How does our immune system distinguish between self and foreign invaders?

Our immune system acts like a detective that knows who belongs and who doesn’t.

Imagine you have a toy box full of your favorite toys, these are your cells. One day, some new toys show up, these are foreign invaders, like germs or viruses. The detective (your immune system) checks each new toy to see if it's one of yours or something different.

Your cells have special labels on them, think of them like a name tag. These labels are made of things called proteins, and they tell the immune system, “I belong here!”

When the detective sees a new toy (a foreign invader), it checks if that toy has one of your name tags. If not, the detective knows it's not from your group, it’s an invader! Then the detective calls in help to send that invader back where it came from.

Sometimes the detective gets confused and mistakes a friend for an enemy, that’s when you get sick or have allergies. But most of the time, everything goes smoothly, and you don’t even notice the detective working hard to keep your toy box safe!