Your blood type is most commonly defined by two systems: ABO (blood types A, B, AB, and O) and Rh (+ or -). But these aren’t the only systems; there are more than thirty others.
A hundred and thirty years ago, no-one had any idea that humans had different types of blood. In 1900, an Austrian doctor named Karl Landsteiner mixed the blood of different people in test tubes, and noticed something strange. Sometimes, the blood would clump together – indicating a kind of immune reaction to the foreign blood. But sometimes it would not. He had discovered blood types.
Landsteiner won a Nobel Prize for his discovery. Briefly, all red blood cells are covered in a wide range of antigens. These act as little markers for the body’s immune system. If the cell has a “native” antigen, the immune system knows not to attack it. If the cell has a “foreign” antigen, antibodies in the immune system destroy it.
The antigens that Landsteiner discovered fall into the ABO system, corresponding to the blood types A, B, AB, and O. A blood has A antigens, B blood has B antigens, AB blood has both, and O has neither. We inherit our blood type from our biological parents.
However! The A and B antigens aren’t the only ones. Thirty-seven years after Landsteiner’s discovery, he and a colleague found another important antigen on human blood cells: D. This too can cause antibody reactions, is also inherited, but is not genetically connected to the ABO type. It’s not just a different blood type, it’s a different blood type system: Rh positive (if you have the antigen), Rh negative (if you don’t).
So, these are the two blood type systems everyone knows about: ABO, which gives you the letter(s) of your blood type, and Rh, which is positive or negative. But here’s the thing that blew my mind: there are dozens more. Red blood cells are replete with antigens produced by a whole range of genes.
There’s the Lutheran system: most of the world’s population is Lu(a-b+), but there are tiny pockets of Lu(a+b+), Lu(a+b-), and Lu(a-b-). There’s the Diego system: everyone’s blood is either Diegob or Diegoa; the latter type is only present in Native American and East Asian populations. There’s the Duffy system, important because one variety of malaria attacks Duffy antigens. If you lack that antigen, you’re effectively immune to that type of malaria. And there are dozens more.
If we want to accurately and completely describe our blood type we have to move well beyond A, B, AB, O, positive, and negative. This is important because blood transfusions fail when incompatible blood types are mixed. The process known as cross-matching tests for blood compatibility by mixing the donor and recipient blood. Just as in Landsteiner’s original discovery, if there are any incompatible antigens from any of the different blood type systems that mixture will clump – and a potential medical mishap can be avoided.