Immunisation with inactive pathogen

One person, from the vaccine to the real infection

Step through in order. Ask at each stage: what's different the second time round?

1. 1The vaccine goes inA vaccine carries a dead or inactive form of the pathogen, or sometimes just parts of it (its antigens). It's usually an injection. Some vaccines are inhaled, or given as drops on the tongue.+
2. 2Your body responds to the antigenYour body reacts just as if you'd been infected with the real pathogen. White blood cells get to work: lymphocytes make antibodies complementary to the antigen, and phagocytes deal with the dead or inactive pathogen.+
3. 3Memory cells are left behindMemory cells are produced to defend against that particular antigen. That's long-lasting immunity to that pathogen, even though you've only met its antigen and never the real thing.+
4. 4The real pathogen arrives laterSome time afterwards, you're infected with the pathogen itself. This time your body isn't starting from nothing: the memory cells for that antigen are already there.+
5. 5A rapid, large responseThe memory cells reproduce rapidly and produce lots of antibodies. So the pathogen is usually cleared before it can cause symptoms, and you're unlikely to feel ill.+

Biology · Later infection

The real pathogen arrives. Which route does it take?

A vaccinated person is later infected. Pick the condition and follow that route to its end.

The pathogen that arrives

On A vaccinated person is later infected. 2 branches to choose from.

One condition decides it: does the pathogen still carry the antigen the vaccine was built around?

What you need to know

• A vaccine can give you immunity to a pathogen without you ever having the disease.
• Immunity means you show no signs or symptoms of the disease when you're infected.
• Have a goAna and Ben are both infected with the same pathogen. Ana shows no signs or symptoms. Ben coughs for a week. Who's showing immunity?

  Ana.

  Immunity is about showing no signs or symptoms when infected, so Ben's week of coughing means he isn't showing it.

• A vaccine contains a dead or inactive form of the pathogen, or sometimes just parts of it (its antigens).
• Vaccines are usually given by injection, though some are inhaled or given as drops on the tongue.
• Your body responds to the antigen, just as if you'd been infected with the real pathogen.
• Lymphocytes make antibodies that are complementary to the pathogen's antigen.
• Phagocytes engulf the dead or inactive pathogen by changing the shape of their cytoplasm, then digest it with enzymes.
• Have a goQuick sort: 'releases enzymes' and 'makes antibodies'. Which belongs to a phagocyte, and which to a lymphocyte?

  Phagocyte: releases enzymes (after engulfing). Lymphocyte: makes antibodies.

  Both are white blood cells, so they blur together, but engulf-and-digest is the phagocyte's job and antibody-making is the lymphocyte's.

• Memory cells are produced to defend against that particular antigen, even though you've only met the antigen, never the real pathogen.
• Have a goPredict: a vaccine has only ever put the antigen into you, never the real pathogen. Will you have memory cells for that pathogen: yes or no?

  Yes. Memory cells are produced against that antigen.

  The real pathogen isn't needed. Exposure to the antigen alone is enough to produce the memory cells, which is why a vaccine can protect you at all.

• If you're infected later, memory cells reproduce rapidly and make lots of antibodies, so the pathogen is usually cleared before symptoms.
• Pathogens such as influenza mutate, so a new strain's different antigens may need a new vaccine.

The big picture

A vaccine contains a dead or inactive form of a pathogen, or just parts of it (its antigens). Your body responds to the antigen, lymphocytes make complementary antibodies, and memory cells are produced. If the real pathogen arrives later, the memory cells reproduce rapidly and make lots of antibodies, so the pathogen is usually cleared before it causes symptoms. Pathogens that mutate, like influenza, need new vaccines.

⚠ Watch out

Writing that a vaccine 'gives you antibodies' or 'gives you the disease'. It carries a dead or inactive pathogen, or its antigens, and your own lymphocytes make the antibodies. Also, don't write 'you'll never be ill'. It's usually cleared before symptoms, but not always for every type of disease.

Memory hook

Vaccine = practice run. The antigen goes in, memory cells stay behind, and next time the response is fast and big.

Check yourself

Without looking back: a vaccine puts only the antigen into someone. Which cells make the antibodies? And what changes when the real pathogen arrives later?