ABO Blood Group System
Introduction
Immunohematology is an application of the principles of immunology to the study of red cell antigen and their corresponding antibodies in blood.
On the basis of blood groups, human being can be divided into various categories. Blood from one category person should not be transfused into another category person. Mismatching of blood groups may result into the recipient’s death. The erythrocytes of a person contains some antigen, called agglutinogens. When these antigen are allowed to react with the corresponding antibodies called agglutinins, antigen-antibody reaction are produced. The agglutinogen are of different kinds. For each type of agglutinogen, there is specific agglutinin, which can produce the antigen-antibody reaction. The agglutinations can be classified into some known systems such as ABO, Rh, MNS, Lutheran, Kell, Duffy etc. Karl Landsteiner discovered the fundamental principles of blood grouping in 1900. He is a is associated with the discoveries of Abu system 1900 and Rh System discovered by Karl Landsteiner and Winner at 1940, which are the most important blood group system.
ABO type
The ABO Blood Group System is determined by the presence or absence of the A and B antigens on the surface of the red blood cells. In addition to the A and B antigens, there are also antibodies in the blood plasma that react against the antigens that are not present on the red blood cells. For example, people with blood type A have the A antigen on their red blood cells and antibodies against the B antigen in their plasma.
If incompatible blood is transfused, the antibodies in the recipient’s plasma will react with the antigens on the donor’s red blood cells, causing them to clump together (agglutination) and potentially block blood vessels, leading to serious health complications.
Rh System
The Rh system is determined by the presence or absence of the Rh antigen on the surface of the red blood cells. Rh-positive individuals have the Rh antigen, while Rh-negative individuals do not. The Rh system is important in pregnancy because if a woman is Rh-negative and her fetus is Rh-positive, there is a risk of Rh incompatibility if the fetus’s blood enters the mother’s bloodstream during pregnancy or childbirth. This can lead to hemolytic disease of the newborn, which can cause severe anemia and other health problems.
Other Blood Group Systems
There are over 30 other blood group systems, each with its own set of antigens and antibodies. Some of the other important blood group systems include:
- Kell system: Determines the presence or absence of the K antigen on the surface of the red blood cells. Incompatible transfusions can lead to hemolytic transfusion reactions, and Kell antibodies can also cause hemolytic disease of the newborn.
- Duffy system: Determines the presence or absence of the Duffy antigen on the surface of the red blood cells. The Duffy antigen is a receptor for the malaria parasite, and people who lack the Duffy antigen are resistant to certain types of malaria.
- Lewis system: Determines the presence or absence of the Lewis antigen on the surface of the red blood cells. The Lewis antigen is also found in other tissues and is involved in the immune response.
Blood typing is an important aspect of blood transfusions and can help prevent adverse reactions. Blood banks carefully screen donated blood and perform blood typing before transfusing blood to patients.
PLATELET Count (PLT) Blood
Red Cell Antigens
The red cells consist of hemoglobin lipids, other proteins, enzymes and electrolytes in a stroma content by the cell membrane. The stroma is a combination of lipoproteins, glycoproteins and mucopolysaccharides. Red cell antigens are the broad group factors, which reside on the surface of the membrane. These antigen are mostly lipoprotein or glycolipids. The antigenic characters of red cell are inherited. The antigen detection of all blood groups is based on the principle of hemagglutination. It is a serological reaction of red cells with the corresponding antibody, as determined in the laboratory.
Natural Antibodies
These can be present naturally in the circulating plasma, from birth, without any known antigenic stimulation. These antibodies are generally of IgM type.
Immune Antibodies
These are produced when the red cell carrying corresponding antigen enter into an individual, who normally lack of antigen. (e.g. anti-D). These are usually IgG type antibodies and react best at body temperature 37 degree Celsius.
