an antigen derived from one member of a species that can cause the production of antibodies in some other members of the same species
See isoantigen in American Heritage Dictionary 4
noun
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.