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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

A doctor might say you have “Type A” or “Type O” blood. What does that mean?

It refers to molecules found in your blood cell’s membrane (lipid bilayer)

All cells have various proteins and carbohydrates in the membrane. Each has a separate job.

The ABO system deals with special carbohydrates – chains of sugars.

Let’s look at these molecules floating in the cells 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.

Some of the particles floating here are A, B, or O molecules.

Cell membrane lipid bilayer animation


What do the ABO antigens look like?

These antigens are carbohydrates (chains of sugars)

Let’s examine the five possible types.

ABA blood group molecules

Bombay type blood cell Not ABO
Bombay type

 Bombay type

Types of antigens in the ABO system

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 – Has only the basic four sugars. Missing the fifth sugar.

Bombay – The first three sugars are here, but the fourth sugar (fucose) is missing.

Why do we care?

If you give the wrong type of blood to someone, their immune system doesn’t recognize the antigen.

Thus their immune system assumes that it is an invader, and 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?

There are 50 protein antigens in the Rh blood group system

These are proteins floating in the blood cell membrane.

When it comes to safely donating blood, we only need to know about one of them. It is called Rh(D)

If you have it?           You are Rh +

If you don’t have it? You are Rh –

Rh factor on blood

Why do we care? It let us know 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

What was the original job of these proteins?

These antigens evolved over millions of years- and their existence had 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.

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.

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

Rh incompatibility during pregnancy

This section comes from Immense Immunology Insight

Rh incompatibility occurs when an Rh-negative person is exposed to Rh-positive red blood cells (RBCs).

For instance, a Rh-negative pregnant mother can be exposed to Rh-positive fetal red blood cells (RBCs).

(Mummy red blood cells don’t have the target and baby cells have the target!)

When does that happen? After all, usually, there is no mixing of fetal and maternal blood. But mixing of the mother’s and baby’s blood can occur after fetomaternal hemorrhage during the course of pregnancy, or from spontaneous or induced abortion, from some kind of trauma, from an invasive obstetric procedures, or even from a normal delivery

When this happens, the baby’s RBCs are mixed with mom’s un-targeted RBCs.

Rh-negative mother becomes alloimmunized to the Rh antigen present on fetal red blood cells during the first Rh-incompatible pregnancy. i.e., the mother’s immune system notices the cells with the target!

The first pregnancy is rarely affected because the number of Rh antibodies produced by the mother during primary immunization is low, and the antibodies are usually IgM in nature. IgM can’t cross placental barrier.

(The mother’s white blood cells make arrows, but they are little in number. They are also too big to pass through the placenta.

Think of the placenta as a wall, or a barrier, with really tiny windows, protecting the fetus. This is why, the child’s targeted RBCs are safe and sound in the first pregnancy.

But now… Rh incompatibility in subsequent pregnancy:

When the mother is exposed to Rh-positive fetal RBCs during a subsequent Rh-incompatible pregnancy, the mother mounts a secondary immune response to the fetus’ RBCs.

i.e. the mother’s white blood cells rapidly recognizes the targeted RBCs and bulls eye!

A large number of IgG-class Rh antibodies are produced. The IgG antibodies cross the placenta and make fetal red cells susceptible to destruction.

(This time, the mother’s white blood cells produces many smaller arrows that can go through the small windows in the wall [placenta].)

Note: If the mother is sensitized to the Rh antigen prior to the first pregnancy, say due to some blood transfusion or environmental exposure, the first fetus will be affected.

The fetal RBCs are then destroyed by the fetal immune system. Anemia develops in the fetus with a concomitant increase in unconjugated bilirubin. The anemia and unconjugated bilirubin levels can lead to a number of conditions like jaundice, kernicterus, hepatomegaly, splenomegaly and worst case scenario, eventual death.

The good news is, sensitization is preventable and even after first exposure, subsequent child can be protected from mom’s antibodies by administration of Rh IgG.

When a mother’s blood type conflicts with her developing baby. Immense Immunology Insight

Secondary link to same article


Learn with these apps

learn.genetics.utah.edu: Blood!


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