The Guide
Three lines of defense
A body is warm, wet, and full of food. For a bacterium, a body is a good place to live and multiply. Other living things have been trying to move in for as long as there have been bodies, and the immune system is what stands between you and them.
It works in three lines, and the difference between them is the shape of this whole topic. The first keeps things out and does not care what they are. The second attacks anything that got in, and also does not care what it is. The third works out exactly what this particular invader is, makes a protein that fits it and nothing else, and remembers it afterward. The first two are ready now. The third is slow the first time and fast forever after, and that last fact is the one vaccines are built on.
What the body is defending against
A pathogen is anything that gets into a body and causes disease. The two that come up most are bacteria and viruses, and telling them apart matters outside the test as much as on it.
| Bacteria | Viruses | |
|---|---|---|
| What they are | Living cells, complete in themselves | Not cells — genetic instructions in a coat |
| How they multiply | Divide on their own, anywhere they can feed | Only by taking over one of your cells and making it build copies |
| Do antibiotics work? | Yes — they attack features only bacteria have | No. There is nothing there for them to attack. |
| Examples | Strep throat, tetanus, tuberculosis | Colds, flu, measles, chickenpox |
Two things follow. First, most bacteria are not pathogens at all — your intestines depend on them, as Quiz 14 described, and so does your skin. Second, a virus cannot be treated with an antibiotic, which is why a doctor will not prescribe one for a cold. That is not stinginess. Taking antibiotics when they cannot help still kills off the harmless bacteria you carry, and still gives the surviving bacteria practice at resisting the drug, so the medicine works less well for everyone the next time it is needed.
The three lines
First line: barriers
Skin is the main one, and Quiz 17 covered why a large burn is so dangerous — the wall is gone. But there are openings in the skin that have to stay open, and each has its own arrangement. The airways are lined with mucus that traps particles and with tiny hairs that sweep the mucus back up to be swallowed. The stomach's acid destroys most of what arrives with food. Tears and saliva carry an enzyme that breaks bacteria apart. And the harmless bacteria already living on your skin and in your gut occupy the space and the food, leaving less of both for anything new.
Second line: the general response
Once something is past the wall, certain white blood cells surround and swallow it. They do this to anything that is not part of you, without identifying it, and they are the reason a small splinter usually comes to nothing.
Inflammation is what you see when this is happening. Blood vessels near the site widen and become leakier, so more blood and fluid arrive, bringing white cells with them. The result is redness, warmth, swelling and soreness. Those four are not the infection damaging you. They are the response arriving, which is worth knowing because it changes what you make of a swollen cut: it means the defense found it.
Fever is the same principle applied to the whole body. The temperature is deliberately raised — Quiz 19 will describe how the setting is changed — which slows many pathogens down and speeds the defense up. A fever is generally the body fighting rather than the body failing, though a very high or very long one is still a reason to see a doctor.
Third line: the specific response
Everything alive carries markers on its surface. An antigen is any such marker that the immune system can recognize, and the ones that matter here sit on the surface of an invader. The third line reads those markers and builds something shaped to match.
The builders are white blood cells called lymphocytes, and they work in two ways. Some produce antibodies: proteins that lock onto one particular antigen and nothing else. Antibodies do not kill anything themselves. They stick to invaders, clump them together so they cannot spread, and mark them plainly for the swallowing cells of the second line. Other lymphocytes do a different job — they find the body's own cells that a virus has taken over and destroy them, since a captured cell is a factory for more virus.
This is why being immune to one disease gives you nothing against another. The fit is exact. It is also why the third line is slow the first time: among all your lymphocytes, only a few happen to match this particular antigen, and those few have to be found and multiplied into large numbers. That takes days, and those days are when you feel sick.
Memory, and why the second time is different
When the fight is over, most of the cells built for it die off. Some do not. Memory cells remain in the blood, already matched to that antigen, sometimes for the rest of your life.
Meet the same pathogen again and the memory cells are waiting. The response starts at once and reaches a much higher level, usually finishing the invader off before you feel anything at all. You were exposed and did not get sick, which is what being immune amounts to. The germ was the same as before. What changed is that your body already had cells that match it.
Vaccines
A vaccine gives the body the antigen without the disease. It may contain a killed pathogen, a weakened one, one piece of its surface, or instructions for your own cells to make that piece. In every case the immune system sees the marker, mounts the slow first response, and makes memory cells — and then the real pathogen, if it ever arrives, meets the fast second response instead of the slow first one.
That is the whole mechanism, and it explains the ordinary facts about vaccines. Some need more than one dose, because a first response is not always enough to leave good memory. Some need a booster years later, because memory can fade. A flu shot is needed yearly because the flu virus changes its surface markers, so last year's antibodies no longer fit — the specificity that makes the system work is also its limit.
Why it matters who else is immune. A pathogen stays in circulation by passing from one person to the next. When most people in a group are immune, the chain keeps running into people it cannot infect and breaks. That protects people who cannot be vaccinated themselves — newborns, some people on cancer treatment, some people with immune disorders — because the pathogen is far less likely to reach them at all. It is one of the few pieces of health that is genuinely collective: an individual's protection is partly built out of other people's.
