system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are we allergic to things?
gpt
Your immune system has evolved to fight invaders. We have varied immune responses to foreign organisms, and a few different ways of trying to kill them when they get into our bodies: fever, mucus secretion, swelling, are all side effects from our white blood cells trying to fight them off. 

Sometimes our bodies turn on this immune response to things that wouldn't actually kill us, but have been misidentified by our immune cells as being dangerous. 

So when we get a runny nose from pollen, it's our bodies trying to kill the pollen because it thinks the pollen is trying to kill us.