How does my body spot a germ that sneaks in?

Think about the last time you scraped your knee and felt it sting. That scrape opens a little door in your skin. Germs can slip right through it.

A germ is far too small to see, and it makes no sound at all. It does not wave or knock. So how does your body spot a germ that sneaks in?

Your body has helpers called white blood cells, which are little guards that ride along in your blood. White blood cells travel to every part of you, day and night. They keep moving so they can find anything that does not belong.

Each guard checks things by touching them. Its outside has lots of little feelers, and it bumps gently against everything it meets. With those feelers, it can feel the shapes on the outside of each thing.

Your own body cells all wear the same special shapes on their outside, like a team in matching shirts. When a white blood cell feels those shapes, it knows the cell belongs to you and leaves it alone. Now a white blood cell bumps into the germ from your scraped knee. Do you think the germ wears those matching shapes too?

No, the germ does not wear them. A germ is a different living thing, so it carries strange shapes your body never makes.

Those strange shapes press against the feelers in a brand new way. That new feeling tells the white blood cell a stranger is here. Just like that, your body has spotted the germ, even though nobody can see it.

White blood cells. Your white blood cells feel the outside of everything they bump into, and when they find shapes your own cells never wear, they know a germ has sneaked in.

Sleep well tonight, knowing your little guards are keeping watch inside you.