Why Does Poison Ivy Itch: Urushiol, T-Cells, and the Botany of the World's Most Annoying Rash

We dive deep into the phytochemistry and biological mechanism behind the poison ivy rash, exploring the invisible oil urushiol and the Type IV hypersensitivity reaction it triggers in the human immune system. Discover the plant's clever botanical relatives—like the cashew—and the surprising reasons why the vine's deep rhizome network makes it notoriously resistant to common herbicides. Essential science and history for curious minds looking beyond "leaves of three."