A long-term competent chimeric immune system in a dizygotic dichorionic twin.

We present here a rare case that involved the long-term coexistence of 2 mature, functional, and equilibrated immune systems in a single child after fetofetal transfusion between dizygotic twins. A dichorionic diamniotic pregnancy complicated by twin anemia-polycythemia sequence resulted in the demise of 1 twin. The detection of abnormal vessels on the dichorionic plate strongly suggested the existence of functional vascular anastomoses leading to blood chimerism in the survivor. Genetic, phenotypic, and immunologic analyses at 2 years revealed chimeric lymphoid and myeloid cells in the surviving twin, although no tissue mosaicism was detected, which indicates that early transfusion led to mutual immune tolerance.