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The immune system's job is to prevent or limit infection. Its cells all start in the bone marrow, and organs such as the thymus, lymph nodes and spleen help it work.

The first line of defence is barriers: skin resists entry, sticky mucus traps germs, tears and saliva contain an enzyme that breaks down bacteria, and stomach acid kills many germs we swallow.

If germs get in, the innate response acts quickly. White blood cells such as neutrophils and macrophages engulf and digest germs. The area may become red, swollen, warm and painful: this is inflammation.

Experiment: How far do germs travel?

1. One pupil (the 'germ carrier') has hand cream and glitter on their hands. Nobody else does.
2. The carrier shakes hands with pupil 2, who then shakes hands with pupil 3, and so on down a line of six pupils. One pupil also passes the ball around.
3. Use magnifying glasses to score each person's hands and each object: 0 = no glitter, 1 = a little, 2 = some, 3 = lots.
4. Record the scores in a table. Plot score against position in the line.
5. Everyone washes hands with soap for 20 seconds. Score the hands again.

Person or object	Glitter score (0–3)	Score after washing (0–3)
Germ carrier	Predict: will pupil 6 have any glitter at all?
Pupil 2	Predict: will pupil 6 have any glitter at all?
Pupil 3	Predict: will pupil 6 have any glitter at all?
Pupil 4	Predict: will pupil 6 have any glitter at all?
Pupil 5	Predict: will pupil 6 have any glitter at all?
Pupil 6	Predict: will pupil 6 have any glitter at all?
Ball	Predict: will pupil 6 have any glitter at all?

Questions

1. Name three barriers that stop germs getting into the body.

2. Explain the difference between the fast (innate) and targeted (adaptive) responses.

3. What did your experiment show about how germs spread?

4. Why do we feel ill for longer the first time we meet a germ than the second time?

Sources for every fact on this sheet: echoesof.science/school/immune-system/sources/

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