How does our immune system distinguish between self and non-self cells?

Our immune system acts like a detective who knows exactly which parts of our body belong to us and which are invaders.

Imagine your body is like a big house full of people, you, your family, and your friends. The immune system is the security guard who checks everyone’s ID before letting them stay in the house. If someone doesn’t have the right ID, the security guard kicks them out or stops them from causing trouble.

Every cell in our body has a special ID tag, like a name badge. These tags are made of proteins called antigens. Most of these antigens look just like ours, they're like family members with familiar names.

But when an invader, like a virus or bacteria, comes into the house, it brings its own ID tag, one that looks different from ours. The immune system notices this difference and knows something is wrong. It then sends out special soldiers, called white blood cells, to fight off the invaders.

If the security guard didn’t notice these differences, he might let a thief stay in the house forever, and your body would get sick!