What’s your type?

Blood type, I mean. You may already know that you’re an “O negative” or a chirpy “B positive.” Or one of many other combinations of A, B, O, positive and negative.  But what do those letters mean, why do they matter, and what does the future of blood typing hold?

I know my type. I’m O negative, which means I’m a universal blood donor – and similar to about 8 percent of the population of Australia (where I come from…G’day!). What does that actually mean?

Imagine for a moment you’re taking one of those Magic School Bus dives into a blood stream. You shrink down to until you’re as tall as about 1/10th the width of a human hair, and you are swept along through the blood vessels with the red blood cells that are carrying oxygen to your cells. You shrink down even further, until you are able stand on one of those cells as if you were using it as a surfboard. When you look down, you can see the individual proteins that make up the surface of the red blood cell. These proteins, like signposts, machines, gateways and scaffolding, help the red blood cells maintain their saucer-like structure and do their job.

Some of these proteins are responsible for your blood type. Like all the proteins that your body makes, the ones that decide your blood type come from genes that you inherit from your parents. These particular proteins are called antigens.

Antigens are so-named because they can stimulate the body to make antibodies – so they are ANTIbody GENerating.  If your body sees unfamiliar antigens in your blood, your immune system will recognize them as invaders and try to destroy them.

When we talk about blood types, “A” and “B” refer to two specific antigens on the surface of your red blood cells. These letters technically describe carbohydrate “decorations” on a protein molecule. And because carbohydrates are sugars, we can use sugar decorations to demonstrate the point.  Imagine a blood cell as a donut.

People who have the A antigen on the outside of their red cells are type A (think of A as chocolate icing on your blood-cell donut).

People who have the B antigen on the outside of their cells are type B (think of B as strawberry icing on your blood cell donut).

People who have blood type O don’t have either type of decoration (think of an un-iced, classic donut).

So what about the positive and negative?

That’s another antigen, called the “Rhesus” or “D” antigen, that is completely separate from A, B and O. You may have heard doctors refer this as your “Rh factor.”

Generally, people either have the D antigen (in which case they are “positive”) or they don’t (in which case they are “negative”). I like to visualize this as sprinkles on the icing of the donuts. Like this:

illinois science council blogs blood typing red blood cells donuts

Why do we worry so much about blood types?

It’s because of those antibodies we mentioned before. If you are blood type A, you will develop antibodies to the B antigen and visa versa. If someone has antibodies against a blood group antigen, and they have a transfusion that contains that antigen, the antibodies will attack and destroy the foreign red blood cells. This is not only a waste of a good blood transfusion, but the broken blood cells can overload the kidneys with iron, which can be fatal.

So now we know that matching blood types is important, and we can see why the “O negative” plain donut is the universal donor (there are too few antigens on the surface to provoke a response), and why people with AB positive blood type can take pretty much any blood type (they are used to all the trimmings).

But it gets a bit more complicated. The ABO and rhesus blood group antigens are just the most “clinically significant” antigens, which means they are likely to generate and attract destructive antibodies, and therefore are super important to match in any transfusion. Apart from these, though, there are literally hundreds of different blood group antigens across 36 blood group systems. (Each system is a molecule on a red blood cell’s surface that may contain one or more antigens.)

Since I’m O negative, if I needed a transfusion, you could find an ABO match for me in about 6.5% of the population in the US. If you needed to match all the antigens in my blood, it would be much more difficult (closer to 1 in 10,000).

Going back to our donut analogy, the full picture of antigens on red cells look more like this:

sprinkle m&m carmel chocoloate donut blood type

Doesn’t that look delicious?

Obtaining a perfect match between donor and patient isn’t always necessary, but for people who need regular transfusions, it’s important to match as many blood groups as we can. If we don’t, they may develop antibodies that make it harder and harder to find compatible blood as their treatment continues. For example, people with thalassemia and sickle cell anemia sometimes need many transfusions every year, for life.

The Future of Blood-Typing

Traditionally blood typing is done using antibodies. A pathologist takes a patient’s blood sample and mixes it with an antibody against a certain antigen to see if there’s a reaction, which means only one antigen can be checked per test. You can imagine it can be very time consuming to do a full blood group test. But because blood groups are determined by your genes, we can now use genetic testing to determine many blood groups at once, which can help us solve cases where no test exists.

At the Australian Red Cross Blood Service, one of our research groups is using the latest genetic techniques, including massively parallel sequencing, to characterize new blood group antigens. They have found a number of new blood group antigens in the last few years, and are working hard to improve our ability to quickly and accurately match blood types for all patients.

If you’d like to donate blood, check out one of the websites below to find a location in your part of the planet:


Canada: https://blood.ca/en

Australia:  http://www.donateblood.com.au/

UK: https://www.nhsbt.nhs.uk/donate/

New Zealand: https://www.nzblood.co.nz/


  • Alison Gould

    Alison Gould (BSc (Hons) PhD MRACI CChem) is a Scientific Communications Specialist in the Research and Development Team at the Australian Red Cross Blood Service. You can follow her on Twitter @A2ali.

    View all posts

Cultivated For Your Curious Self

Keep Your Learning Going

Did you enjoy this article? You’re our kind of person. And we think you’ll love these posts from our team of experts.

Total Solar Eclipse on April 8, 2024

Total Solar Eclipse on April 8, 2024

On April 8th, 2024, a total solar eclipse will sweep across North America, from Mexico to the Maine-Canadian border. For those who experienced the spectacular solar eclipse of 2017, this one will be similar, crossing the United States from west to east and passing through or near several major metropolitan areas. And while its path is quite different this time, Carbondale, Illinois, a reasonable destination for Chicago-area residents, will once again be on the line of totality.    

Just a little background on eclipses:  Lunar and solar eclipses are not uncommon – they each occur about twice a year when the moon is crossing the ecliptic, the path of the sun in the sky.

Two women representing the Illinois Science Council at an event.

Don’t Have the Time? Donate Today.

We know you’re busy. but you can still help. We’re an independent 501c3 nonprofit, and all donations go to bringing science to the community.

Donate Today