Human Blood Group System
There are nearly 300 blood group system so far discovered. The ABO Blood Group System and Rh are the major, clinically significant and the most important of all the blood group systems. All people (with few exceptions) of ABO system can be divided into 4 major groups in this system and they are A group, B group, AB group and O group. This depends on the reactions obtained by mixing their red blood cell with 2 different reagents known as anti-A and anti-B. For example in cell grouping or Forward grouping, by using known anti-A and anti-B, the results obtained are as follows.
| Sr. No. | Red cell sample | Reagent | Reaction | Cell group |
| 1 | Red blood cells Red blood cells | Anti-A Anti-B | Agglutination No agglutination | A group |
| 2 | Red blood cells Red blood cells | Anti-A Anti-B | No agglutination Agglutination | B group |
| 3 | Red blood cells Red blood cells | Anti-A Anti-B | Agglutination Agglutination | AB group |
| 4 | Red blood cells Red blood cells | Anti-A Anti-B | No agglutination No agglutination | O group |
ABO Blood Group System
The presence of A, B or O antigens on the red cells is determined by the inheritance of the allelic genes A, B & O on chromosome 9, which are inherited in pairs as Mendelian dominant’s. The cellular expression of A and B antigen is determined by a further gene,, the H Gene. It is inherited independently. The H gene codes for an enzyme that converts a carbohydrate precursor into H substance.
The A and B genes code for specific enzymes, glycosyl transferase which convert H substance into A and B antigens. The O gene produce an inactive transferase so that H substance persists unchanged in Group O. About 20% of Group A and group AB subjects belong to Group A2 and Group A2B respectively, and the reminder belong to Group A1 and Group A1B.
Almost all individual A, B, O and AB. possess an H gene and therefore produce H substance (antigen). H antigen is the chemical processor to A and B antigen. When an individual produces H substance, it will be used to synthesise A substance if he has an A gene. B substance (antigen) will also be synthesised from H gene is present. Since O people lack A or B genes, they cannot produce A or B antigen, but they retain the H substance on their RBC which behave as a very weak antigen.
Rarely an individual is born lacking an H gene. This person genetically a receive hh can not produce H substance and therefore is unable to produce either A or B antigen even if he posses the A or B genes. His RBC therefore lack H or B antigens and he is described as a Bombay or Oh type individual. He produces, consequently, anti H, anti A, and anti B natural agglutinins. In the Oh Bombay phenotype, the H genotype is ‘silent’ (hh) and no H-transferase is produced. Hence no H substance is made and therefore A and B genes, If present can not be expressed. These individuals have anti-A and-B and anti-H in their blood all active at 37 degree Celsius and can be safely transfused with other Oh blood.
ABO Antibodies
ABO antibodies are usually IgM, which are cold reacting. These antibodies do not cross the placenta and can bind complement. The majority of anti-A from a Group B individual and Anti-B from a Group A individual contains IgM antibody predominantly, with minor amounts of IgG or IgA present. The immune form of anti-A or anti-B can be produced by individuals exposed to foreign red cell stimulation, either by transfusion or pregnancy.
Serum from group O individual contains not only anti-A and anti-B but also anti A,B. Anti-AB from group O individual has been reported to be a mixture of IgG and IgM or IgG, IgM and IgA. Anti-A, B will cross the placenta more frequently than anti-A or anti-B, confirming the presence of IgG. The immune form of anti-A,B can be produced by O individuals exposed to A or B red cells by either transfusion or by pregnancy. These anti-A, B immune antibodies are predominantly IgG.
Inheritance of the ABO Groups
The theory of the inheritance of the ABO blood groups was first described by Bernstein in 1924. Each Individual inherits one ABO gene from each parent and these two genes determine which ABO antigens are present on the red cell membrane.
One position, or locus, on each chromosomes number 9 is occupied by an A, a B or an O gene. The O gene is considered an amorph. No deductible antigen is produced in response to the inheritance of this gene. The designation A or B referred to phenotypes, whereas AA, BO and OO denote genotypes. In the case of an O individual, both phenotype and genotype are the same, because that individual would have to be homogeneous for the O gene.
