Allergen [al-ler-jen] What it is: A substance that causes an allergic reaction in the body. This reaction can be topical or systemic. VIDEOS RELATED TO THIS TERM LEARN MORE TERMSENAMELDIP NAILSOVEREXPOSUREBASE COATPH VALUECARPAL TUNNEL SYNDROMEBRITTLENESSUV GELHARD GELSCOLOR WHEELROUND NAILANALOGOUS COLORSEXTENSIONCOLORED ACRYLICLIGHT-CURED GELSLINER BRUSHORGANICALUMINUM OXIDEINSURANCEACRYGEL PrevNext