Lesson Explainer: Blood Groups and Rhesus Factors Biology

In this explainer, we will learn how to explain the classification of blood groups and the importance of this classification, and apply knowledge of blood groups and Rhesus factors to practicing medicine safely.

The blood that constantly travels around the human body is incredibly important: it transports nutrients to where they are needed, delivers oxygen to respiring cells, and aids the removal of waste products. In an average adult human body, there are around 5.7 litres of blood! In the vast majority of cases, an adult will be able to easily donate up to 500 mL of blood without suffering any negative side effects.

Blood group, or blood type, is an example of a trait in humans that is controlled by a person’s genetics. There are three alleles for blood type: IA, IB, and IO. The interaction of these alleles results in four options for blood group: A, B, AB, and O. Table 1 shows how the combination of alleles gives rise to the different blood groups.

Table 1: The alleles present in a person’s genotype and the resulting blood group.

Blood GroupABABO

Key Term: Alleles

Alleles are the different versions of a single gene. For example, the gene for blood type has three possible alleles: IA, IB, and IO.

The classification of a person’s blood type depends on two different substances found in their blood: antigens and antibodies. Antigens are substances that can cause an immune response in the body. This means that some antigens are recognized as foreign and the body can attack them. One of the ways the body fights antigens is by making special proteins called antibodies. Antibodies attach to one specific type of antigen, which helps the immune system identify and destroy foreign cells or structures.

Definition: Antigen

Antigens are substances that can trigger an immune response.

Definition: Antibodies

Antibodies are proteins produced by the immune system in response to antigens.

Antigens found on the surface of red blood cells can be divided into two types: A and B. The allele IA indicates that a person has type A antigens on the surface of their red blood cells, and the allele IB indicates they have type B antigens. If a person has the alleles IA and IB, they will have both type A and type B antigens on their red blood cells. This is an interesting genetic property known as “codominance.” The allele IO indicates that there are neither type A nor type B antigens on the surface of the red blood cells.

The antibodies present in the blood plasma can also be divided into two types: anti-A and anti-B. Anti-A antibodies are produced by those with blood type B. This is because antibodies target antigens they recognize as foreign. Type A antigens would be recognized as foreign by the blood cells of a person having type B blood, so in response they produce anti-A antibodies. An easy way to remember this is that “anti” means “against,” so anti-A means antibodies that are “against” type A blood! Figure 1 outlines how the combination of antibodies and antigens can be used to determine a person’s blood group.

Figure 1: A diagram showing the antigen types and antibodies present in the different blood groups. The antigen types present are type A, type B, both, or neither. The antibodies present are anti-A, anti-B, both, or neither.

Example 1: Recalling the Antigens and Antibodies That Determine Blood Groups

Consider this partially complete table of blood groups and the antigens and antibodies found within blood.

Blood GroupABAB?

  1. What is the missing blood group?
  2. What are the missing antigens?


Part 1

Blood group in humans is a genetic trait controlled by the presence of certain alleles in a person’s DNA. Blood groups in humans are categorized into four types: A, B, AB, and O.

Humans with blood type A will produce type A antigens, and their antibodies will be categorized as anti-B. A way to remember this is that to be “anti” something is to be against it, so to protect the body, type A blood cells produce antibodies that are “against” type B antigens; therefore, they are “anti-B”! Blood type B has a similar pattern, and these red blood cells will express antigens that are type B, and the blood will contain antibodies that are anti-A. However, If a person has blood type O, this means they have no type A or type B antigens on the surface of their red blood cells—you can remember this by “O = zero”! However, these individuals will have antibodies of both types, anti-A and anti-B, in their blood to recognize foreign antigens and react against them as a defense mechanism.

In our table, we can see that we have blood groups A, B, and AB, but O is missing from the list. Therefore, the missing blood group is O.

Part 2

If a person has blood type AB, this means their red blood cells possess both antigen type A and antigen type B on their surface. However, because they possess both of these antigens, neither type of the antibodies that target these antigens will be present in their blood, as this would be detrimental for the person’s health.

Therefore, the missing antigens are both A and B.

Understanding blood groups, the alleles that determine them, and the substances produced by the blood is crucial in different areas of science and medicine. Understanding how blood groups are inherited can be used to help determine the paternity of a child in family disputes; for instance, a child with blood type O could only be born if both parents possessed at least one IO allele each. If a child with blood type O is born to a mother who has the allele IIOO and a “father” with IIAB, the child is very likely to not be his! The genetics of blood groups can be used to study the evolution of humans. Although the alleles for blood type A (IA) or B (IB) are dominant over the allele that gives blood type O (IO), blood type O is actually the most frequently occurring; in the US, around 44% of individuals have blood type O.

