See also: Reagin English Noun reagin (countable and uncountable, plural reagins) (biochemistry) An antibody. (archaic) A substance elaborated by the spirochetes of syphilis and yaws, having the property of uniting with lipoids, and of fixing complement. Derived terms antireagin rapid plasma reagin reaginic References American Illustrated Medical Dictionary (1919). A Practical Medical Dictionary (Stedman, 1922). Anagrams gainer, Gearin, anigre, Regina, regian, regain, Angier, Arenig, inrage, raigne, regina, Negari, earing, graine, in gear, Aigner, GainerWikiwand - on Seamless Wikipedia browsing. On steroids.