the.com/allergen
A harmless substance your immune system treats like a home invasion.
means Any molecule that triggers an allergic reaction by tricking the immune system into overreacting to something basically innocent.
from From German Allergen, coined by pediatrician Clemens von Pirquet in 1906 from Greek allos, meaning other, plus ergon, work — literally altered reaction.
Usual suspectsPollen, peanuts, dust mites, shellfish, cat dander.
Tiny troublemakersMost are proteins under a millionth of a meter.
The overreactionBody fights pollen as if it were plague.