ABO blood group system is the most important blood group system because: when antigens are expressed on the surface of erythrocytes, the corresponding antibodies are always present in plasma, stimuli for production of antibodies are represented by a variety of environmental factors such as bacteria (ex .: E. coli) that express the surface structure almost identical to ABO carbohydrates; what is formed is a mixture of IgM antibodies (predominantly) and IgG, active at 37ºC and able to activate the complement, and the high density of antigenic sites on erythrocyte membrane allows a large number of antibody binding resulting in incompatible blood transfusion in case of ABO system , reactions to acute hemolytic transfusion.

There are three alleles in the ABO system (A, B, A) with Mendelian transmission; A and B are co- dominant alleles, and A is a recessive allele. These three genes cause four phenotypes: A, B, AB and O, individuals with phenotype A and B can be heterozygous (AO or BO), or homozygous (AA, BB, respectively). ABO system antigens are located in the oligosaccharide chains that are part of glycosphingolipids. Genes of the ABO system do not encode directly antigens, but enzymes that add specific sugars to an erythrocyte membrane substrate (substance H).

All immunocompetent individuals produce natural antibodies (isoagglutinins or izohemolysines) to lacking ABO (H) group antigens. Anti-A and anti-B are usually detectable 3-6 months after birth and at age 5 antibody titer peaks and lasts throughout adulthood. IgM antibody titer may decrease gradually with advanced age.

Recommendations for determining blood group ABO:

– before transfusion;

– antepartum and postpartum immunohematological monitoring of mother and child;

 – in blood donors;

 -for ABO compatibility group in kidney and heart transplantation.

Determination method – direct method / globular (Beth-Vincent) to determine ABO blood group sera using anti-A, anti-B and anti-AB, with the following variants:

The result of blood group determination should be read by two people and, if possible, be confirmed on a second blood sample.