reagin

re·agin

noun \rē-ˈā-jən, -gən\

Definition of REAGIN

1

: a substance that is in the blood of individuals with syphilis and is responsible for positive serological reactions for syphilis

2

: an antibody (as IgE in humans) that mediates hypersensitive allergic reactions of rapid onset

—re·agin·ic \ˌrē-ə-ˈjin-ik, -ˈgin-\ adjective