Blood groups (clinical analyzes)



The blood group is one of the many characteristics of an individual such as, for example, the color of the eyes and hair; it is genetically determined and inherited from both parents. The classification of blood groups occurs on the basis of particular molecules, called antigens, present on the surface of some blood cells, the red blood cells. Over 700 types of antigens have been classified, which can be grouped into more than 30 systems. Of these, the best known are the ABO system and the Rh (rhesus) system.

The ABO System is composed of four groups (A, B, AB, O) characterized by the presence of antigens A and B. If antigen A is present on a red blood cell, group A is present; if antigen B is present, group B; if both are present; the AB group. If neither of the two antigens is present, we have group O (group "zero" also called "O", from the initial of the German word "ohne", which means "without").

Each of these blood groups is further subdivided into two categories based on the presence, or not, on the red blood cell membrane of a particular antigen belonging to the Rh system. The Rh factor can therefore be positive (Rh +) or negative (Rh-).

There are also the so-called "rare groups": a person is defined as a "rare group" when his antigenic structure is found in at most 1 subject every 1,000-5,000. In Italy there are banks of rare groups that have a register and a stock of (frozen) blood belonging to these groups.

Defining an individual's blood group is very important in transfusion medicine; it is essential, in fact, to ensure that there is compatibility between the person who needs a transfusion and the blood that is transfused. In case of ABO incompatibility life-threatening reactions can occur. In fact, a person with blood type O has anti-A and anti-B antibodies. Consequently, if you are transfused with blood belonging to groups A, B, or AB, your antibodies will react with the transfused red blood cells, destroying them. Similarly, an individual Rh- who is transfused with Rh + blood will produce anti-Rh antibodies with the risk of a serious reaction, especially if he receives a subsequent transfusion with Rh + blood.

The identification of the Rh factor is particularly important during pregnancy since a mother could be incompatible with the fetus. If the mother is Rh- but the father is Rh +, the fetus could be positive for the Rh antigen. The mother's organism could then develop antibodies against the Rh + antigen which, crossing the placenta, would cause the destruction of the blood cells of the fetus, with the development of the so-called fetal-neonatal haemolytic disease (MEFN). MEFN rarely occurs in the first pregnancy, while the risk increases in subsequent pregnancies.

The test

The determination of the blood group is carried out on a small amount of blood, usually taken through a needle inserted into the vein located inside the elbow.

All the direct tests for the determination of the blood group (performed with different methods: slide, plate or microplate, test tube, coupons with micro columns of gel or microspheres, or with an automatic instrument) are based on the finding, or not, of a blood reaction in contact with two different types of immune serum containing anti-A or anti-B antibodies (agglutination test).For example, two drops of blood are placed on a slide: a drop of "anti-A" serum is added to one of them and a drop of "anti-B" serum on the other. If no reactions occur, the blood in test belongs to group O (zero). If you observe the formation of small masses that precipitate (agglutination) with the anti-A serum, the blood is of group A, if it reacts with the anti-B serum it is of group B; if the agglutination reaction is observed with both sera (anti-B and anti-A), the blood belongs to group AB.

There indirect determination ABO blood group test is an additional test that does not replace direct verification. The indirect test looks for the presence of anti-A and anti-B antibodies which are naturally present if the corresponding antigens are absent on the person's red blood cells. It consists of testing the subject's serum against known group A and group B red blood cells.

Similar procedures apply for the determination of the Rh factor.


The test results allow you to determine your blood group (ABO and Rh systems).

The spread of the groups varies according to the different areas of the world. In Italy the distribution is as follows:

0 +

A +

B +

AB +

0 -

TO -

B -

AB -









About 85% of the Italian population is Rh +.

The results of the identification (typing) of the blood group will also allow the doctor to determine which type of blood can be transfused / received without risk to the patient who needs it.

The compatibility diagram for ABO and Rh systems is shown in the following table:

Blood group and Rh type

Can receive blood from ...

A +

A +, A -, O +, O -

TO -

A -, O -

B +

B +, B -, O +, O -

B -

B -, O -

AB +

AB +, AB -, A +, A -, B +, B -, O +, O -

AB -

AB -, A -, B -, O -

O +

O +, O -

OR -

OR -

Blood of group O negative is the so-called universal donor since it can be transfused, without danger of unwanted reactions, to people belonging to any blood group.


Dean L. Blood Groups and Red Cell Antigens. Bethesda (MD): National Center for Biotechnology Information (US); 2005. Chapter 5, The ABO blood group

Reid ME, Lomas-Francis C. The Blood Group Antigen Facts Book. Second ed. 2004, New York: Elsevier Academic Press

Klein HG and Anstee DJ. Mollison's Blood Transfusion in Clinical Medicine, 12th edition. Wiley-Blackwell, 2014

In-depth link

International Society of Blood Transfusion (ISBT)

American Red Cross. Blood types

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