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You are either blood type A, B, AB or O. But, did you know that your blood type is determined by microscopic antigens found on your red blood cells? In this lesson, you will learn about the different blood groups and why they do not mix.

Red Blood Cell Antigens

We previously learned that red blood cells are the most common type of blood cell. Red blood cells are also referred to as erythrocytes, which comes from the Greek words erythros (meaning red) and cyte (meaning cell). The plasma membrane that surrounds each of your red blood cells contains special proteins that are placed there based on your unique genetic makeup.

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In this lesson, you will learn about these genetically determined proteins and the role they play in deciding your blood type.Your immune system recognizes foreign particles that are not normally in your body as antigens. If an antigen is present, it stimulates your immune system to release antibodies to provide immunity against the specific antigen. As mentioned earlier, there are genetically determined proteins found on the plasma membrane of your red blood cells. These special proteins are antigens – and what we refer to as red blood cell antigens.


Your body tolerates your own red blood cell antigens, but if your blood were to be transfused into another person with different red blood cell antigens, that person would have some big problems. That other person’s antibodies would recognize the transfused red blood cells as foreign and bind to them. If this occurs, it leads to a process called agglutination, where the red blood cells clump together and interfere with blood circulation due to the fact that this clump of cells can clog up small blood vessels in the body.This term is easy to recall if you know that the root of the word comes from the Latin language and literally means ‘to glue.’ While it’s strange to think about your blood being able to stick together like glue, this is exactly what happens when a blood transfusion goes wrong.

There are many different red blood cell antigens, and these allow your blood to be categorized into different blood groups. Let’s take a look at one of the most important blood groups: the ABO blood group system.

ABO Blood Group System

The four major blood types.

Blood types

The ABO blood group system is a classification system for blood that depends on the presence or absence of an A or B antigen on the red blood cells. ABO blood groups are determined by the types of antigens you inherited from mom and dad, specifically, type A or type B. If you inherited both types of antigens, then you ended up with type AB blood. In the event that both antigens are absent, you would end up with type O blood.

Therefore, we see that there are four major blood groups or blood types: A, B, AB and O.Your red blood cell antigens are formed before your antibodies form. The interesting thing is that the antibodies that get produced by your body are based on the antigen not present on your red blood cells. Therefore, if you have type A antigens stuck on the outside of your red blood cells, you will develop only anti-B antibodies. This is a very good thing, if you think about it, because if you had type A antigens on your red blood cells, and your body formed anti-A antibodies, they would destroy your own blood.

Let’s look at this further and consider type B blood. Being blood type B means you inherited type B antigens, and the red blood cells in your body would have B antigens attached to them. Your immune system would be used to seeing B antigens and would tolerate them just fine. However, your immune system would not be familiar with A antigens because they are not present on your red blood cells. Your body would form antibodies against A antigens, called anti-A antibodies, and this mixing could lead to clumping or agglutination of the blood. When you have type B blood, it’s almost like the B red blood cells own the neighborhood, and they do not want type A blood cells moving in.

In fact, they create anti-A antibodies to stand guard and keep the As away.If you were born with type O blood, we know that both antigens are absent from your red blood cells. Therefore, your blood would form both anti-A and anti-B antibodies. This would be an issue if you ever needed a blood transfusion because you could only accept blood from someone else who has type O blood. Any other blood type (A, B, or AB) would have antigens that react with your antibodies. Here, we see that having type O blood is even more exclusive than having type A or type B.

When type O blood owns the neighborhood, neither type A nor type B blood cells can move in. In fact, type O blood cells create anti-A and anti-B antibodies to stand guard and keep the other blood cells away.

People with AB blood types can receive blood from AB-, A-, and B-types.
AB blood type

Contrast that to someone who has an AB blood type. That person’s immune system is used to seeing both A and B antigens. They would not have antibodies against any type of blood.

So, this person could receive not only AB-type blood but also A- or B-type blood. Therefore, we can think of AB blood being friendly to the other blood types. When they own the neighborhood, they’re happy to have type A and type B come visit.

Universal Donor and Universal Recipient

People with an AB blood type, as well as those who have A or B blood type, can receive blood from a type O person because that person produces red blood cells with no antigens that can react with their immune system. You already learned that someone with type O blood can receive blood from another type O person, so that means a person who is type O can donate blood to anyone and, therefore, they’re called ‘universal donors.’ Similarly, since people who are type AB can receive blood from any type, they are called ‘universal recipients.’

Lesson Summary

Let’s review. The ABO blood group system is a classification system for blood that depends on the presence or absence of A and B antigens on red blood cells.

Depending on which of these genetically determined proteins or antigens, known as red blood cell antigens, you inherited, you will have one of four blood types: type A, type B, type AB or type O.If you have type A blood, your red blood cells have the A antigen, and your body will produce anti B-antibodies. If you have type B blood, your red blood cells will have B antigens, and your body will produce anti-A antibodies. If you have type AB blood, your red blood cells will have both A and B antigens, and therefore produce no antibodies.

This means you can receive a blood transfusion from any other type, making you a universal recipient.If you have type O blood, your red blood cells will not have any antigens and therefore produce both anti-A and anti-B antibodies. This means you must be careful and only receive blood from other type O people. But, because your blood has no antigens to react with the immune system, you can donate blood to anyone.

This makes you a universal donor.It’s important to get the right blood type during a blood transfusion. If you get the wrong type, it will cause the blood cell antigens to react with the antibodies and clump together. This is a phenomenon called agglutination, where the red blood cells clump together and interfere with the blood circulation.

Learning Outcomes

At the end of this lesson, you will be able to:

  • Explain how blood types are classified in the ABO blood group system
  • Identify which blood type is the universal donor and which is the universal recipient
  • Describe what happens when a person receives the wrong type of blood through a transfusion

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