One of the most important applications for understanding blood groups in humans is the ability to carry out safe blood transfusions. Because of the different antibodies and antigens produced by the different blood types, transfusions can only happen safely in specific ways. Figure 2 outlines which blood transfusions can be carried out.

Key Term: Blood Transfusion

A blood transfusion is the process of taking blood from a donor, processing it, and then inserting it into the body of a patient in need of blood. This is a fairly common and safe procedure.

Figure 2: A diagram outlining the transfusions that can be safely carried out between the blood types of humans.

As we can see from Figure 2, people belonging to blood group O can donate their blood to be received by any other blood group. For this reason, people with blood type O are often referred to as “universal donors.” However, people with blood type O can only receive blood that is type O. People with blood type AB can receive blood from any other blood group; for this reason, type AB is often referred to as a “universal receiver.” However, people with blood type AB can only donate to others with blood type AB.

Example 2: Interpreting Diagrams of Blood Transfusions

Blood can be transfused between individuals. The diagram provided shows which blood group can receive blood from or donate blood to other groups.

Which of the following statements is correct?

  1. People with blood type AB can receive any other blood type.
  2. People with blood type B can donate blood to any other person.
  3. People with blood type O can only receive blood that is type AB.
  4. People with blood type O cannot receive any blood transfusions.
  5. People with blood type A can only receive blood that is type A.


Understanding blood types is crucial to performing safe and effective blood transfusions. If people receive a blood transfusion using a type that is incompatible with their own, it can initiate an immune response that could make them very ill. The diagram provided gives a simplified summary of what blood transfusions can take place, which are indicated by the direction of the arrows. For instance, we can see that blood type O can be donated to people of any blood group, and that those with blood type AB can receive any other type of blood. Those with blood type A can donate to those with blood type AB or A and receive blood from A and O blood types. Similarly, those with blood type B can donate to those with blood type B or AB and receive blood from their own blood type and from O. Those with blood type AB are known as “universal receivers”—they can accept blood donations from any other blood type, including their own. However, they can only donate blood to other AB individuals.

Let’s look at the statements. We can discount option E, as we can see from the diagram that people with blood type A can receive blood type O as well as blood type A. We can also eliminate option D, as people with blood type O can receive blood transfusions, just only from the same blood type. Option C is also not correct for this reason. Option B is incorrect, as we can see from the diagram that people with blood type B cannot donate blood to those with blood type O. However, option A is correct—those with blood type AB can receive blood from any other donor. It is for this reason that people with blood type AB are often called “universal receivers.”

Therefore, the correct statement is A, people with blood type AB can receive any other blood type.

Next, let’s look at how a person’s blood group can be determined using laboratory methods.

We know that each blood group has certain antigens on the surface of the red blood cells and will contain specific antibodies that target antigens that are not found on these red blood cells (known as “foreign” antigens). By using a reaction called agglutination, we can easily determine which blood group a person belongs to. The method is outlined below:

  1. A sample of blood is taken from the person who needs their blood type determined.
  2. Two large drops of blood are placed on clean, sterile glass slides.
  3. On one drop of blood, anti-A antibodies are dropped.
  4. On the other drop of blood, anti-B antibodies are dropped.

Agglutination means to “clump together,” and this is a visible change that happens when antibodies encounter the antigen they are specialized to attack and fight against. If a sample of blood shows agglutination when mixed with particular antibodies, this indicates that the antibodies are targeting the antigens that they recognize as “foreign” and are binding to them as a mechanism of defense.

Key Term: Agglutination

Agglutination means to “clump together.” This is a process that occurs when an antigen is mixed with its corresponding antibody.

The principle of this experiment is that if a sample of type A blood, which has type A antigens on the surface of its red blood cells, is mixed with anti-A antibodies, agglutination will take place. Likewise, if type B blood is mixed with anti-B antibodies, it will agglutinate. A simplified diagram of what we expect to see is outlined in Figure 3.

Figure 3: The results of agglutinating a sample of each blood type with antibodies anti-A and anti-B.

Example 3: Identifying the Agglutination Patterns of Different Blood Types

Blood samples can be tested for blood groups by agglutination. A small sample of blood is mixed with antibodies A and antibodies B. The table provided shows some results.

Blood TypeABABO
Anti-AAgglutinationNo agglutinationAgglutinationNo agglutination
Anti-BNo agglutinationAgglutinationNo agglutination

What is the missing result?


