Blood group systems

This section will cover the major blood group systems, some of the other blood group systems and will also include information on HLA and notes on platelet antigens. Although some references will be made to the molecular structures, the detailed molecular structures and recent advances in DNA technology are not within the scope of this publication. By the end of the section, the student should be able to describe the following in simple terms: Blood group terminology Functions of blood groups ABO and H blood group systems ABO grouping inheritance of ABO blood groups ABO blood group frequencies production of ABO antigens H-deficient phenotypes and Bombay Oh phenotype Subgroups within the ABO system ABO system antibodies Clinical significance of ABO system: clinical significance in transfusion clinical significance in haemolytic disease of the fetus and newborn Lectins (plant agglutinins) ABH secretion Unique features of the ABO system Rh blood group system Rh genetics and inheritance molecular studies terminology frequencies Rh typing Rh antigens Clinical significance of Rh system clinical significance in transfusion clinical significance in haemolytic disease of the fetus and newborn Unique features of the Rh system Other major blood group systems (MNS, P, Kell, Duffy, Kidd, Lewis, Lutheran): date of discovery well known antigens antigen frequencies antibody characteristics significance in transfusion significance in haemolytic disease of the fetus and newborn features and practical application MNS P Kell Duffy Kidd Lewis Lutheran I blood group system Additional blood group systems/collections/antibodies reacting with high and low frequency antigens Polyagglutination Human leucocyte antigen system disease association transplantation transfusion pregnancy parentage testing Human platelet antigen systems Fetomaternal alloimmune thrombocytopaenia. Currently, 29 different blood group systems are known, nine of which are considered to be the major blood group systems. In addition there are various blood group antigens that have been allocated to collections, low incidence antigens (700 Series) or high incidence antigens (900 Series) according to the International Society of Blood Transfusion (ISBT) Committee on Terminology for the Red Cell Surface Antigens, which is the committee responsible for the terminology and allocation of antigens to the appropriate system. There are strict criteria for: The allocation of blood group antigens to a new blood group system (the antigen must be shown to be an inherited character defined by a human alloantibody, the gene encoding it must have been identified and sequenced, and the chromosomal location must be known). The allocation of a new specificity to an existing system. The establishment of a collection (requires two or more antigens that are related serologically, biochemically or genetically, but do not fulfil the requirements for a blood group system). Inclusion into the 700 series (incidence of less than 1% of the population and distinct from other systems and collections). Inclusion into the 900 series (incidence of > 90% in most populations tested and distinct from other high frequency antigens). Various terminologies have been used to describe the different blood group systems and their antigens and respective antibodies ever since the ABO blood group system was first discovered in 1900. In 1980 an ISBT committee was tasked to devise a genetically based numerical terminology for red cell antigens. This is an ongoing process and new information regarding the antigens and apparent new antigens are reviewed by the committee on a regular basis. The numerical terminology was primarily designed to facilitate computer input. The alternative/popular terminologies are commonly used, both in everyday communication, in the laboratories and in publications. Note: The term group or type can be used interchangeably when discussing blood grouping or typing. Further notes on Rh terminology will be found in the Rh section. The number of antigens per blood group system, collection, and series varies tremendously from 1 in the P system to more than 30 in the Rh and MNS systems. Table 6·1 shows the major blood group systems. Note that H antigen is in a separate system, H Blood Group System 018, and is not part of the ABO system. Table 6·2 gives information on the blood group systems other than the nine major systems. The structures of the different blood group systems and their antigens have been studied extensively, and a wealth of information is available particularly since the development of molecular genetic techniques. Less is known about the actual function of the blood groups. The red cell is a complex structure, and the red cell membrane contains many surface proteins that are anchored to the membrane or cross the lipid bilayer one or more times. Many of the proteins on the surface of the red cells are polymorphic and carry the different blood groups. The functions of some of the red cell membrane proteins have been identified, and other functions have been deduced from the structures of the protein. Studies on the null phenotypes which occur in most blood