THE UNBORN Child was in a tough situation. Its mom’s doctors, at a UK medical clinic, realized something was off about the fetus’ blood, so they chose to play out an emergency C-section weeks before the child was expected. Yet, notwithstanding this, and ensuing blood bondings, the child experienced a mind drain (hemorrhage) with crushing outcomes. It unfortunately died.

It wasn’t clear why the hemorrhage had occurred. Yet, there was a sign in the mother’s blood, where specialists had seen a few weird antibodies. Some time later, as the surgeons attempted to figure out more about them, a sample of the mother’s blood showed up at a lab in Bristol run by specialists who study blood gatherings.

They made a frightening discovery: The lady’s blood was of an ultrarare type, which might have made her child’s blood incongruent with her own. It’s conceivable that this prompted her immune system to deliver antibodies against her child’s blood — antibodies that then, at that point, crossed the placenta and hurt her baby, at last prompting its death. It might appear to be improbable that something like this could occur, yet quite a while back, before specialists had a clear understanding of blood groups, it was considerably more common.

Through studying the mother’s blood sample, along with a number of others, scientists were able to unpick exactly what made her blood different, and in the process confirmed a new set of blood grouping—the “Er” system, the 44th to be described.

You’re probably familiar with the four main blood types—A, B, O, and AB. But this isn’t the only blood classification system. There are many ways of grouping red blood cells based on differences in the sugars or proteins that coat their surface, known as antigens. The grouping systems run concurrently, so your blood can be classified in each—it might, for instance, be type O in the ABO system, positive (rather than negative) under the Rhesus system, and so on.

Thanks to differences in antigens, if someone receives incompatible blood from a donor, for example, the recipient’s immune system may detect those antigens as foreign and react against them. This can be highly dangerous, and is why donated blood needs to be a suitable match if someone is having a transfusion.

On average, one new blood classification system has been described by researchers each year during the past decade. These newer systems tend to involve blood types that are mind-bogglingly rare but, for those touched by them, just knowing that they have such blood could be lifesaving. This is the story of how scientists unraveled the mystery of the latest blood system—and why it matters.

It was back in 1982 that researchers first described an unusual antibody in a blood sample that hinted that this mystery blood type was out there. The scientists couldn’t go much further than that at the time, but they knew that the antibody was a clue pointing toward some unknown molecule or structure that prompted the person’s immune system to generate it.

In the years that followed, more people with these unusual antibodies turned up—but only now and again. Generally, these people surfaced thanks to blood tests containing the mysterious and rare antibodies. Eventually, Nicole Thornton and her colleagues at NHS Blood and Transplant in the United Kingdom decided to look into what might be behind the antibodies. “We work on rare cases,” she says. “It starts off with a patient with a problem that we’re trying to resolve.”