How does your immune system know which cells to attack?

Your immune system acts like a detective who knows exactly which bad guys to catch.

Imagine you're playing hide and seek at school, and there are two kids hiding, one is your friend, and the other is the teacher. You know to run after the teacher because you can tell them apart. Your immune system does something similar when it sees all the cells in your body. It knows which ones are friendly (like your own cells) and which ones are unfriendly (like germs or viruses that have sneaked in).

Every cell in your body has a kind of special password on its surface, like a badge. Your immune system's detectives, called white blood cells, check these passwords.

• If the password is familiar, they know it’s one of your own cells and let them go.
• If the password is new or changed (like when a germ takes over your cell), the detectives know something is wrong, and they attack!

It's like having a fingerprint scanner at the door. Only people with the right fingerprint can come in, and everyone else gets locked out, or caught by the detective!