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ABO blood group system


The inheritance of blood types from the ABO system is an example of multiple alleles in the human species.

The discovery of blood groups

Around 1900, Austrian physician Karl Landsteiner (1868 - 1943) found that when blood samples from certain people were mixed, the red blood cells would clump together to form clots. Landsteiner concluded that certain people have incompatible blood, and indeed, subsequent research has revealed the existence of different blood types in different individuals in the population.

When, in a transfusion, a person receives a type of blood that is incompatible with their own, the transferred red blood cells clump together as they enter the circulation, forming compact clumps that can clog the capillaries and impair blood circulation.

Agglutinogens and Agglutinins

In the ABO system there are four blood types: THE, B, THEB and O. These types are characterized by the presence or absence of certain substances in the red cell membrane, agglutinogens, and the presence or absence of other substances, agglutinins, in blood plasma.

There are two types of agglutinogen, A and B, and two types of agglutinin, anti-A and anti-B. Group A people have A agglutinogen A in red blood cells and anti-B agglutinin in plasma; those in group B have red cell agglutinogen B and plasma anti-A agglutinin; people in group AB have A and B agglutinogens in red blood cells and no plasma agglutinin; and people with gripo O have no agglutinogens in red blood cells, but they do have both agglutinins, anti-A and anti-B, in plasma.

Determination of blood groups using anti-A and anti-B sera.

Sample 1- blood type A.
Sample 2 - Blood Type B.
Sample 3 - Type AB blood.
Sample 4 - Type O blood.

See the table below for compatibility between different blood types:

ABO Substances % Can receive from
Types Agglutinogen Agglutinin Frequency A + B + AB + 0+ THE- B- AB- O-
AB + A and B Does not contain 3% X X X X X X X X
A + THE Anti-B 34% X X X X
B + B Anti-A 9% X X X X
O + Does not contain Anti-A and Anti-B 38% X X
AB- Ae B Does not contain 1% X X X X
THE- THE Anti-B 6% X X
B- B Anti-A 2% X X
O- Does not contain Anti-A and Anti-B 7% X

Possible types of transfusion

Agglutinations that characterize the blood incompatibilities of the system occur when a person possessing a particular agglutinin receives blood with the corresponding agglutinogen.

Group A individuals cannot donate blood to group B individuals because red blood cells A, when they enter the bloodstream of receptor B, are immediately agglutinated by anti-A present in them. The reciprocal is true: Group B individuals cannot donate blood to Group A individuals. Neither A, B, or AB individuals can donate blood to O individuals, since they have anti-A and anti-B agglutinins, which agglutinate red cells carrying agglutinogens A and B or both.

Thus, the really important aspect of transfusion is the donor red cell agglutinogen type and the recipient plasma agglutinin type. Type O individuals can donate blood to anyone because they do not have agglutinogens A and B in their red blood cells. Individuals, AB, on the other hand, can receive any type of blood because they do not have plasma agglutinins. Therefore, individuals in group O are called universal donors, while type AB individuals are universal recipients.

How does Blood Group Inheritance occur in the ABO System?

The production of agglutinogens A and B are determined, respectively, by the genes. I THE and I B. A third gene called i, conditions the non production of agglutinogens. It is therefore a case of multiple alleles. Among the genes I THE and I B there is co-dominance (I THE = I B)but each of them dominates the gene i (I THE > i and I B> i).

Phenotypes Genotypes
THE I THEI THE or I THEi
B I BI B or I Bi
AB I THEI B
O ii

From this knowledge it is clear that if a blood type A person receives type B blood the red blood cells contained in the donated blood would be agglutinated by the recipient's anti-B agglutinins and vice versa.