An allergic reaction can develop if a person merely enters a room in which they inhaled pollen before, even if there is no longer any pollen in the room.
Here’s why: The first step occurs when the person inhales pollen inside a certain room. In the second step, the brain associates the allergy trigger, or pollen, with the spatial environment. The next time the person enters the same room, the body calls up the learned allergic reaction, even if the pollen is not there any more.

Learning while sleeping

But the association does not always develop. The brain only associates the allergen with the room if the person sleeps after inhaling the pollen. The research team believes that a specific region of the brain called the hippocampus, which depends on sleep in order to work, is involved in this learning process.

They were surprised at how quickly the body learns the maladaptive reaction, says study director Luciana Besedovsky of the University of Tübingen. “During the experiment, the allergen only needed to be administered once for the allergic reaction to be associated with the environment.”

The study also partly explained why allergy symptoms often occur as a kind of placebo reaction in the absence of the allergen.

Source

Rijkhoek KG. Being in the wrong place can set off an allergic reaction. Press release from Eberhardt Karls University of Tübingen, 5 May 2020.

Original publication

Besedovsky L et al. Human sleep consolidates allergic responses conditioned to the environmental context of an allergen exposure. Proceedings of the National Academy of Sciences, 4 May 2020.