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Karl Landsteiner discovered the ABO human blood types in 1901. Until then, blood had been assumed to be the same for everyone, thus leading to many tragic consequences of blood transfusions.

ABO blood groups are determined by the gene I (isoagglutinin). There are three types of alleles of gene I, namely, IA, IB and Io. Proteins produced by the IA and IB alleles are called A antigen and В antigen.

Whether your blood type is A, B, AB or O, depends on whether you have, or don’t have, antigens (A and B) on the red blood cells.

People with blood group A have A antigen on the surface of their RBCs, and antibodies to antigen В in their plasma. Whereas, people with blood group B, have B antigen on the surface of their RBCs, and antibodies to antigen A in their plasma. 

Importance of Blood Type Study

Knowing your blood group gives deeper insight into your genetic blueprint, and the most important reason to know your blood type is to be prepared in case you’re ever in an emergency situation.

Blood Transfusion

Transfusions between blood groups can be deadly, so knowing the blood type of donors and recipients is very important.


Blood typing is especially important for pregnant women. If the mother is Rh-negative and the father is Rh-positive, the child will likely be Rh-positive. In these cases, the mother needs to receive a drug called RhoGAM.

Paternity Test

Illegitimacy is a major problem in many countries, and raises important questions relating to the rights of children.

Assessing paternity by blood testing is done, whereby the genetic makeup of the alleged father’s blood is compared with that of the mother and child.

In Forensic Medicine

Blood is one of the most important biological traces that are often found on the crime scene. The blood found can then be tested and can help in solving the case.

Theory Involved

Later, in 1940, Landsteiner discovered the Rh factor. There are many blood group systems known today, however, ABO and Rh blood groups are the most important ones considered during blood transfusions.

Blood cells contain inherited antigenic substances (proteins, carbohydrates, glycoproteins/lipids) on their surface.

Presence or absence of those substances and antibodies determines blood group/ type of an individual (Figure 1).

Figure 1. ABO blood groups
Figure 1. ABO blood groups

Landsteiner explained that the reactions between the RBCs and serum were related to the presence of antigens on the surface of RBCs and antibodies in the serum.

Agglutination occurred when the RBC antigens were bound by the antibodies in the serum (Figure 2).

Figure 2. Blood group testing by slide agglutination method
Figure 2. Blood group testing by slide agglutination method

He called the antigens A and B, and depending upon which antigen the RBC expressed, blood either belonged to blood group A or blood group B.

A third blood group contained RBCs that reacted as if they lacked the properties of A and B, and this group was later called O after the German word “Ohne”, which means “without”.

Later, AB blood group, was added to the ABO blood group system for the individual whose RBCs expressed both A and B antigens (figure 3).

Figure 3. ABO blood ypes
Figure 3. ABO blood types

RBCs antigens are inherited from parents. A and B antigens are inherited co-dominantly over O. Bernstein’s “three allele model” is used to determine a person’s blood type.

Rh Factor

The Rh system classifies blood as Rh-positive or Rh-negative, based on the presence or absence of Rh antibodies in the blood (Figure 4). The Rh factor is a type of protein on the surface of red blood cells.

Figure 4. Rh factor. Blood cells with antigen D and without it
Figure 4. Rh factor. Blood cells with antigen D and without it

Rh antigens are non-glycosylated, hydrophobic cell membrane antigens expressed in 85% of the human population (Rh positive, or Rh+).

Individuals with alterations or a deletion of the Rh protein are considered Rh negative (Rh–).

Formation of Rh protein is controlled by a dominant gene (R). Thus, RR (homozygous) and Rr (heterozygous) persons are dominant and are Rh positive, whereas, rr (homozygous) are recessive and are Rh negative.

Blood Group Antigen
A Has only A antigen on red cells (and B antibody in the plasma)
B Has only B antigen on red cells (and A antibody in the plasma)
AB Has both A and B antigens on red cells (but neither A nor B antibody in the plasma)
O Has neither A nor B antigens on red cells (but both A and B antibody are in the plasma)

In addition to the A and B antigens, Rh factor, which can be either present (+) or absent ( – ) plays very important role helping make decisions during blood transfusion or organ donation.

Rh negative blood is given to Rh-negative patients, and Rh positive blood or Rh negative blood may be given to Rh positive patients.

  1. The universal red cell donor has Type O negative blood type.
  2. The universal plasma donor has Type AB positive blood type.

Blood Donation/Transfusion

Blood types are very important when a blood transfusion is necessary. In a blood transfusion, a patient must receive a blood type compatible with his or her own blood type (Figure 5).

Figure 5. Blood donation chart showing recipient and donor
Figure 5. Blood donation chart showing recipient and donor

Transfusion with incompatible blood types can be fatal as red blood cells form clots that block blood vessels.

Therefore, it is important that blood types be matched before blood transfusion.

Compatibility chart of different blood types:

Blood Type Donate Blood To Receive Blood From
A+ A+, AB+ A+, A-, O+, O-
O+ O+, A+, B+, AB+ O+, O-
B+ B+, AB+ B+, B-, O+, O-
AB+ AB+ All blood types
A- A+, A-, AB+, AB- A-, O-
O- All blood types O-
B- B+, B-, AB+, AB- B-, O-
AB- AB+, AB- AB-, A-, B-, O-

Type O-negative blood is called the universal donor type because it is compatible with any blood type.

Type AB-positive blood is called the universal recipient type as they can receive blood of any type.

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