What is Immunohematology?

Blood transfusion is the best and immediate solution for protecting a patient suffering with chronic health problems due to reduced blood or blood components in circulation.

Replacement of total blood or its components such as blood cells, fluids etc. through intravenous administration is the best and most effective therapeutic tool when judiciously applied.

Transformation of total blood allows transfer of all components of blood such as blood cells platelets, serum proteins etc.

Until 1901 blood transfusion was not very safe, many patients died due to incompatible blood transfusion.

German immunologist Landsteiner in 1901 identified the existence of blood group antigens on human red blood cells, and presence of natural antigens in serum during his experiments with different red cells and plasma factors.

He discovered that blood clumping was an immunological reaction, which occurs when the receiver of blood transfusion has antibodies against the donor’s blood cells.

Karl Landsteiner’s work made it possible to determine blood groups and thus paved the way for blood transfusions to be carried out safely. For this discovery he was awarded Nobel Prize in Physiology / Medicine in 1930.

The disparity in human blood is due to the presence or absence of certain protein molecules called antigens and antibodies.

The antigens are located on the surface of the red blood cells and the antibodies are in the blood plasma.

Different types and combinations of these molecules are present in different individuals. Blood group is inherited from parents. The blood group antigens are inherited according to a simple Mendel System.

There are more than 20 genetically determined blood group systems known today, but the ABO and Rh systems are the most important ones used for blood transfusions.

Not all blood groups are compatible with each other. Mixing incompatible blood groups leads to clumping or agglutination of RBCs which is hazardous for individuals.

According to the ABO blood group system, there are four different kinds of blood groups: A, B, AB and O involving three allelic genes called A, B and O genes. The genes for O antigen are not expressed where as A and B genes are dominant and expressed in the form of antigens on the surface of RBCs. Since both A and B genes are co dominant in the presence of both the genes RBCs express both A and B antigens on their surface.

Thus a person with “AB” blood group is heterozygous with both A and B genes but “O” group person is homogenous genetically with two recessive O genes. A person with “A” or “B” group may be homozygous (AA or BB) or heterozygous (AO or BO).

In fact O blood group individuals produce a substance called “H antigen”, that acts as a precursor for A and B substances. The H locus is located on chromosome 19. It contains 3 exons and it encodes a fucosyltransferase that produces the H antigen on RBCs.

In the presence of A or B genes the precursor is converted into A, B or AB substances and in the absence of A or B genes H substance alone is expressed on the RBCs of the blood of the ‘O’ blood group. In addition to A, B and H substances/ antigens Lea (Lewis a) and Leb (Lewis b) antigens, present on different locus also can be inherited. All the genes code different glycosyl transferases to act on precursor molecules of antigens. If glycosyl transferase adds a fucose molecule to the terminal glucose residue, the precursor becomes H substance and presence of H substance alone on the RBCs makes the blood group as ‘O”.

In blood group A glycosyl transferase links N-acetyl-D-galactosamine to the terminal galactose of H substance. But in blood group B glycosyl transferase enzyme adds one more galactose to the terminal galactose of H substance. In the presence of both A and B genes glycosyl transferase act in both ways on the precursor molecules, and results in the production of both A and B substances.

Landsteiner and Wiener in 1937 identified another important agglutinogen “Rh” on RBCs. They are expressed as part of a protein complex in the RBC membrane. This complex is only expressed in cells of the erythroid line, and therefore Rh antigens are only expressed in RBCs.

Since antibodies raised against the Rh antigens reacted with RBCs of Rhesus monkey, the newly discovered RBC antigens are referred as “Rhesus antigens” The blood with Rh antigens on RBCs is referred as the Rh+ (Rh positive) blood group and the blood without the Rh antigens on RBCs is considered as Rh- (Rh negative) Unlike A, B antigens Rh antigens do not have any natural antibodies.

Hence development of Rh antibodies in Rh- individuals occur only after interaction with Rh-i- antigens. Being the main cause of hemolytic disease of the newborn (HDN) it has remained of primary importance in obstetrics. In general majority (about 85%) of people are Rh+.

Significance of the Rh blood group is related to the fact that the Rh antigens are highly immunogenic. The complexity of the Rh blood group antigens begins with the highly polymorphic genes that encode them. There are two genes, RHD and RHCE that are closely linked. They are 97% identical, and are located next to each other on chromosome 1.

Numerous genetic rearrangements between them has produced hybrid Rh genes that encode a myriad of distinct Rh antigens. The most important antigen produced by one of the dominant genes D is “antigen D”. Antigens produced by RHCE are not much important immunologically. Rh- blood group shows the presence of homozygous condition (dd) for receive gene.

The Rh” factor is widely studied in reproduction immunology because; in normal conditions Rh factor does not have any natural antibodies. Hence there may not be any transfusion reactions if Rh⁺ blood is transferred to Rh individual. After interaction with the Rh⁺ antigens, the Rh” person’s immune system starts producing antibodies against Rh⁺ antigens.

Majority of antibodies formed against the Rh antigens are of the IgG type. Since the production of antibodies takes time, delayed type transfusion reactions may develop in the recipient.

If the same individual receives Rh+ blood again, an immediate transfusion reaction develops due to the presence of antibodies produced following the previous exposure to Rh + antigens.

In women Rh” factor plays a very important role during pregnancy. The second baby of Rh” mother and Rh⁺ father face several complications due to the formation of antibodies against Rh+ antigens of first baby. (Since Rh⁺ genes are dominant in nature, most probably babies of Rh~ mother and Rh⁺ father get Rh⁺ blood group) The complications due to the presence of Rh⁺ antibodies may sometimes be severe ‘Resulting miscarriage or death of the baby through “erythroblastosis fetalis” Development of Coombs test in 1945, the advent of transfusion medicine, and understanding of hemolytic disease of the newborn led to the discovery of more blood groups.

Besides A, B, O system other groups such as Kidd, Kell, Duffy, MNS and Lewis groups have been identified. But these groups are of no importance for blood transfusion, but have got medical importance.

A total of 30 human blood group systems are now recognized by the International Society of Blood Transfusion (ISBT). A complete blood type would describe a full set of 30 substances on the surface of RBCs, and an individual’s blood type is one of the many possible combinations of blood-group antigens.

Continuity of work in blood transfusion immunology suggests the presence of about 300 different antigens on the RBCs that are ignored during transfusion due to the absence of natural antibodies and their poor antigen city.

Delayed transfusion reactions even with compatible blood transfusion occur in patients who have under gone several transfusions. These delayed reactions are due to the presence of minor blood group antigens.

The antibodies that are produced after exposure to minor blood group antigens are of IgG type. IgG antibodies are not very good in complement fixation, hence complement mediated RBC lysis is incomplete and the completion of RBC destruction takes place in extravascular sites.

Phagocytes and macrophages help in the completion of RBC lysis. The extravascular lysis of RBCs prevents release of haemoglobin into the circulation or plasma. The pre transfusion testing (cross matching) helps to detect potentially harmful antibodies in a patient before transfusion and to select red cell units that will not react with them.

The natural antibodies of A, B blood group antigens are mostly IgM type. Binding of IgM antibodies with the antigens present on the surface of RBCs leads to activation of compliment system and destruction of the target cell.

Hence release of haemoglobin into the circulation takes place leading to various health problems such as jaundice, haemoglobinuria, renal failure, fever or even death in severe conditions.