group systems have contributed to the information. The ABO, H, I, P1 and P blood groups are carbohydrate structures on the red cell membrane glycolipids and glycoproteins and less is known about their function. Table 6·3 provides a list of the functions of the blood groups. The format for describing the blood group systems in this publication is as follows: Antigen frequencies are given as approximate percentages, simply to make them easier to remember. This will sometimes result in the total being slightly more or less than 100%. The figures for black people apply to published data, or to surveys performed in southern Africa. When an antibody is stated to cause haemolytic disease of the fetus and newborn (HDFN) and when it has an optimum reaction temperature of +37°C using the IAT technique, it is presumed to be a type IgG antibody. When an antibody is described as a saline agglutinin and when it reacts best at cold temperatures, it is presumed to be a type IgM antibody. The two most clinically significant blood group systems are the ABO and the Rh blood group systems. Although the ABO and H are two different blood group systems genetically they will be described together as they are closely related, both at the biochemical and phenotype level. The ABO system is the most important blood group system in transfusion therapy and was the first blood group system to be discovered. This great contribution to medicine was made by Landsteiner in 1900 when he observed that 'the serum of healthy humans not only has an agglutinating effect on animal blood corpuscles, but also on human blood corpuscles from different individuals'. The following year, in 1901, Landsteiner was able to recognize two antigens on the red cells by separating and mixing the cells and sera of several individuals. He called the antigens A and B. Those individuals with the A antigen on their red cells were called group A; those with the B antigen, group B. Many individuals lack the A and the B antigens and were termed group C, which was later termed group O. The least common group, called AB, was found by several of Landsteiner's students in 1902. Group AB individuals have both A and B antigens on their red cells. Landsteiner found that the serum of an individual always contained antibodies to the antigen, which was not present on that individual's red cells. Thus, group A individuals will have anti-B antibodies in their serum and group B individuals will have anti-A antibodies in their serum. These facts became known as Landsteiner's Rule which states, '(In the ABO system) the antibody to the antigen lacking on the red cells is always present in the serum or plasma.' The regular presence of anti-A and/or anti-B antibodies means that it is critical for patient safety and good transfusion practice that ABO groups are performed, recorded and interpreted correctly. ABO incompatibilities are responsible for the majority of serious and/or fatal transfusion reactions and are usually caused by technical, clerical or administrative errors. The ABO system is unique in that whenever the A or B antigens are not present on the red cells, the corresponding antibody is present in the serum/plasma. Anti-A and anti-B alloagglutinins are therefore often referred to as being 'naturally occurring'. ABO grouping can therefore be performed by: Typing the red cells for the presence or absence of the A and/or B antigens. This is known as forward grouping. Testing the serum/plasma for the presence or absence of anti-A and/or anti-B antibodies. This is known as reverse grouping. The forward and reverse grouping results should correlate; refer to Landsteiner's rule. The general population can then be divided into four ABO groups as shown in Table 6·4, based on the forward and reverse grouping. It should be noted that the anti-A,B produced by a group O individual is different from anti-A+B, which is a mixture of anti-A from one source and anti-B from another source. Anti-A,B detected in group O individuals is an antibody that will react with group A and group B cells. The monoclonal anti-A,B reagents available commercially will detect the weak A group Ax. The ABO genes are located on chromosome number 9. The inheritance in the ABO system is controlled by various alleles: A1, A2, B and O and a series of rare alleles A3, Ax and Am (etc). The O allele (which does not produce an antigenic product) is recessive to the A and B alleles, which are co-dominant. The ABO phenotype is shown by ABO grouping laboratory tests on a blood specimen but this is not necessarily the genotype of the individual. For example, blood of phenotype A1 can represent one of several possible genotypes: A1A1, A1A2, A1A3, A1Ax, A1Am (etc) or A1O. Although each individual has two ABO genes, serological tests do not reveal the O allele in the A and B phenotypes, nor can an allele producing a weak form of A be recognized if an allele higher in the scale of A antigen production is simultaneously present. The genotype can however be determined by DNA analysis of the gene or may be determined by family studies. Table 6·5 shows ABO blood group/phenotype with possible genotypes (simplified), including some of the