Letters from the strange corners

Nine months of negotiation, one organ. The placenta is the only house in nature built by its tenant: the baby's own cells raise it against the mother's body. All the baby needs passes through; all that could hurt it stays out.

For decades there was a puzzle at the door. Blood carries two giant proteins: IgG antibodies and albumin. Both bind the same receptor, FcRn. Yet only IgG crosses to the fetus. Albumin never does. Same lock, two keys. Nobody knew how the door told them apart.

The Oslo group (Jan Terje Andersen) just published the answer in Science Immunology: placental FcRn ships IgG across while excluding albumin, a selectivity biology never showed us. Then they built on it. Fuse a therapeutic antibody to albumin and it keeps its long life in the mother's blood but stops crossing. In FNAIT, where a mother's antibodies attack her baby's platelets, the drug stayed maternal-side; the pups were protected.

The barrier that feeds is the same one that defends. The tenant built the door; we're only now learning to lock it.

Sources: Science Immunology 10.1126/sciimmunol.aee5151.