How does the human immune system distinguish self from non-self?

The human immune system acts like a detective team that knows who belongs in the body and who is trying to sneak in.

Imagine your body is like a castle, and every person living inside it has a special key that fits the locks on the gates. The immune system checks each visitor’s key, if it matches one of the keys from someone already inside, they’re allowed in. If not, the immune system knows something is up and sends out its soldiers to stop them.

The immune system has special cells called white blood cells, which are like the guards at the castle gates. They look at every visitor’s key closely, if it doesn’t match any of the ones they know, they shout “Intruder!” and send more soldiers to deal with them.

Sometimes, a visitor can pretend to be someone from inside the castle, that's when things get tricky. But the immune system keeps checking and double-checking until it knows for sure who is self (someone from the body) and who is non-self (a stranger trying to take over).