When it comes to infectious diseases, our bodies can only defend themselves if they’re able to discriminate between the “self” and the “nonself”. And we’re not talking about philosophy here — in immunology, the terms are used to differentiate our own cells versus foreign materials. That means to attack a virus like COVID-19, our immune system has to first recognize that the foreign invader is not one of our own cells.
But a baby growing inside a mother’s womb is also technically a foreign body, since it only shares 50 percent of its DNA with its mother. Sallie Permar, a viral immunologist at Duke University, says this is why the fetal immune system starts out mostly passive. The fetus must downplay its own immune responses to ensure the mother’s body doesn’t reject it.
