system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come when some people get in contact with poison Ivy, oak, or sumac; they break out in blisters. What is happening for this reaction to occur?
gpt
I type this with a hand covered in poison ivy =( 

It's caused by an oil called "Urushiol". Yes, Poison Ivy, Sumac, and Oak all have the same stuff that makes the rash, although it may be different amounts/concentrations.

In 5 year old terms, what happens is this oil gets in contact with your skin, and over the course of 1-3 days, it causes a reaction with your skin cells. It bonds with the proteins in your skin cells, which makes your immune system react as if it's an invader since it can no longer identify it as skin. In an effort to get rid of this foreign substance, your body forms the itchy/blistery/flaky "dermatitis" that we know so well. 

TL;DR Urushiol oil messes with the proteins of your skin cells, tricking your body into thinking it's not skin. Your immune system tries to get rid of it, resulting in the itchy/blistery/flaky skin.