i·so·an·ti·gen 
(ī′sō-ăntĭ-jən)
(ī′sō-ăntĭ-jən)n.
A protein or other substance, such as histocompatibility or red blood cell antigens, that is present in only some members of a species and therefore able to stimulate isoantibody production in other members of the same species who lack it. Also called alloantigen.
i′so·an′ti·genic (-jĕnĭk) adj.
i′so·an′ti·ge·nici·ty (-jə-nĭsĭ-tē) n.