Why the second meeting with a germ goes so much better than the first

How to use this simulation

1. The left panel shows the blood. Red spiky balls are antigens (the germ). Blue Y shapes are antibodies. Below them are B cells, a plasma cell that makes antibodies, and a row of memory cells.
2. The right panel is the graph of antibody level against days. The amber line is a person who was never vaccinated and the blue line is a person who was vaccinated on day 0. The playhead sweeps along the days, and the blood scene follows it.
3. Press play and watch the infection day. For the unvaccinated person, the antibody level stays at zero for several days, then rises. For the vaccinated person, it rises almost at once and reaches a much higher peak.
4. Use the buttons to switch between vaccinated and unvaccinated. The readings below show the lag, the time until the protective level and the peak for the chosen person.
5. Drag the infection-day slider to see how antibody left over from the vaccine fades with time. Drag the memory-boost slider to make memory stronger or weaker, and the protective-level slider to move the green line.
6. Press reset to return to the defaults, then check your numbers against the solved problems below.

The germ that was beaten before it started

Two children are exposed to the same germ in the same week. The first was vaccinated last year. The second never was. The first barely feels ill, because her blood already carries antibodies, and within a day or two of the germ arriving, she is making far more. The second child has to start from nothing. He has no antibody ready, and it takes his body days to find the right B cells and build a force. During that wait, the germ multiplies freely, and he gets sick.

The two children have the same immune system. The difference is memory. A vaccine gives the body a safe, harmless preview of the germ. The body answers it, and keeps a record of how.

This page lets you run that story as a graph. You can see the delay, the size of the response, and how both depend on whether the body has met the antigen before.

Two lines of defence

The body has a fast, general defence and a slow, specific one. Both run at once during an infection.

Words you need first

Immunology has a short vocabulary. Learn these before reading the graph.

The primary response: starting from nothing

When an antigen arrives for the first time, very few B cells fit it. Each B cell carries a receptor for one antigen shape, and only a handful of cells in the whole body match. Those cells must be found, helped by T cells, and told to multiply. Then they must grow into plasma cells and start making antibody. All of that takes time.

In the model, the first meeting has a lag of 5 days with no antibody at all. The level then rises over 8 days to a peak of 60 units on day 13, and falls with a half-life of 12 days. The model uses arbitrary units, so the shape matters more than the number.

The long wait is the danger window. In real infections, the germ multiplies during the lag, and symptoms often appear before the antibody level is high enough to stop it.

The secondary response: memory at work

After the first infection or vaccine, some B cells do not become plasma cells. They become memory cells and stay for a long time. When the same antigen returns, there is already a large army of the right cells, so they multiply and make antibody at once.

In the model, the second response has three changes. The lag shrinks from 5 to 2 days. The rise takes 4 days instead of 8. The peak is 5 times higher, 300 units, and the half-life doubles to 24 days. The three changes stack: earlier, taller and longer.

The memory-boost slider controls just the height of the peak. In real life, how much stronger the second response is depends on the antigen, the vaccine and the person, so the number is a teaching choice, not a fact about any vaccine.

Why memory makes it faster

The reason is a rule called clonal selection. An antigen does not make B cells fit it. It finds the few that already fit and tells them to multiply. All the copies, called a clone, carry the same receptor.

Some of the clone become short-lived plasma cells, which make the antibody and then die. Others become long-lived memory cells. Memory cells wait in the lymph nodes and spleen. If the same antigen comes back, those cells start from a bigger group, so each step comes sooner.

This is also why immunity is specific. Memory for one antigen does nothing for a different one. A new germ with a different surface starts a fresh primary response.

Reading the graph

Look at the antibody graph and ask these questions of it.

• Where is the infection? The dotted vertical line. For the vaccinated person there is a second dotted line at day 0, the vaccine.
• Why does the blue line rise twice? The first bump is the primary response to the vaccine. The second, much taller, is the secondary response to the real infection.
• Why is the blue line not zero when the infection arrives? The vaccine’s antibody is fading but not gone. At the default infection day it has dropped to 9.4 units, below the green protective level of 20.
• What does the green line mean? It stands for the level of antibody needed to stop the germ. Above it the person is protected in the model; below it the germ has room to multiply.
• Which curve is higher and sooner? The vaccinated one, on both counts. That is the whole point of vaccination.

