Pull out your blood donor card or your health insurance card: next to the letter that designates your blood group (A, B, AB or O), there’s this small, discreet sign, a “+” or a “–.” We read it without ever really noticing it. Yet this seemingly innocent symbol tells a story more than eighty years old, born in a laboratory thanks to a small monkey from Asia. A discovery that transformed medicine and saved countless newborns. Explanations.
When a rhesus monkey unveiled the secret of our blood
We are in 1940. Two researchers, Karl Landsteiner and Alexander Wiener, are studying the mysteries of human blood. To understand certain phenomena that remained unexplained, they use the blood of a macaque species well known in laboratories: the Macaca rhesus, or rhesus monkey. By injecting red blood cells from this primate into laboratory animals, they observe a particular immune reaction, revealing the presence of a factor previously unknown on the surface of blood cells.
This factor, they naturally name the “rhesus factor”, in homage to the small primate that brought it to light. This name, chosen almost out of convenience, would nevertheless cross decades and appear on billions of blood group cards around the world. A nice way of saying that the famous “+” or “–” you carry comes, in a sense, from a monkey.
This famous “+” and “–”: what your donor card really reveals
concretely, what does this little sign mean? The rhesus factor, today known to scientists by the more precise name of antigen D (or RH1), is a kind of microscopic marker that may be present or absent on the surface of your red blood cells. Imagine a tiny label stuck on each of your blood cells.
If you carry this label, you are Rh-positive. If you don’t have it, you are Rh-negative. It’s as simple as that. And this is where the announced figure makes sense: about 85% of humans possess this antigen D and are therefore “+,” often without ever reflecting on what this symbol actually means. A vast majority of us thus carry, unknowingly, the imprint of an experiment conducted with a macaque more than eight decades ago.
The invisible battle between a mother and her baby
This discovery is far from a mere laboratory curiosity. It helped illuminate a medical tragedy that had remained unexplained until then: hemolytic disease of the newborn. The mechanism is as fascinating as it is formidable. When a Rh-negative mother carries a Rh-positive baby, her body may treat the baby’s antigen D as an intruder. The maternal immune system then begins producing defenses against her child’s red blood cells.
This true invisible battle, this blood-type incompatibility between mother and child, could lead to severe complications, sometimes fatal. By describing this mechanism of isoimmunization, Landsteiner and Wiener offered medicine a crucial key. With this understanding, it became possible to monitor at-risk pregnancies and prevent this blood conflict. The rhesus factor thus helped save the lives of thousands of babies.
An animal heritage that still runs in our veins
Karl Landsteiner was no stranger to breakthroughs. A few years earlier, he had already been awarded the Nobel Prize for the discovery of the ABO system, the very one that determines whether you are A, B, AB, or O. This work earned him the nickname the father of transfusion medicine, as his research made blood transfusions safer.
The story also holds a final twist. Shortly after the discovery, it became apparent that the experiment had actually highlighted two distinct systems: the antigen D on one side, and another system, today called the LW system, on the other. A nuance that reminds us how much human biology still holds surprises, even in what we believed to be fully understood.
Next time you look at your blood type card, this little “+” or “–” will no longer seem so trivial. Behind it lies a macaque, two visionary researchers, and an advancement that continues, today, to protect mothers and their children. So, one question remains: how many other secrets do our red blood cells still keep hidden?
