Why do people develop allergic reactions to harmless substances?

People get allergic reactions because their body thinks harmless things are dangerous.

Imagine you're playing with your favorite toy every day, it's normal, friendly, and nothing bad happens. But one day, someone gives you a new toy that looks just like the old one. Your body might think this new toy is different, even though it’s not. It gets scared and sends out tiny soldiers to fight it off. This makes your nose run, your eyes water, or your skin itch, all signs of an allergic reaction.

Your body has a special group of helpers called white blood cells that protect you from germs and other dangers. Sometimes they get confused and think something harmless, like pollen, chocolate, or even a pet, is actually a threat. They shout "Alert!" and cause a big fuss in your body.

It's like when you mistake your brother for the mailman and give him a hug instead of opening the door. Your body makes a mistake, but it’s not trying to be mean, it just wants to keep you safe!