When the system misfires
Two failures come up often enough to name. An allergy is a strong response to something harmless — pollen, a food, a bee sting. The machinery is working; it is aimed at the wrong thing, and the damage comes from the response rather than the trigger. An autoimmune disease is a response against the body's own cells, as though a marker of yours had been read as foreign.
Barriers keep things out and are never choosy. A general response inside the body swallows anything foreign, with inflammation and fever as the visible signs of it working. The specific response reads an invader's antigens, builds antibodies that fit only those, and leaves memory cells behind, so a second meeting is answered fast and large. A vaccine supplies the antigen without the disease so the memory is there in advance.
Worked Examples
Two questions, worked through
Start with what is causing the cold. Colds are caused by viruses, and a virus is not a cell — it is genetic instructions in a coat, reproducing only by taking over one of your own cells.
Now ask how an antibiotic works. It attacks features that bacteria have and human cells do not: a bacterial cell wall, the bacterium's own machinery for building proteins. A virus has none of that. There is no target.
Antibiotics act on bacteria, and a cold is caused by a virus, so there is nothing for the drug to work on. The refusal is not about rationing; taking one anyway would kill useful bacteria and give the rest practice at surviving the drug. On the test, the moment you see the word virus, antibiotics are the wrong answer.Look at what the graph in the Guide compares. Both meetings are with the same pathogen; the two curves differ because of what the body brought to them.
At six, no cells matched that antigen yet, so the few that did had to be found and multiplied — days, during which she was ill. At twenty, memory cells were already in her blood, matched and waiting, so antibodies rose immediately and in quantity, and the virus was cleared before it could establish itself.
No — the virus was the same. The difference was memory cells from the first infection. Whenever a question offers you a change in the germ and a change in the person, check which one the evidence actually shows.The Quiz
Ten questions
Answer all ten, then press the button at the bottom. Nothing is timed.
Use the three lines of defense diagram from the Guide above: barriers, the general response, and the specific response, each with a note on whether it is choosy and how fast it acts.
1.Stomach acid destroying bacteria in swallowed food belongs to which line?
2.According to the diagram, which line is the only one that is choosy about what it attacks?
3.A cut on the hand becomes red, warm, swollen and sore within a few hours. What does that best indicate?
4.The diagram says the third line takes days the first time but hours afterward. What explains the difference?
Antibody levels in one person's blood, measured in units, after a first injection of a vaccine and again after a second injection of the same vaccine given months later.
| Days after the injection | After the first | After the second |
|---|---|---|
| 0 | 0 | 4 |
| 5 | 2 | 60 |
| 10 | 15 | 180 |
| 20 | 10 | 140 |
5.On day 10, how does the level after the second injection compare with the level after the first?
6.What best explains the difference between the two columns?
7.A student says the table proves this person is now protected against infections in general. What is wrong with that?
A carpenter steps on a nail that goes through the sole of his boot. At the clinic the wound is cleaned out, and the nurse asks when he last had a tetanus shot. It was more than fifteen years ago, so she gives him a booster.
Tetanus is caused by a bacterium common in soil and dust, which grows well in a deep wound where little air reaches. It produces a toxin that acts on nerves. The vaccine contains a harmless form of that toxin, which prompts the body to build antibodies against it and leave memory cells behind. Protection from the vaccine weakens over the years, which is why boosters are given.
8.What does the booster do?
9.He says he has never had tetanus in his life, so he does not need the shot. How should that be judged?
10.Which conclusion does the passage best support?
The Key
Answers, and the trap in each one
Your Score
What the number means
| 8 to 10 | Solid. Check topic 18 on your map, and go on to Quiz 19. |
|---|---|
| 6 or 7 | Close. Read the whole key, then take this again in a few days before you check the box. |
| 5 or fewer | Worth another pass. Write the three lines down the side of a page with two notes each: choosy or not, and how fast. Then say in your own words what a vaccine contains and what it does not. |
Misses on 1 to 4 usually mean the three lines have run together; separating them by speed fixes it, since barriers are instant, inflammation takes hours and antibodies take days. Misses on 5 to 7 are arithmetic and the specificity rule. Misses on 8 to 10 tend to come from mixing up what kills a pathogen with what prepares you for it, and holding those two apart is most of what this topic asks.
Onesimus, and what he knew
When smallpox reached Boston in 1721, one man in the town already knew a way to blunt it. Onesimus was an enslaved West African, the property of the minister Cotton Mather, and some years earlier he had told Mather that in his homeland people were deliberately given a small dose of the disease through a cut in the skin. They fell mildly ill, recovered, and could not catch it again. He said it had been done to him as a child.
Mather pressed the practice on Boston's doctors during the epidemic and was met with fury; someone threw a bomb through his window. One physician, Zabdiel Boylston, tried it. Of the roughly two hundred and forty people he inoculated, about one in forty died. Of those who caught smallpox in the ordinary way that year, about one in seven died.
The method was older than any of them and had traveled from West Africa and from Turkey by word of mouth and by people carried against their will. Seventy-five years later Edward Jenner found a safer version using cowpox, which gave the practice its modern name and eventually took smallpox off the earth entirely. The knowledge that started it in Boston belonged to a man whose own name we know only because his owner wrote it down.