ABO Full Form: Blood Grouping System

ABO Full Form: Background Information

In the 20^th century, a scientist of Austria known as Karl Landsteiner, observed that the red blood cells of some individuals show agglutination by the serum of other individuals. After the observation and some experiments he noticed the pattern of agglutination and divided blood into groups and this initiated the blood group system ABO, a landmark discovery for which Landsteiner was awarded the Nobel prize.

The reaction between red blood cells and serum were actually related to the markers known as antigens present on RBCs and antibody against the marker present on the serum. Agglutination reaction occurs when antigen present on RBCs is bound with the antibodies present in the serum. Landsteiner named antigens as A and B, depending upon which one is expressed, that belongs to either blood group A and Blood group B.  Landsteiner also explained a third type of blood group if RBCs lack the properties of antigen A and B then its called as O blood group termed on German word Ohne which means without. Afterwards, a fourth blood group AB was also added in the ABO blood group system which explains when both antigens A and B are expressed in the RBC..

ABO Full Form: Determination of Blood Group:

The blood group in ABO system antigens are encoded by one genetic locus, ABO locus which has three allelic forms A, B, and O. a child receives one allele from the three alleles of each parent which give rise to six possibilities in genotype and four possibilities in phenotype.

ABO Full Form: Biochemical Nature of ABO System

The four principle phenotypes of the ABO blood group system are A, B, O, and AB.

1. After the discovery of the fact, that in blood group A, RBC reacts a particular antibody which later known as antiA1 antibody, blood group divided into two phenotypes A1 and A2.
2. RBC having A1 phenotypes reacts with anti-A1 and makes near about 80% blood type, RBC having A2 phenotype does not react with anti-A1 antibody and makes about 20% blood type.
3. A1 containing red blood cells reacts five times more as A antigen than A2, so A1 and A2 are interchangeable.
4. Our immune system forms antibodies ABO blood group whichever antigen is not found on the individual’s RBCs.
   1. So in this system, a group of antigens A individuals will have anti-B antibodies,
   2. A group of Antigen B individuals will have Anti-A antibodies.
   3. In the blood group, O individuals have both types of antibodies anti-A and anti B in serum but no antigen in RBCs.
   4. And in the blood group, AB individuals have both antigens A and B have their RBC and no antibodies in serum.

5. In serum, ABO antibodies forms naturally. The production of these antibodies is stimulated when the immune system encounters the missing ABO blood group antigen in food or in microbes. This reaction happens at an early age because identical sugars similar to ABO antigen are found throughout in nature.

6. The three alleles of ABO, A encodes glycosyltransferase, N-Acetyl glucosamines that produces A antigen, B allele encodes glycosyltransferase, D Galactose which creates B antigen, O allele encodes the non-functional enzymes so no A or B are formed.

ABO Full Form: Expression of ABO Antigens

1. ABO antigens are expressed on human tissues and they are present mostly on epithelial and endothelial cells. Studies reported that each human RBC expresses near about 2 million ABO blood group antigens other blood cells like T-cell, B cell, plasma cell, and platelets also have ABO antigen that absorbed from the plasma. 
2. ABO locus encodes specific enzyme glycosyltransferase which synthesizes A and B antigens on RBC.
3. For the synthesis of the A/b antigen, a precursor H antigen plays an important role. In RBCs enzyme synthesizes H antigen encoded by the H-locus (FUT1). And in secretion saliva and other enzymes that synthesise is encoded by Se locus (FUT2).
4. The location of ABO locus is on chromosome 9 at 9q34.1-q34.2; this locus contains 7 exons that span more than 18 kb of genomic DNA. Exon 7 contains most coding sequences. Exon 6 contains deletion which results in loss of enzyme activity found in allele O.
5. The difference between the A and B allele occurs by seven nucleotide substitutions, in which four translate into different amino acids in gene products, R176G, G235S, L266M, G268A. at position 266 and 268 determines A or B specific of encoded glycosyltransferase.
6. Difference of O allele from A is because of the guanine position of 261. Deletion cause frameshift which results in the loss of enzyme activity in the O allele

ABO Full Form: Clinical Significance of ABO Antigens

1. A number of diseases have been linked to the susceptibility to a person’s ABO phenotype, a claim also states that gastric cancer is common in blood group A and gastric and diagonal ulcers occur more often in blood group O individuals.
2. A correlation between ABO phenotype and level of blood clotting protein; factor VII and von Willebrand factors indicates that individuals having blood group O contain 25% less FVIII and vWF in their plasma and low level of this factor cause excess bleeding.

ABO Full Form: Clinical Significance of ABO Antibodies

ABO antibodies have clinical significance because of their natural occurrence and highly reactivity

Transfusion Reactions

1. In routine blood typing practices, cross-matching of blood products should prevent the adverse transfusion reaction caused by ABO antibodies error, that can even result in the death of the patients.
2. If any recipient having blood group O is transfused with non-O group, the naturally occurring antibodies in recipient serum binds to their antigen on blood which is transfused and cause intravascular haemolysis which can result in acute haemolytic transfusion reaction. This may result in disseminated intravascular coagulation, shock, acute renal failure, and death.

Haemolyses in Newborn

1. Haemolytic disease in newborns (HDN), most cases arise because of ABO incompatibility.
2. HDN is caused by antibodies ABO exclusively occurs in infant blood group A or B who are born with the mother of group O.
3. This reaction occurs because of the formation of anti-A and anti-B in individuals of blood group O which tend to be IgG type. While, anti-A and anti-B found in the serum of group B and A individuals, respectively, tend to be of the IgM type.
4. Although uncommon, cases of HDN have been reported in infants born to mothers with blood group A2 and blood group B.

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