Thing That Causes a Reaction, Like Pollen, Gluten

This page is the CodyCross answer for the clue below from House of Horrors, Group 1115, Puzzle 1 (No Electricity).

Question is: Thing That Causes a Reaction, Like Pollen, Gluten

and the answer is: ALLERGEN

Word Meaning:

ALLERGEN — (medicine) A substance which causes an allergic reaction.

Definitions from Wiktionary.