ABO and Rh TypingA blood group is usually an inherited character of the erythrocyte (red blood cell). Researchers and scientists have identified 29 blood group systems with a total of 262 antigens in humans. Antigens may be defined as molecules that are found on the red cell surface and are capable of producing an antibody. The red cell antigens can be of various forms; they can be glycolipids, carbohydrates, proteins and glycoproteins. Thus, its presence on the membrane of the red cell ensures that the cell integrity is maintained and improves on the functions of the membrane.
The first blood group system that was discovered by Karl Landsteiner in 1901 was the ABO system and this blood group system play a major role in transfusion medicine (Daniels, 2005). The ABO system can further be grouped into four major groups: A, B, AB, and O. The antigens that are found in the ABO system are usually made of type II oligosaccharides that H transferase adds a fucose that results in the formation of the H substance. Thus, the H substance contributes in the formulation of antigens A and B.
Under the condition of the inherited gene, the transferase of A and/or B will then include a terminal sugar to the H substance. Hence, the A transferase will add a D-N-acetyle-galactosamine (Gal-NAc) while the B transferase will add the D-galactose (Gal). Not only can the ABH antigens can be found on the blood cell but can be found in many cells in the body and especially the body fluids. Even though the ABH antigens are found in body fluids and various cell membranes, the normal physiologic function of the glycoprotein is not clear.
For example, those individuals who do not have any ABH antigens commonly referred to as Bombay phenotype usually have normal functions and survival of the red cell. The antigen(s) that are present on the membrane of the red blood cells (phenotype) determines the name of the blood group as illustrated in Table 1.The benefit of the ABO system in transfusion medicine lies in the fact that the antibodies against the ABH antigens are usually present in the plasma or serum of a health person and thus does not require and form of red cell stimulation; it is usually produced because of environmental stimulation.
Moreover, the individuals who lack A or B antigen in their plasma, these individuals will have IgM antibody in their plasma. In most instances, these antibodies through activation of the classic complement pathway can cause intravascular haemolysis, which may result in fatal transfusion reactions due to incompatible blood transfusion or may result to hyperejection in case of transplantation of body organ. Thus, this illustrates the importance of ABO cross matching before any transfusion can take place. Determination of the presence or absence of the antigens and antibodies can be achieved by laboratory techniques such as serological blood grouping.
The test for antigens (testing red cells) is forward test while for the antibodies (testing antiserum) is reverse test, which in most cases is a confirmatory test. Another important aspect in the blood group after the ABO is the Rh blood group system. Rh blood group system is the most polymorphic blood group system. This blood group system has five major antigens D, C, c, E and e that are encoded by two genes RHCE and RHD that are found in chromosome 1.
The RhD is a crucial antigen since its absence will lead into the formation of the antibodies that will be against the D antigen. It is capable of inducing immune responses in 80% of D-negative persons who are transfused with 200 ml of D-positive blood (Learoyd, 2005). Thus, it is important for RhD typing to be done before any transfusion can take place; for example, D-negative patients receive only D-negative blood products.
The production of the Rh antibodies may be attributed to immune response that is attributed to pregnancy state. RH antibodies can cross the placenta since it is usually IgG and may contribute in causing haemolytic disease of the foetus; hence, it is paramount to determine the Rh phenotype of D-negative mothers’ foetus.