Vaccines and boosters

A vaccine contains something the immune system can recognise, but that cannot cause the full disease. It might be a weakened germ, a killed germ, a harmless piece of the germ’s surface or a set of instructions for making such a piece. Whatever the type, the body treats it as an antigen and starts a primary response.

The antibody from a single dose fades. That is why some vaccines need a booster. A booster is a second meeting. It calls up the memory cells, produces a taller peak and builds a bigger and longer-lived memory.

Notice the cost-benefit: the first dose is slow and small, but it is the one that creates the memory. After that, the body is ready.

What the sliders change

Each slider moves one part of the picture. They do different jobs.

What the model leaves out

This is a teaching model. Real immunity is richer, and you should not use these numbers for any real decision.

• There are several classes of antibody, and the early class differs from the later one. The model merges them into one level.
• T cells, which are vital to a full response, are not drawn on the graph.
• Real lag times, peaks and half-lives depend on the germ, the vaccine, the dose and the person. These are invented round values.
• The model does not simulate the germ’s growth, only a simple clearing rule in the picture on the left.
• Some germs change their surface, so memory for an old version may not fit a new one. The model assumes the same antigen both times.

Try it in the simulation

Spend ten minutes on these experiments. Each is a question an examiner could ask.

• Choose unvaccinated and watch the infection day. How long does the level stay at zero? Compare with the lag reading.
• Switch to vaccinated. What is different about the lag, the peak and the time to the protective level?
• Drag the infection day to its minimum. Is the vaccinated person protected the moment the infection arrives? Why?
• Drag it to its maximum. What has happened to the vaccine antibody by then, and does memory still help?
• Set the memory boost to its smallest and then its largest value. Which readings change and which do not?
• Raise the protective level to its maximum. How does that change the time to protection for each person?
• Switch off the comparison line to study one person at a time.

Solved problems

Each solution states the rule first, then substitutes the numbers. Every figure is computed by the same code that drives the simulation.

Common mistakes

These are the slips that cost marks every year.

• Saying vaccines contain antibodies. Most contain an antigen. The body makes the antibodies.
• Mixing up memory cells and antibodies. Antibodies fade; memory cells persist.
• Writing that the secondary response is only larger. It is also faster, and it lasts longer.
• Thinking one vaccine protects against every germ. Immunity is specific to the antigen.
• Confusing the lag phase with the time to the protective level. One is when antibody starts to rise, the other is when it is enough.
• Saying the immune system “learns” in the first minutes. The first response takes days.
• Forgetting that the protective level is a line on the graph, not a property of the antibody itself.

Where you see it in real life

Childhood vaccination schedules are built on this graph. A first dose creates memory. Later doses, called boosters, are timed to lift the level again before it fades too far.

Blood tests that count antibodies against a germ are read as a snapshot of this curve. A high level soon after exposure suggests a recent response. A low but steady level suggests old memory.

Some infections give lifelong protection after one illness, because memory lasts. Others, where the germ keeps changing its surface, can infect the same person again, because the old memory does not fit the new antigen.

Antivenom and some emergency treatments give ready-made antibodies from outside. That gives protection at once, but only for a short time, because the body has made no memory of its own.

Exam corner

Typical questions, and what a full-mark answer contains.

Revision summary

A first meeting with an antigen gives a primary response: a lag (5 days in the model), a slow rise to a modest peak (60 units) and a decay with a half-life of 12 days. A later meeting gives a secondary response, driven by memory cells: a shorter lag (2 days), a faster rise, a higher peak (300 units) and a longer half-life (24 days). A vaccine causes the first meeting safely. At the default setting, the vaccinated person reaches the protective level 2.5 days after infection and the unvaccinated person 8.1, and the vaccinated peak is 5.1 times higher. Antibodies fade; memory cells last.

Frequently asked questions