cow.

A vaccine works by giving the immune system a safe preview. It may hold a weakened germ, a killed germ, a piece of a germ, or instructions for cells to make one harmless germ protein. The immune system sees the antigen and builds B cells, T cells and memory cells. The person gains immunity, protection from the disease, without having to get sick first.

Vaccines are tested in large studies before they are approved, and they keep being watched afterward. Like any medicine, they can have side effects. Most are mild, such as a sore arm or a day of tiredness, which are signs the immune system is working. Serious reactions are rare. Your doctor or nurse can answer questions about which vaccines are recommended and why.

Picture a baby too young for the measles vaccine, and a classmate being treated for cancer whose immune system is weak. Neither can be fully protected by vaccines. Yet they can still be shielded. If nearly everyone around them is immune, the measles virus has almost no one to jump to. It dies out before it reaches them.

This shielding is called herd immunity, or community immunity. It works because a germ spreads from person to person. Each immune person is like a missing link in the chain. The more links are missing, the harder it is for the germ to travel. The share of people who must be immune depends on how contagious a germ is. A contagious disease is one that spreads easily between people.

Measles is one of the most contagious diseases known. Experts estimate that about 95 of every 100 people need to be immune to stop its spread. Diseases that spread less easily need a lower share. When vaccination rates in a community drop, outbreaks can return. That happened with measles in several U.S. communities in recent years. Public health workers track these rates to keep communities safe.

Every spring, Luis gets itchy eyes and a runny nose when the trees bloom. Tree pollen is harmless to most people. But Luis’s immune system treats it as an invader. An allergy is an immune reaction to something that is not actually dangerous. The harmless thing that sets it off is called an allergen.

In an allergy, the body makes a special kind of antibody against the allergen. These antibodies sit on cells that hold a chemical called histamine. When the allergen shows up again, those cells release histamine. It causes sneezing, itching, watery eyes or hives. Common allergens include pollen, dust mites, pet dander, some foods, and insect stings. A study led by Dr. Ruchi Gupta in Chicago, published in 2018, estimated that about 8 in 100 U.S. children have a food allergy.

Most allergies are an annoyance that can be managed. A doctor can help find the allergen and suggest ways to avoid it or medicines to ease the signs. A few people have a severe, whole-body reaction called anaphylaxis, which can make breathing hard. It needs emergency care right away. People with severe allergies may carry an epinephrine auto-injector and a plan made with their doctor. Friends who know the plan can help.

Normally, the immune system learns to leave the body’s own cells alone. It learns this while immune cells are developing. Sometimes this learning slips. In an autoimmune condition, the immune system attacks some of the body’s own healthy tissue. Auto means "self." Scientists are still studying why this happens. Genes and things in the environment both seem to play a part.

Here are a few examples, named simply. In type 1 diabetes, the immune system attacks the cells in the pancreas that make insulin, the hormone that lets cells use sugar. In celiac disease, eating gluten, a protein in wheat, leads the immune system to harm the lining of the small intestine. In rheumatoid arthritis, it attacks the lining of joints, causing pain and swelling.

Autoimmune conditions are not caused by anything a person did, and they are not contagious. Many people live full, active lives with them. Treatment depends on the condition. A person with type 1 diabetes takes insulin. A person with celiac disease eats gluten-free food. Doctors, nurses and dietitians help each person make a plan that fits their body.

For about 2,000 years, many European and Middle Eastern doctors followed the idea of the four humors. They thought illness came from an imbalance of four body fluids. From the 1860s to the 1880s, Louis Pasteur in France and Robert Koch in Germany showed that specific germs cause specific diseases. This germ theory replaced the humors because experiments supported it again and again.

In 1882, the Russian scientist Élie Metchnikoff was studying see-through starfish larvae in Italy. He pushed a tiny rose thorn into one. The next day, he saw moving cells gathered around the thorn. He guessed these cells defend the body by swallowing invaders. He named them phagocytes. Around the same time, Paul Ehrlich in Germany studied substances in the blood, later called antibodies. In 1908, the two men shared the Nobel Prize for their work on immunity.

Science keeps building on earlier work. Microscopes, lab tests and large clinical trials have shown how B cells, T cells and memory cells work. A clinical trial is a careful study that tests a treatment in people, often comparing it to a group that does not get it. Each new finding is checked by other scientists. That is how the ideas in this chapter became trusted.

Between about ages 10 and 16, many young people grow several inches in just a few years. Bones get longer and denser. Muscles grow. All that building takes nutrients, the substances in food the body uses for energy, growth and repair. A growing teen often needs more food than a younger child, and that is normal.

