Besides their ability to cause cell-to-cell fusion, they can also suppress the antiviral immune response of a host. This has obvious value to an invading virus. It has other value, less obvious, to a mammal for use in its placenta. Both the fetus and the placenta of an individual mammal carry a different genome from the mother’s. Half their DNA comes from the father. If the mother’s immune system were entirely on alert, her white blood cells might attack the fetus and reject it. Part of the role of the placenta, a uniquely adaptive organ among placental mammals, is to keep peace between the
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