View full lesson: http://ed.ted.com/lessons/why-do-some-people-have-seasonal-allergies-eleanor-nelsen

Ah, spring. Grass growing, flowers blooming, trees budding. For those with allergies, though, this explosion of new life probably inspires more dread than joy. So what’s behind this annual onslaught of mucus? Eleanor Nelsen explains what happens when your immune system goes rogue.

Lesson by Eleanor Nelson, animation by TED-Ed.