allergen (n.)

"condition caused by hypersensitivity of the immune system to typically harmless substances," 1911, from German Allergie, coined 1906 by Austrian pediatrician Clemens E. von Pirquet (1874-1929) as an abstract noun from Greek allos "other, different, strange" (from PIE root *al- (1) "beyond") + ergon "work, activity" (from PIE root *werg- "to do").

"substance that causes production of an antibody," 1908, from German Antigen, from French antigène (1899), from anti(body) (see antibody) + -gen.

also hypoallergenic, 1950; see hypo- + allergen + -ic.