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At some point—like, if you've donated blood— you might have been asked about your blood type. And even if you don't know what yours is, you're probably aware that you could have.

A, B, AB or O blood, and that your blood can be positive or negative. But that's not the whole story, because there are potentially millions of blood types out there. There are so many possible blood types because of the way blood types are defined.

You see, your blood type is defined by the antigens present in your blood. Antigens are anything that can elicit a response from your body's self-defense system, though your immune system normally ignores the ones that belong to you. And you can find antigens on cells throughout your body, with different cells having different combinations of antigens.

The ones that matter for blood classification are found on the surfaces of your body's red blood cells. Simply put, your particular blood type depends on which antigens are or aren't there. Like, if you have AB blood, that means you have both the A and B antigens in the ABO blood group.

You could be just A, or just B, or if you're O, you don't have either. But those ABO antigens are just two of over 600 blood antigens identified by the International Society of Blood Transfusion, and the list hasn't stopped growing yet. Many of these antigens fit into blood group systems, like ABO, each of which is defined by a gene at a single site or by multiple genes that are closely related.

And all of us have these genes— your type for a particular group depends on how your genes translate into the antigens that end up on the outside of your blood cells. There are 36 blood group systems currently recognized. So your full blood type, if written out, would include all 36 of these groups and the variants you have or don't have for all of those 600-plus antigens.

Which is why we can say that there are millions of potential blood types. Of course, there are only 8 common ones, and that's because many of these antigens are found in practically everyone, while others are present in only a few individuals. For example, the SARA antigen has only ever been seen in two families, while 99.96% of people have the Vel antigen.

And blood groups can get really complicated, too. Just look at the Rh blood group. That's the group that gives you a positive or negative blood type— like, if you're AB+, the positive part generally means you have the Rh antigen called Rh(D).

But, to make things more confusing, whether your blood is considered positive or negative may depend on how much of the antigen you have in your system, since that can impact what sorts of antibodies your immune system makes to protect you. And D is just one of over /60/ known antigens in the Rh group. So positive or negative doesn't even begin to capture your overall Rh blood type.

In fact, one of the rarest blood types in the world occurs if you have none of the antigens in the Rh group. If you're one of the about 50 people with this blood type— which is known as Rh null— your body will reject the blood from practically anyone else. And the Rh group isn't the only blood group where having no antigens can be a matter of life or death.

Another example is the Diego group. Its antigens are proteins that help the lungs and kidneys perform essential functions. Like with the ABO group, there are two primary antigens, a and b, that determine a person's Diego blood type.

But unlike the ABO group, where “O” or the lack of antigens is most common, there has only been one documented case of someone lacking both of these key Diego antigens. Sometimes, though, having no antigens can help you out. For example, one of the malarial parasites uses antigens in the Duffy blood group to target and infiltrate cells, so having no antigens from that group can make you more resistant to the disease.

But it also means there's a chance that your body will attack another person's blood if you're given a transfusion, since the cells you receive could have Duffy antigens on them that your body sees as foreign. The good news is that despite all the potential blood types you could have, for the most part, the ABO-Rh blood typing we're used to does a pretty good job of matching people's blood. With this system, if you receive blood from someone with the same ABO-Rh type, there's a 99.8 percent chance your blood will be compatible with your donor's.

For some reason, your body's immune system doesn't go after every antigen equally, so you don't usually need to know what version of every single known antigen you have. And if you do want to be extra sure, there are ways that doctors can tell if you have a rare blood type. For example, they can screen for unexpected antibodies that could potentially target donated red blood cells, or perform crossmatching, where your blood is mixed with a donor's to see how the two react.

Doing these two steps racks your safety margin up to 99.95 percent. And if you do happen to have a rare blood type like Rh null, don't fret. Efforts like the International Rare Donor Panel work hard to make sure you can get the blood you need, no matter where in the world you are.

One person needing a transfusion had a rare blood type delivered from the UK to Cameroon. That's about four thousand miles away! So even if your blood is literally one in a million, you can be pretty confident that you'll be able to find someone whose blood matches yours.

And, I guess, you could say that's one of the parts of being human — having something in common with other people while being unique in your own special way. Blood types ultimately boil down to microscopic differences between red blood cells. And if you were to zoom in to see those differences and really watch what goes on in your own bloodstream, you'd see a bustling and bizarre world.

In fact, when you look at pretty much anything under a powerful enough microscope, all sorts of wonderfully fantastic lifeforms are revealed. And that's kind of the point of our new sister channel produced by Complexly: Journey to the Microcosmos. Journey to the Microcosmos is all about bringing you closer to the world of very, very small things in a relaxing way.

It pairs James Weiss's incredible microscopic footage with Hank Green's soothing narration and Andrew Huang's meditative music, so you can just kick back and marvel at the microscopic realm. You can check it out for yourself at the link in the description. And as always, thanks for watching! [ outro ].