rare alleles. Many populations have been studied world-wide, and it has been shown that the frequency of the ABO blood group genes varies between different populations. Note the variation shown in Table 6·6 as an example of ABO blood group distribution. The ABO red cell antigens expressed on the red cells are dependent on the presence of both the H gene, and the ABO genes. The loci for the H and ABO genes are not linked although they are related and they are therefore two separate blood group systems. The H, A and B genes do not code directly for red cell antigens, but for enzymes known as transferases. The H-transferase adds the sugar l-fucose to a precursor substrate, which is a carbohydrate chain already present on the red cell membrane. Once this has been performed, the A- and B-transferases can act. The A-transferase adds another sugar called N-acetyl-D-galactosamine, which results in the expression of A antigen on the red cells. Similarly, the B-transferase adds the sugar D-galactose and the cells then also express B antigen. These red cells type as group AB. Group A antigen is expressed when H and A transferases are the two enzymes present; group B antigen is expressed when the H and B transferases are the enzymes present, and in group O only H transferase is present. Figure 6·1 shows a simplified diagram to indicate the structural differences in the molecules that result in ABH antigen expression. Simplified diagram to indicate structural differences in ABH antigen composition. The expression of A, B or AB antigens results in a relative masking of the H antigen. Thus, A1, B or A1B cells express only small quantities of H. The A2 allele is less effective than the A1 allele in masking the H determinant, and A2 cells therefore have considerably more H antigen and less A antigen than do A1 cells. The O allele in double dose leads to the expression of H specificity alone, resulting in group O individuals having abundant H antigen. The amount of H antigen that is present in red cells of different groups, from left or right in decreasing order, is as follows: most H antigen: O → Weak A → A2→ A2B → B → A1→ A1B → least H antigen. The A, B and H antigens are detectable long before birth. The ABH antigen strength usually peaks at between 2 and 4 years of age and then remains relatively constant in most individuals. It may not be possible to distinguish between group A1 and A2 groups at birth as the antigens may not yet be fully expressed. Although the ABO and H are two different blood group systems genetically (H Blood Group System: Number 018), they are closely related at the biochemical and phenotype level. The H-deficient phenotypes are very rare and include a total deficiency in H antigen (Bombay or Oh phenotype) or a partial deficiency (Parabombay). The Bombay or Oh phenotype, in which the cells lack the H antigen, arises when the individual has not inherited the very common gene H. As there is no H gene present, the H-transferase enzyme is absent. The precursor substance remains unchanged and no molecules of l-fucose are present on the precursor substrate in the red cell membrane. The individual may have inherited the A and/or B genes, which code normally for the appropriate transferases. However, without the single terminal carbohydrate (sugar) l-fucose at the end of the substrate protein, these transferases are non-reactive. The Bombay Oh phenotype therefore results when the individual has inherited a double dose of a rare recessive allele, known as h. The gene h does not code for H transferase. Individuals who have inherited HH or Hh genes produce normal amounts of H transferase. Bombay Oh individuals are extremely rare. Those who were originally shown to carry the trait were Indians whose ancestors originated in Bombay, hence the name Bombay Oh. Their cells are not agglutinated by anti-A, -B, -A,B or -H. Bombay Oh individuals have powerful anti-H, -A and -B antibodies in their serum. They should therefore be transfused only with type Oh blood. Table 6·7 shows the difference between blood group O and blood group Oh. About 10 years after the discovery of the ABO groups, the subgroups of A were described. It was observed that not all group A bloods tested gave similar results with anti-A. Those unusual bloods which gave weaker reactions became known as 'weak A'. Furthermore, it was realized that the common A antigen occurred in two forms: A1 and A2. Later studies on transferase enzymes of group A1 and group A2 individuals showed that less antigenic sites are produced in group A2 individuals as the enzyme is less effective in converting the precursor H substance into A antigen. However, with the use of monoclonal anti-A blood grouping reagents, little if any difference between the reactions of group A1 and group A2 cells can be detected in the laboratory. About 99.9% of all group A bloods from white people and about 96% of group A bloods from black people, in one survey, were either group A1 or A2, with group A1 being more frequent than group A2 in both populations. A higher incidence of 'weak' A was detected in the black people. The anti-A found in the serum/plasma of group B individuals consists of two