The U.S. Department of Agriculture uses a simple picture called MyPlate, introduced in 2011. It shows a plate about half filled with fruits and vegetables, about a quarter with grains, and about a quarter with protein foods, plus a serving of dairy or a calcium-rich food. Whole grains, like brown rice or whole-wheat bread, give more fiber than white bread. Calcium from milk, yogurt, fortified soy drinks or leafy greens helps build strong bones.

Healthy eating is about patterns over time, not about one perfect meal. A balanced diet has many kinds of food, including foods from your family’s culture. Drinking water instead of sugary drinks most of the time helps too. Skipping meals can leave you tired and unfocused. If you have questions about food, allergies or your body, a doctor, school nurse or dietitian can help.

Jalen stayed up past midnight on his phone. At 6:30 the next morning, his alarm rang. In math class, he could not focus, and he snapped at a friend. Many students know this feeling. Sleep is not wasted time. While you sleep, your brain sorts and stores what you learned that day. Your body releases growth hormone and repairs muscles.

Sleep experts at the American Academy of Sleep Medicine recommend 8 to 10 hours a night for teens ages 13 to 18, and 9 to 12 hours for children ages 6 to 12. In puberty, the body clock also shifts. Teens often feel sleepy later at night than younger children do. This shift is a normal part of growing up, called a change in circadian rhythm, the body’s daily clock.

Good sleep habits can help. Go to bed and wake up at about the same time each day, even on weekends. Put screens away about an hour before bed, since bright light can signal the brain to stay awake. Keep the room dark and cool. Caffeine from soda, coffee or energy drinks can make sleep harder. If you often cannot sleep, talk to a parent or doctor.

Physical activity does not have to mean a team sport. Walking the dog, dancing in your room, biking to a friend’s house and playing tag all count. The U.S. Department of Health and Human Services, in its Physical Activity Guidelines of 2018, advises children and teens ages 6 to 17 to get 60 minutes or more of activity every day.

Most of that time should be aerobic activity, movement that makes your heart beat faster and your breathing quicker, like running, swimming or dancing. On at least three days a week, the guidelines also include muscle-strengthening activity, like climbing or push-ups, and bone-strengthening activity, like jumping. Bones grow stronger when they carry weight and land from jumps.

Activity helps more than muscles. It strengthens the heart and lungs. Many studies link it to better mood, lower stress and better sleep. Every body can move in its own way. A student who uses a wheelchair might play wheelchair basketball or do seated strength work. A physical therapist, coach or doctor can help anyone find activities that fit their body and that they enjoy.

By about age 6, your brain is already close to its adult size. But it keeps changing inside for years. Brain scans show that pathways used often get stronger and faster, while ones rarely used get trimmed away. This makes the teen years a powerful time for learning new skills.

Different parts finish at different times. The prefrontal cortex, the area just behind your forehead, helps you plan, weigh choices and control impulses. Research suggests it keeps developing into the mid-20s. Areas linked to emotions and rewards become very active in the teen years. That can make excitement and friends feel especially important. This is a normal stage, not a flaw.

Because the brain is still wiring itself, what it is exposed to matters. Learning, sleep and exercise help build strong pathways. Addiction is a condition in which a person keeps using a substance even when it causes harm, because the brain has come to crave it. Scientists have found that the younger a person starts using nicotine, alcohol or other drugs, the higher the chance of addiction later.

A vape, or e-cigarette, is a battery-powered device that heats a liquid into an aerosol that a person breathes in. The liquid often has flavors like fruit or candy. Many young people think vapes are just flavored water. But most vapes sold in the United States contain nicotine, the same addictive drug found in tobacco. Some labeled nicotine-free have been found to contain it.

In 2016, the U.S. Surgeon General issued a report on e-cigarettes and young people. It explained that nicotine exposure during the teen years can harm the developing brain. It can affect attention, learning and mood, and it makes addiction more likely. Some vapes deliver a lot of nicotine, so a person can become dependent quickly. Dependent means the body feels uncomfortable without it.

The aerosol is not just harmless water vapor. It can contain tiny particles and chemicals that are breathed deep into the lungs. The good news is that most teens do not vape. In a 2023 national survey, about 90 in 100 high school students said they had not vaped in the past month. People who want to quit can get free help from a doctor, a school counselor, or programs like the text line This Is Quitting.

Alcohol is a depressant, a drug that slows down the brain and body. Even small amounts can slow reaction time and affect judgment and balance. That is one reason it is illegal to drive after drinking, and why it is illegal to buy alcohol in the United States before age 21. Larger amounts can make breathing slow down, which is a medical emergency.

