Home » Biology » Physiology and Anatomy » Blood » Blood type

Blood type


When a doctor talks about your blood type,  he or she is referring to two things:

• your blood type in the ABO system,

• your blood type in the Rh factor system

These systems let us know who you can safely donate blood to, and who you can receive blood from.

Why is this so important? Before Karl Landsteiner discovered ABO human blood groups in 1901, it was thought that all blood was the same. This misunderstanding led to fatal blood transfusions.

His discovery of the ABO system allowed doctors to safely transfuse blood from a donor into a patient.

This system was great, but not perfect. Occasionally donors had a bad reaction to donated blood: Apparently there was another difference between blood types that was still unknown.

Later, in 1940, Landsteiner discovered this unknown difference: Some people had a certain protein in their blood, while others didn’t.  People who have it are labeled Rh+.  People without are labeled Rh-.

(The protein was called Rh because it was first discovered in rhesus monkeys.)

Combining info from the ABO and the Rh system, finally allowed doctors to make a fantastically safe system allowing blood donations.

Karl Landsteiner ABO blood types Nobel Prize

A, B, O system

These letters refer to types of proteins found in a blood cell’s membrane (lipid bilayer)

Here we see proteins and receptors in a blood cell’s membrane.

antigens on cell membrane

antigens on cell membrane

Is the above picture complete? LOL, no. There’s tons of stuff going on in and near the membrane. We’re not going to study it all.  But it is beautiful to see a more complete picture:

Cell membrane lipid bilayer animation

The ABO system has five possibilities

A – The A antigen is in the membrane

B – The B antigen is in the membrane

AB – Both the A and B antigen is in the membrane

O – The base of the antigen exists in the membrane, but without extra groups attached to it.

Bombay – The  antigen is entirely missing.


What do these five possibilities look like?

ABA blood group molecules

Bombay type blood cell Not ABO
Bombay type

 Bombay type

Why do we care?

If you give the wrong type of blood to someone, their immune system doesn’t recognize that antigen. Thus their immune system assumes that it is an invader, and so mounts an immune response to destroy it.

That response would attack blood throughout your body, causing your body to shut down and die.

What are the Rh proteins?

Just refers to whether you have a certain protein in your blood or not.

There are 50 protein antigens in the Rh blood group system, but for our purposes we only care about one of them, called Rh(D)

If you have it?           You are Rh +
If you don’t have it? You are Rh –

Why do we care? Helps us figure out who it is safe to give blood to, for blood transfusions.

Are there any other blood types?

Yes! There are 346 known red blood cell antigens and 33 platelet antigens. But don’t worry, as far as blood transfusions are concerned, these aren’t a big deal. We can skip by them in high school biology.

Learn with these apps

learn.genetics.utah.edu: Blood!

Who can you donate blood to?

blood-types If you are this type then you can donate blood to

Are there really just 2 cell surface proteins, and just 4 types?

LOL, no, the human body is way more complicated than that!

There are 346 known red blood cell antigens and 33 platelet antigens.

But don’t worry, as far as most blood transfusions are concerned, these aren’t as much of a big deal, and we can skip by them in high school biology.

Honors: What was the original job of these proteins?

These antigens evolved over millions of years- and this has nothing to do with blood transfusions.

They likely had an important job at some point in the distant past. But today they usually don’t usually seem critical – after all, people with blood type O don’t have the A or B antigen. And people with Bombay blood type are missing these antigens altogether.

The basic idea works like this: Molecules in our cells evolve, thru natural selection, if they help an organism survive. The antigens we’re looking at here presumably had some useful job at one point. Yet cells have many thousands of molecules, all interacting in many ways. So not every molecule is necessary all the time.  Yet sometimes they become useful when circumstances change.

In this case, evidence suggests that these molecules became the targets of certain pathogens (viruses at some points, bacteria at other points.) Over time, people with certain alleles (versions of these genes) were more likely to die – and hence, those genes became rarer over time. Yet other people with different alleles were more likely to survive – and hence those alleles became more common over time.  The articles below show that the alleles for ABO blood types do offer some protection against certain diseases (yet, they also aren’t absolutely necessary, as people with the Bombay blood type show.)

Further reading

Why do we have blood types? By Carl Zimmer. BBC Future

An integrative evolution theory of histo-blood group ABO and related genes. Scientific Reports Oct 2014

Smithsonianmag.com The-mystery-of-human-blood-types-86993838/?no-ist

Independent.co.uk Why-do-we-have-blood-types-9622054.html

Learn with these apps

learn.genetics.utah.edu: Blood!


%d bloggers like this: