Thanks to Brilliant for supporting this episode of SciShow.

Go to Brilliant.org/SciShow to learn more about their course The Chemical Reaction. [♪♪♪Intro♪♪♪]. You were born with a blood type.

And whether it's AB positive, O negative, or anything in-between, that's almost definitely the blood type you'll have your entire life. ...almost definitely. In some cases, your blood type can change. And one reason it happens has to do with, of all things, cells inside your bones.

Usually, when we're talking about blood types, we use the A-B-O and Rh systems. This refers to different antigens, or markers, on the surface of your red blood cells. Type A blood has the A antigen, type B has the B antigen, type AB has both, and type O has neither.

Meanwhile, the positive or negative sign usually attached to blood types refers to a protein called Rh factor. Positive means you have it, negative means you don't. Whatever you have, though, blood type is genetically determined.

What you're born with is generally what you get. And what type you have can be good to know. After all, if our bodies recognize blood that isn't compatible with our blood type, like during a transfusion, it will launch a potentially-deadly response against it.

So if there's anything that can change blood type, that's important to understand. So far, we know of one big way that can happen: hematopoietic stem cell transplants. These cells, or HSCs for short, are found throughout your body, mostly in your bones.

And since they can be damaged or destroyed by cancer and other diseases, occasionally, someone will need new ones. Those new cells can come from elsewhere in someone's body, but they can also come from a donor. And when they do, that donation can change a patient's blood type.

Because here's the key: When HSCs settle down in your bones, they produce more specialized cells, including red blood cells. So if you get an HSC transplant from someone with genes for a different blood type, you're going to churn out different blood. Overall, these procedures might not sound familiar, but there's likely a kind of HSC transplant you've heard of: Bone marrow transplants.

See, despite the name, these transplants aren't about completely replacing the tissue inside certain bones. They're specifically about giving someone new hematopoietic stem cells — for instance, to replace cancerous ones. Now, I mentioned earlier that the body can attack incompatible blood cells.

So it kind of seems like an HSC donor should always have the same blood type as their recipient. But there's an exception here. Unlike with solid organ transplants, doctors aren't just looking at A-B-O blood type in this case.

Instead, they're making sure people have the same human leukocyte antigens, or HLA type. Like the A-B antigens, your body uses these markers to tell whether certain cells belong to you. But when it comes to HSC transplants, these antigens are actually more important than the ones on blood cells.

That's because the stem cells that become our red blood cells don't have A-B antigens yet, but they do have HLAs. So if there's a mismatch, the transplant might see some complications. And in some cases, it's easier to deal with a changing blood type than not having a transplant at all.

Really, this is a reminder that our bodies are amazing, complicated systems — and before we go messing with them, it's helpful to know how everything works. Because even understanding something that seems as basic as blood type can have life-saving implications. If you liked this episode, you might also like Brilliant's course The Chemical Reaction.

It's based around puzzles and patterns to help you understand how molecules act and by the end, you'll know enough to make predictions about chemical systems. Which is pretty impressive. Like all of Brilliant's other courses, this one is hands-on and interactive, and it's available offline through their iOS and Android apps.

If you want to check it out, you can go to Brilliant.org/SciShow. And the first 200 people to sign up there will get 20% off their annual Premium subscription. [♪♪♪Outro♪♪♪].