Other drugs affect the brain in different ways. Cannabis, also called marijuana, can affect memory, attention and coordination. It is legal for adults 21 and older in Illinois since 2020, but not for teens. Some of today’s cannabis products are much stronger than in the past. Prescription medicines are safe when used as a doctor directs. Taking someone else’s pills, or taking more than directed, can be dangerous.

One serious risk today is fake pills. Some pills sold outside pharmacies look like real medicine but contain fentanyl, a very strong opioid. A tiny amount can stop a person’s breathing. Only take medicine from a pharmacist or trusted adult. If someone seems very sleepy, will not wake up or is breathing strangely, call 911 right away. Illinois has a law that protects people who call for help in an overdose.

When Aisha’s friends joined the robotics club, she joined too, and she found she loved building. When Omar’s friends started skipping lunch to look thinner, he felt pushed to skip it too. Both are examples of peer pressure, the influence that people your own age have on what you do. Peers are people in your age group, like classmates and friends.

Peer pressure can be spoken, like "Come on, just try it." It can also be unspoken. Nobody says a word, but you notice what everyone is wearing or doing and feel you should match. People often overestimate how many peers are doing a risky thing. Survey data, like the vaping numbers you just read, often show that far fewer are doing it than it seems.

Wanting to belong is a normal and healthy human need. The key is noticing the pull and asking yourself a few questions. Is this safe? Does it match who I want to be? Would I be okay if my family knew? Choosing friends who respect your choices makes hard moments easier. Positive peer pressure, like friends who study together or cheer each other on, can help you grow.

Remember Dev at the party. He did not give a speech. He said, "No thanks, I’m good," and suggested pizza. These are refusal skills, ways to say no that you plan and practice ahead of time. Practicing helps because in a stressful moment, the brain reaches for what it already knows.

Here are steps many health teachers use. Say no clearly and early, and use a firm voice. Give a short reason if you want: "I have a game tomorrow." Suggest something else to do. If the pressure keeps going, leave. Body language matters too. Stand tall, look at the person, and keep your voice steady. You do not need to explain or argue.

A backup plan helps. Some families use a code word, like Dev’s sister. A teen texts it, and the adult calls with a reason to leave, no questions asked in the moment. A true friend will respect your no. If someone keeps pushing or makes fun of you, that tells you something about the situation. Walking away is a strong choice, not a weak one.

The night before a big test, Priya’s stomach felt tight and her heart beat fast. That is stress, the body’s response to a challenge or demand. When the brain senses a challenge, it releases hormones like adrenaline and cortisol. They speed up the heart and breathing and give a burst of energy. In small doses, stress can help you focus and rise to the moment.

Anxiety is a feeling of worry or unease about what might happen. Everyone feels it sometimes. When worry is strong, lasts a long time, or gets in the way of school, sleep or friends, it is worth talking to a doctor or counselor. Anxiety is very common and very treatable. Counseling and other care help many people feel better.

Some coping skills help calm the body. Slow breathing, like breathing in for four counts and out for six, can slow a racing heart. Moving your body, getting enough sleep, spending time outside, and talking with someone you trust all help. Breaking a big task into small steps can make it feel possible. Different skills work for different people, so it helps to try a few and notice what works for you.

Sometimes feelings get too heavy to carry alone. A student might feel sad for weeks, lose interest in things they used to enjoy, or have thoughts of hurting themselves. These are signs to reach out right away. Asking for help is a sign of strength and good sense, just like going to a doctor for a broken arm.

Who can help? A trusted adult is a grown-up who listens and helps keep you safe, like a parent, relative, teacher, coach, school nurse or school counselor. You can say something simple: "I’ve been feeling really down, and I need to talk." If a friend tells you they are thinking about hurting themselves, do not keep it a secret. Tell a trusted adult right away. That is what a good friend does.

Help is also available any time, day or night. In July 2022, the United States launched 988, the Suicide and Crisis Lifeline. Anyone can call or text 988, or chat online, for free. Trained counselors listen and help people who are in emotional distress or worried about someone else. If someone is in immediate danger, call 911.

Look at a class photo from fifth grade and another from eighth grade. Many students look very different. That is because of puberty, the stage when a child’s body gradually grows into an adult body. It is guided by hormones, chemical messengers made by glands and carried in the blood.

Puberty often starts sometime between about ages 8 and 14. Everyone’s timing is different, and early, middle and later timing are all normal. Common changes include a growth spurt, stronger body odor, and skin changes like pimples. Feelings can also shift more quickly. Daily showers, deodorant, and washing your face help with some of these changes.

Questions about puberty are normal and important. The best people to ask are a parent or guardian, a school nurse or a doctor. They can give correct information that fits you. Your health class may also cover it. Because every body grows on its own schedule, comparing yourself to friends is not a useful measure.