Scientists in Norway have uncovered a surprising trick the placenta uses to protect the growing baby, and it could pave the way for a new generation of medicines that are safe for pregnant women to take. According to the study published in Science Immunology, the placenta is far pickier about what it lets through than researchers had realised.
A Growing Problem in Modern Medicine
Biologic medicines (drugs made from proteins rather than simple chemicals) have transformed the treatment of serious conditions like cancer, autoimmune diseases, migraine, and inflammatory disorders. Many of these medicines are based on antibodies called IgG.
But these drugs are not always suitable for pregnant women. IgG antibodies are actively carried across the placenta to the developing baby, which is helpful when the mother is passing on protective antibodies against infections, but problematic when those antibodies are actually drugs. Many women of reproductive age take these medicines, yet doctors have very little solid evidence about whether they are safe to use during pregnancy.
The Placenta’s Hidden Selectivity
For decades, scientists have known that a protein called the neonatal Fc receptor (FcRn) binds to two of the most abundant proteins in the bloodstream: IgG antibodies and albumin. What they couldn’t fully explain was why only IgG ends up reaching the baby, while albumin is largely left behind.
Now, researchers led by Professor Jan Terje Andersen at the University of Oslo and Oslo University Hospital have shown that the placenta actively distinguishes between the two.
“This is first and foremost a discovery of how the placenta works. For decades, we have known that FcRn binds both IgG and albumin, yet only IgG reaches the fetus by an FcRn-dependent mechanism. We show that the placenta has a remarkable ability to distinguish between these two soluble proteins, revealing a level of biological selectivity that was previously unrecognized,” Andersen said.
In other parts of the body, FcRn does something helpful for both proteins: it protects them from being broken down, which is why IgG and albumin both stay in the blood for a long time. But in the placenta, FcRn seems to work like a bouncer, letting IgG through the door while quietly turning albumin away.
How They Worked It Out
To crack the mystery, the team combined studies in specially engineered mice with experiments on human placentas donated straight after childbirth. Using a technique that pumps fluids through the placenta outside the body, they could watch in real time which proteins moved across and which didn’t. Across all their tests, the result was the same: IgG went through, albumin didn’t.
From Discovery to Medicine Design
This is where the discovery gets exciting. If albumin can’t cross the placenta, then attaching albumin to a therapeutic antibody might act like a “stay out” sign, keeping the drug from reaching the baby, while still letting it do its job in the mother.
The researchers tested exactly that. When they fused albumin onto antibody-based drugs, the resulting medicines still lasted a long time in the bloodstream (thanks to FcRn protection elsewhere in the body) but were much less able to cross into the fetus. Even better results came when they used a specially engineered version of albumin called QMP, which binds more precisely to the human FcRn.
They then put the approach to the test in a mouse model of a serious pregnancy complication called fetal and neonatal alloimmune thrombocytopenia (FNAIT), where the mother’s antibodies attack her baby’s platelets. The engineered antibodies dramatically reduced fetal exposure and the harm that came with it.
A New Way of Thinking
For pregnant women needing treatment for chronic diseases, decisions currently come down to weighing benefits against unknown risks. This discovery offers something different.
“Rather than asking whether existing biologic medicines are safe to use during pregnancy, our findings show that we can now design them differently,” Andersen said. “The placenta selectively transfers protective IgG antibodies while preventing albumin from crossing. By understanding this, we now have the opportunity to develop a new generation of biologic medicines that combine long-lasting efficacy with improved safety during pregnancy. This study shows how fundamental discoveries in biology can directly inspire design of better medicines.”
Nilsen J, Sand K, Al-Khabbaz J, van Ligtenberg L, et al. Fusion of IgG antibodies to albumin inhibits transport across the placenta. Science Immunology (2026). https://doi.org/10.1126/sciimmunol.aee5151