isoantigen

from The American Heritage® Dictionary of the English Language, 5th Edition.

• 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun an immunologically active material (especially a protein or polysaccharide) that is present only in some individuals of a particular species; responsable for the formation of antibodies