A person can have their blood group determined by a sample of blood being taken and mixed with antibodies A and antibodies B. A person’s blood group indicates the type of antigens that are on the surface of their red blood cells, so a person with blood type A will have antigen type A on the surface of their red blood cells. Antibodies that are “anti-A” target these type A antigens—you can remember this by “anti” meaning “against,” so antibodies that are “anti-A” will be against “A” antigens! When the antibodies target the antigens, they agglutinate. This means they bind to the antigens and cause the red blood cells to clump together. The missing cell in the table is asking us, “what happens when a sample of blood that is type B (so it has type B antigens) is mixed with antibodies that are anti-B?” We know that anti-B will target the B type antigens and cause agglutination.

Therefore, the missing result is “Agglutination.”

We know that, on the surface of red blood cells, there are type A antigens, type B antigens, or neither. However, there can also be another type of antigen present—the Rhesus factor antigens. Rhesus factor antigens are so called because a very similar antigen was discovered on the red blood cells of Rhesus macaques!

For the Rhesus factor antigens, there are only two choices. Either a person has these antigens on the surface of their red blood cells (and is Rh+) or does not have these antigens (and is Rh). In the US, around 85% of the population is Rh+. The inheritance of Rhesus factors is controlled by three pairs of genes. The allele that determines if a person is Rh is recessive, whereas the allele for Rh+ is dominant. Dominant alleles are those always expressed when present in a person’s genetic makeup, whereas recessive alleles are only expressed when the dominant allele is absent. This means only one dominant allele in these three pairs of genes is needed to make a person Rh+.

Understanding Rhesus factors is incredibly important. Firstly, a person with Rh blood should only receive blood transfusions that are also Rh. This is because the body may start to make antibodies that target the Rhesus antigens on the Rh+ blood, and this can have negative side effects leading to severe illness or, potentially, a fatality. Secondly, it is important to know when a female with Rh blood is pregnant with a baby that has, or is likely to have, Rh+ positive blood.

When an Rh mother is carrying an Rh+ baby, her blood may start to mix with that of the unborn child via the placenta. Her immune system will recognize the Rhesus antigens of the baby’s blood as foreign and start to produce antibodies to target them. If this is the first pregnancy, the baby and mother are unlikely to suffer any negative consequences of this. However, if the mother becomes pregnant again, and again the baby is Rh+, the antibodies she produced during the first pregnancy are ready to target the fetus, and this could lead to jaundice, infection, and anemia and could potentially lead to the death of the unborn child.

Luckily, this devastating outcome can be prevented. If the mother is identified as being Rh with an Rh+ baby, she can be injected with a protective “serum” during pregnancy and in the 72 hours following the birth of the first child. This serum helps break down the Rh+ blood of the baby in the mother’s system, thus preventing the formation of antibodies, which means that she can safely carry a second Rh+ baby.

Example 4: Recalling the Risks of a Rh Mother Being Pregnant with an Rh+ Baby

The flowchart shows the series of events that can occur if a Rhesus-negative mother is pregnant with a Rhesus-positive child.

What would the most likely statement to complete the flowchart be?

  1. The antigens produced by the baby can attack the antibodies of the mother.
  2. The child will inherit Rh antigens from its mother and become Rh.
  3. The antibodies produced by the mother can attack the red blood cells of the baby.
  4. The mother will obtain Rh+ antigens from the child and become Rh+.


Rhesus antigens are antigens found on the surface of red blood cells. If they are present within a person, this person is Rhesus-positive, but if they are not present, they are Rhesus-negative. The mixing of Rhesus-negative blood with blood that is Rhesus-positive can be dangerous; a Rhesus-negative person may respond to Rh+ blood by producing antibodies that target these antigens, and this can lead to negative reactions and illness. This is particularly worrying during pregnancy, as the unborn baby can be fatally harmed.

We can see from the flowchart that, during her first pregnancy, the body of an Rh mother begins to make antibodies to target the Rhesus antigens present in her baby’s blood. The baby is unharmed, and the mother gives birth. However, during the second pregnancy, her body produces these antibodies much faster. We know that these antibodies will recognize the Rhesus antigens in her baby’s blood as foreign and begin to attack them. If left untreated, this could cause jaundice, anemia, and even death for the fetus.

Therefore, the most likely statement to complete this flowchart is C, the antibodies produced by the mother can attack the red blood cells of the baby.

Let’s summarize what we have learned about blood groups and Rhesus factors.

Key Points

  • The human blood groups are A, B, AB, and O.
  • Alleles for blood types A and B are codominant and are both dominant over the allele for blood type O.
  • Blood groups are identified by the antigens on the surface of red blood cells and the antibodies produced by the blood.
  • Rhesus factor antigens are also present on the surface of red blood cells in people who are Rh+.
  • Rhesus factors are controlled by three genes and are inherited independently of blood groups.
  • Understanding Rhesus factor antigens is important for safe pregnancies and births when mothers are Rh.

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