Olivia: You already know that your blood type is important. If you’ve lost a lot of blood, getting a transfusion of the wrong stuff can be deadly. But did you know that whether you’re A, B, AB, or O can also put you at higher risk for things like malaria, cholera, and heart disease?

The blood type you have is the result of a specific kind of antigen -- in this case, a type of sugar -- on the surface of your red blood cells. If you have the A antigen, you’re type A; if you have the B antigen, you’re B. If you have both, you’re AB, and if you have neither, you’re O.

But here’s the thing: There’s more to your blood’s alphabet soup than just those three letters. You also have another kind of antigen -- one you’ve probably never heard of: The H antigen. The A and B that you always hear about are really extra sugars that get added to antigen H.

And what’s more, all of these sugars aren’t just on your red blood cells. They also appear in your guts, and in other compounds that are swimming around in your blood, where they interact with pathogens and toxins and even parts of your own immune system, to make you either more or less vulnerable to certain infectious diseases.

For example, it turns out that having type O blood can help you, if you contract malaria. One of the big dangers of malaria is when your red blood cells begin to clump together, forming characteristic flower-shaped patterns known as rosettes. They form when an infected red blood cell sticks to uninfected red blood cells -- a process that’s helped along by A and B antigens. As a result, people with A, B or AB blood tend to develop more and bigger rosettes if they get malaria.

These cell clusters can get lodged in tiny blood vessels -- often in your brain -- and block blood flow. Which is bad enough, but when rosettes get tucked away like this, it also prevents the infected cells from being cleaned up by your body’s natural defenses.

All of this means that people with A or B or AB blood are at higher risk for a severe case of malaria than people with type O. But, type O has its downsides, too. You may fare better with malaria if you’re an O, but you’ll probably do worse against certain strains of the bacteria that cause cholera.

During an outbreak of cholera in Peru in the early ‘90s, people with type O blood were 8 times more likely to be hospitalized. And it turns out that type O blood is least common in places like the Ganges River Delta, where cholera has been making people sick for centuries.

While scientists still don’t fully understand what’s going on here, one idea is that having A or B antigens might help prevent the cholera toxin from binding as firmly to some of your cells. But this protection doesn’t take place in your blood. Instead, it’s the result of antigens on the cells that line your intestines. That’s where the cholera toxin does its work, making your cells pump out water and electrolytes, and causing the diarrhea that makes cholera such a fast killer.

For people who have A’s and B’s on these cells, the cholera toxin can still bind to them. But it binds even more strongly to the H antigen. And since H is the antigen that Type O people have, O’s are at greater risk for a more severe case of cholera.

Finally, the antigens that determine your blood type can also affect your risk for heart disease. Here, it is the antigens in your blood that call the shots. But not the ones on your red blood cells.

Instead, the key is the antigens on something called your von Willebrand factor. It sounds like the name of a German techno-pop band. But von Willebrand factor is a protein that helps your blood form clots.

Obviously, you want to have enough von Willebrand factor in your blood to stop bleeding in case of an injury. But having too much of it in your circulation can create clots in places you don’t want... and trigger a heart attack, or a stroke.

Thankfully, your body routinely sweeps out some of this factor. Scientists haven't quite figured it out yet but for some reason if your von Willebrand factor has either the A or B antigens on them, that clearance is harder to do. As a result, people with type A, B or AB blood have about 25 percent more of this clotting factor in their blood. This may explain why researchers have consistently found higher rates of heart disease, heart attack, and stroke in people with A, B, or AB blood, compared to those with O.

Now, in the grand scheme of things, your blood type is only a bit player when it comes to what diseases you might get. Eating too many hamburgers and not working out, for instance, are almost certainly more damaging to your heart than having A or B antigens on your blood cells. But scientists hope to figure out why certain blood types help protect you from some conditions, while making others worse. The hope is that, one day, everyone will be able benefit, whether you’re an A, B, AB, or an O.

Thanks for watching this episode of SciShow, which was brought to you by our patrons on Patreon. If you want to help support this show, just go to patreon.com/scishow. And don’t forget to go to youtube.com/scishow and subscribe!