separate antibody specificities, anti-A and anti-A1, the latter being specific for the A1 type. Group A or AB individuals who lack the A1 component may form an irregular, cold reacting anti-A1 antibody in their serum. The lectin Dolichos biflorus or monoclonal anti-A1 reagents, are usually used to type red cells for the A1 antigen. A number of further subgroups of group A and subgroups of group B also occur. The subgroups are caused by genetic variations that result in a variety of weakened expressions of the antigens. The subgroups cannot be detected when inherited with a normal A or B gene. The subgroups may be detected in the laboratory when weak or unexpected negative results are obtained with the forward grouping and/or anomalous results with the reverse grouping. The term weak A covers a large range of reactivity, some bloods giving clear (although weak) results and other bloods giving such weak reactions that detection may prove difficult. The weak A types include A3, Am, Ax, Abantu, Ael and Aend. Weak A type A3 gives a characteristic mixed field agglutination pattern when tested against polyclonal anti-A and anti-A,B antiserum. However, stronger agglutination is observed when using most monoclonal blood grouping reagents. Table 6·8 compares reactions between group A and subtypes. Anti-A1 may or may not be produced, although it is often produced by group Ax individuals. Note that type A3 shows mixed field agglutination with anti-A and anti-A,B and that type Ax reacts macroscopically with monoclonal anti-A,B. In weak AB types, the B antigen may occur with any of the weak A subtypes. Subgroups of group B are suspected when the expression of the B antigen is weak or cannot be easily detected. Subgroups of B are rare and are found mainly in populations where the frequency of group B is high as in African and Chinese populations. The subgroup cannot be detected if inherited with a normal B allele. The weak B subgroup may be inherited with an A allele giving rise to a normal A, weak B phenotype, ABweak. This is caused by the action of enzymes which break down the group A antigen N-acetyl-d-galactosamine to galactosamine which is similar to the structure of the group B antigen immunodominant sugar (d-galactose). Some anti-B reagents react with this acquired group B antigen and a group A individual could be incorrectly grouped as group AB. It is important to select anti-B grouping reagents carefully to ensure that they do not react with acquired B cells. The condition is rare but may be associated with gastrointestinal bacterial disease or caused by bacterial contamination of a blood sample. The individual's red cells often become polyagglutinable. Healthy adults who lack a particular ABO group antigen on their red cells usually have the corresponding antibody in their serum as a result of stimulation from the environment, such as exposure to certain bacteria or food that may contain A-, B- or H-like substances. Additional exposure to the antigen can result in more potent antibody formation. This immune response may be prompted by: Presence of ABO incompatible fetal red cells in the maternal circulation during pregnancy and at delivery of A or B that may be found in either in the or in the The transfusion or of ABO incompatible red cells. that are weak or in adults may occur in weak subgroups of A or with low of serum age or with are not normally detected in newborn and after of to exposure to A- and in the ABO antibodies are detected in blood they are usually agglutinating IgG antibodies of maternal Table shows the grouping results of a group B newborn and an of of Individuals of phenotypes A2, A2B and weaker subgroups of A may have anti-A1 in their serum/plasma. This antibody will react with group A1 cells. Anti-A1 is usually a cold reacting which is not of clinical As it reacts it is to cause transfusion reactions or It a clinically significant antibody. Anti-A1 in the serum/plasma of about of A2 individuals and of A2B individuals. The antibody more as the strength of the A antigen therefore weak A weak individuals are more to have anti-A1 in their serum/plasma than A2 individuals. As individuals of group A1, A1B and B have very little H antigen on their red cells, they sometimes in their serum. This antibody can be recognized by reaction with group O red cells, a weaker reaction with group A2 cells and usually a to react with group A1 or group B red cells. of this which is by individuals who are not H is usually and in the serum of some group A1, B and A1B individuals. Group O serum is not a simple mixture of anti-A and It cannot be by using either group A or group B cells and is a antibody known as anti-A,B. Various have been to this and it that the anti-A,B produced by group O individuals a structure common to both A and B antigens. all the blood group systems, the ABO is the most important in transfusion the alloagglutinins are normally present in the absence of the corresponding antigen. reactions when incompatible bloods are mixed with each not only in but also in an transfusion of group A blood into a group B patient may be the anti-A in the blood of the group B patient react with the group A