How vaccines work: how a shot trains your immune system
Antigens and antibodies: how your immune system knows a germ by its shape
Your body can tell a measles virus from a flu virus, and both from your own cells. It has no eyes in your blood. It feels shapes, with a tiny Y that fits only one of them.
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Let's shrink down and go inside your body, on the day a measles virus gets in.
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The virus is covered in small spikes. The tip of each spike has a shape that belongs to measles. Every germ wears shapes like this, and each kind of germ has its own.
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A flu virus wears other shapes. Your own cells have shapes too, and your body knows them as its own. A shape that lets your body recognise a germ is called an antigen.
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Your body answers with antibodies. An antibody is a tiny Y-shaped protein. The tips of its two arms have a shape too. Both tips fit the same antigen, very closely.
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When the tips fit, the antibody sticks. Many antibodies cover the virus, so it can no longer get into your cells. They also mark it, so other defenders come and destroy it.
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The same antibody does nothing to a flu virus. The shapes don't match. So each germ needs its own antibodies.
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Antibodies come from white blood cells called B cells. Each B cell makes one kind of antibody. It wears some on its surface, and later it can send out many more.
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But there are countless germs, and new ones keep appearing. Your body has never met most of them, and it has never met measles. So how can it have an antibody that fits a germ it has never seen?
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Quick check
Cowpox and smallpox.
Show the answer
Their antigens are so alike that the same antibodies fit both. The antibodies his body made for cowpox could also stick to smallpox.
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Go further
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