Clinically significant anti‐ KEL RBC alloantibodies are transferred by breast milk in a murine model

Background and Objectives Fetuses affected by maternal RBC alloantibodies may have prolonged anaemia after birth, leading one to question whether maternal alloantibody transfer may occur outside the placenta. In response to a recent publication describing breast milk transfer of clinically significant amounts of maternal antiplatelet IgA antibodies from mother to nursing infant, we hypothesized that maternal RBC alloantibodies may also be capable of being transferred in breast milk. Materials and Methods The presence and clinical significance of breast milk alloantibody transfer were tested through a series of pregnancy, fostering and transfusion experiments, using a murine model in which transgenic RBC s express the human KEL glycoprotein. Results Maternal anti‐ KEL immunoglobulins, induced through transfusion or pregnancy, were detected in the aqueous phase of breast milk. Further, efficient transfer of maternal anti‐ KEL IgG and IgA to nursing pups was observed in fostering experiments. The breast milk‐acquired alloantibodies were clinically significant in wild‐type pups in a transfusion setting, binding to ‘incompatible’ KEL RBC s and leading to premature clearance from the circulation. Although breast milk‐acquired alloantibodies also bound to the RBC s of transgenic KEL ‐positive fostered pups, no anaemia resulted. Conclusions Taking these murine data in combination with recently published human data of maternal antiplatelet IgA antibodies in breast milk leading to sequelae in some infants, it is theoretically possible that maternal anti‐ RBC IgA alloantibodies may also be transferred in human breast milk and may lead to sequelae in some infants under some circumstances.