<genetics, immunology> Individuals of a species differ in alleles (are allogeneic) and the antigenic differences will cause an immune response to allografts. The antigens concerned are often of the histocompatibility complex and are referred to as alloantigens. This entry appears with permission from the Dictionary of Cell and Molecular Biolo [..]

This is the chemical that induces the immune response in the woman during pregnancy. The alloantigen is provided by the father to the baby’s placenta.