How does the immune system differentiate between self and non-self?

The immune system acts like a detective who knows exactly what belongs to your body and what doesn’t.

Imagine you have a toy box full of toys that are yours, these are the cells in your body. Now, when something new comes into your toy box, like a toy from a friend, it's non-self, and the detective (your immune system) checks if it’s friendly or not.

Your cells have special labels on them called proteins, think of them like name tags. These proteins tell the immune system, “I belong to you!”

If something doesn’t have one of these labels, or has a label that says “I don’t belong to you,” the immune system knows it's time to act, just like when you see a toy you don’t recognize and decide to check if it’s yours or not.

If the detective finds something harmful, like a virus or bacteria, they send in special soldiers (like white blood cells) to fight it off. These soldiers know how to capture or destroy anything that doesn’t belong, keeping you healthy and happy.