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How vaccines train your immune system

The short answer

A vaccine shows the immune system a harmless piece of a germ. Only the immune cell whose lock fits that piece wakes up, divides and makes antibodies shaped to fit it. Some of those cells stay behind as memory, so when the real germ returns, the body answers faster and can stop it before it spreads.

2:02The human bodyIntermediatevaccinesimmune systemantibodiesmemory cellslymph node

Chapters and transcript

0:00
The first time, it is a race.
A germ enters the body and multiplies while the immune system is still on its way. One immune cell finds it and takes it in.
0:18
It cuts the germ into pieces.
Inside the cell the germ is broken into pieces, and one piece is held up on the cell's surface, where other immune cells can see it.
0:28
It carries the piece to a lymph node.
The cell carries that piece to a lymph node, where many lymphocytes gather, each with its own lock.
0:38
Only the cell that fits wakes.
Each lymphocyte carries one lock. Only the one whose lock fits the piece is activated.
0:50
It divides, again and again.
The activated cell divides, so there are many copies that all carry the same lock.
1:02
Only this shape fits.
The copies make antibodies: Y-shaped proteins that fit this one piece of the germ.
1:14
Most go quiet.
When the threat has passed, most of these cells calm down.
1:26
Long after, they still remember the shape.
A few cells stay behind as memory cells, ready to respond again.
1:38
The same germ.
When the same germ returns, the memory cells wake quickly and antibodies cover it before it can multiply.
1:50
The first time, a race. This time, a wall.
The first meeting is a race. Later, the defence is already in place.
1:56
Learned once. Ready next time.
The body keeps the shape of the germ, so the next meeting is faster.

Sources

Every figure in the film comes from these. The animation is illustrative.