'allergen' originates from the Greek word 'allergia,' where 'allos' meant 'other' and 'ergon' meant 'work.'

'allergia' transformed into the German word 'Allergen,' and eventually became the modern English word 'allergen.'

Initially, it meant 'a reaction to a foreign substance,' but over time it evolved into its current meaning of 'a substance causing an allergic reaction.'