re·a·gin 
(rē-ājĭn)
(rē-ājĭn)n.
1. An antibody found in the blood of individuals having a genetic predisposition to allergies.
2. A substance present in the blood of individuals having a positive serological test for syphilis.
[REAG(ENT) + -IN.]
re′a·ginic (rē′ə-jĭnĭk) adj.
re′a·gini·cal·ly adv.