cells, agglutination and of the cells and a haemolytic transfusion Individuals of blood group O are termed blood as their blood can usually be into of other ABO groups group They do not have A or B antigens on their red cells to react with antibodies within the circulation of the Their anti-A,B antibodies are not to the red cells of the if blood is the alloagglutinins are ABO However, blood from group O which contains immune anti-A and/or -B, may only be transfused into group O group This is these have potent alloagglutinins with which may cause haemolytic reactions when into with A, B or AB antigens on their red cells. The of anti-A and anti-B in blood group O blood can be by the transfusion of group O red cell In it is to a patient with blood of the ABO group and to of group O blood for group O and for group A1B individuals are They have both A1 (the of A and B antigens on their red cells. They usually lack ABO antibodies in their serum. of the ABO blood group antigens on the cells, there are no antibodies in the group AB to react with that the alloagglutinins in the of the are low they will not be to the A and B antigens on the red cells of the group AB Some individuals produce high anti-A and/or of a mixture of IgM and IgG with characteristics in the presence of This immune anti-A and/or -B in can cause ABO with of although the fetus is in ABO within a of birth. for more information. human and animal red cells. have been for these and the latter term used for those which red cell Note that these are not antibodies. Some are described in The most lectin anti-A1 is found in Dolichos the A1 and A1B it reacts less with A2 cells and very with A2B cells. The can therefore be by as a specific anti-A1 can be from the of or the common is for the of group O group O In addition to being present on the red cells, A, B and H antigens are present on most other cells as Blood group of the ABO group as the red cells may also be found in the serum/plasma and are detectable in the and other of most individuals. The is controlled by the and genes at the is the gene and is responsible for the secretion of H. of the general population ABH the form of in in all their The ABO group of a may be determined by testing the to the presence or absence of A, B and H The of the population are termed Table shows the antigens according to ABO The critical unique of the ABO blood group system is that other blood group systems, the anti-A and/or anti-B alloagglutinins are present in the serum/plasma of healthy when the corresponding antigen is absent. As the ABO antigens are the the ABO group must be considered in Some the must be ABO In ABO is of the lack of expression of ABO on cells, but to be such as of the red cells or Note: red cell typing may be The of blood transfusion practice is to blood of the ABO It is critical that the ABO group on all from a patient or a is as ABO group can have fatal The discovery of the Rh groups by Landsteiner and in together with the of and in the discovery in the blood grouping field since Landsteiner described the ABO system in 1900. In and described the of a fetus a haemolytic reaction when transfused with blood. The who some antigen, must have been by fetus that this antigen, having inherited it from the When the ABO blood was the maternal antibody with this antigen on red cells. In Landsteiner and having with the blood of a discovered that the resulting antibodies agglutinated not only the red cells but also the red cells of about of white people. Later showed that the red cell antigens detected by the antibody and the animal antibody were not and to two different blood group systems. The blood group system detected by the antibodies is known as Rh and the antigen is called The antigen originally described by Landsteiner and is in the blood group system. The two systems are serologically, biochemically and genetically different from one The for the Rh genes is on chromosome 1 and is linked to the gene for The for is on chromosome It was realized that the Rh antibodies produced in humans were not as simple as they first and that many sera contained antibodies of more than one Many related antigens were found by in and in the of This to the discovery of the major Rh antigens, C, and The Rh blood group system has been shown to be one of the most complex blood group systems with more than antigenic having been described. individuals are either or and the expression or absence of the antigen on the red cells results from the presence or absence of an gene. A individual may two genes, one from each or one gene from either The two of antigens and and and are controlled by the various genes. The and alleles are inherited as a gene complex or different were for the genetics and inheritance of the Rh blood group system but these have been by molecular genetic studies. In the the results of and in that some reactions were and he therefore that there were of alleles and and and that the genes, if must be very closely for no been The was and although it not in the Rh system it was to Although this that antibodies to all the antigens described are able to be in individuals lacking the corresponding antigen, no has ever