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Human blood can be classified into different blood group systems, e.g. ABO blood group, MN blood group and Rh blood group.
All of these blood groups in man are under genetic control, each series of blood groups being under the control of genes at a single locus or of genes that are closely linked and behave in heredity as though they were at a single locus.
1. ABO Blood Group:
If we consider the immunity reactions in connection with the ABO blood group then we find that some of them contain ‘natural’ antibodies against some others.
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Following is the antibody content of ABO blood group:
Similarly, if we consider the presence of antigen in the red blood cells of different ABO blood group people, then we find:
Due to the presence of different antigens and antibodies in the blood groups of A, B,
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AB and O, all types of blood cannot be mixed together because of their agglutination reaction as following:
When a blood transfusion is made, it does not harm if the donor’s blood contains antibodies against the recipient’s for the donor’s blood is small in amount compared with the total volume of the recipient’s and, therefore, the antibodies are diluted.
But it would do harm if the recipient’s blood had the antibodies, since now the amount of antibody is relatively large. A person of blood group O, for example, could not be a recipient of blood from any other group but his own, since his serum agglutinates all corpuscles but his own, though he might be a donor for any group since nobody’s blood contains antibodies against his corpuscles.
Genetics of the ABO Blood Group:
We do not know which of the four blood groups is the normal one. In genetics it is generally accepted that individuals with normal traits are the most numerous than in any others. For easier understanding, if we consider the O group as the normal, then A and B group arose from O group as a result of two dominant mutations (one for each group), the mutant gene can be given the symbols A and B, respectively. Both of these genes arose in the same locus from one of the normal genes in O group.
If we designate the normal gene by using the symbol +, then three genes +. A, and B occupy the same locus and are multiple alleles. Since + gene is a recessive, so O group must be homozygous for +/+, and since the A and B mutant genes are dominant one, so the combinations for A group either A/A or +/A and similarly for B group, B/B or +/B. Blood group AB, on the other hand, is always the
hybrid, A/B (This is an example of the phenotypical expression of co-dominance).
Some geneticists also proposed that the inheritance of A, B, AB and O blood type in man is determined by a-series of three allelomorphic gene of which i for neither antigen, IA for antigen A, IB for antigen B. IA & IB show complete dominance over i.
Sub-Divisions of A, AB and B Blood Groups:
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The blood corpuscles of A blood group have been subdivided into two sub-groups known as A1and A2 but of these two subgroups A2 is less common. It has been found that A1 corpuscles are not agglutinated by A2 serum, nor the vice versa; but both A1 and A2 corpuscles are agglutinated by B serum and O serum.
It has also been further noted that two more sub-groups of A (apart from A1 and A2) have been identified which are A3 and A4but both of these groups are rarer than A2. Each of the A sub-groups is determined by a separate gene and the genes for all the four sub-groups are alleles.
Similarly, the group B serum contains at least two kinds of antibodies, one agglutinates the corpuscles of both A1 and A2 group and other agglutinates only A1. AB blood group has also been divided into A1B, A2B, A3B and A4B.
So, the gene ‘I’ is a multiple allele (which determines the antigen production) and can produce 15 genotypes and 10 phenotypes of blood groups, which are:
Mode of Inheritance:
If both of the parents in a given family are of O blood group, all the children of them must have O group of blood like their parents. If on the other hand, both of the parents are of a group and both happened to be hybrid (A/+) then they may have some children with O blood group.
So, in this way, if we know the blood groups of a child and his/her mother then we can legitimately claim or test the probable blood group of the child’s father.
Following table is the summarised form medicolegal application of the blood groups :
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The following table (Table 13.1) is the mode of inheritance of blood group to the children from the parents:
Special Genetic Cases of ABO Blood Group:
It has been’ found that some persons have A or B antigens in their body secretions also (from eyes, nose, salivary gland and mammary gland) and are known as secretors. Persons who are secretors have water-soluble antigen which can pass out of the red blood corpuscles and thus it is present in the body secretions.
But in case of non-secretors, antigens are only alcohol-soluble and cannot be dissolved out in the secretions. So, the secretors can be identified by test on the blood as well as on the body secretions. This secretor trait is inherited as a dominant gene ‘S’ while the non-secretor trait is inherited by the homozygous recessive allele ‘s’. It has been estimated that almost 77% of U.S.A. populations are secretors.
Similarly, another antigen known as ‘H’ antigen, also identified on the erythrocytes which can be demonstrated by agglutinations by anti-H serum. This antigen is believed to be an intermediate between antigen A and B. The dominant gene H is responsible for the production of H antigen and the geotypes are as follows :
It is interesting to note that the individuals whose blood gives no reaction with Anti-A or Anti-B or Anti-H belong to very rare group and are known as “Bombay phenotype”, because it was first described in a very small group of people in the Bombay city.
2. MN Blood Groups:
The blood corpuscles of different people may contain either one or the other, or both M and N and these antigens have no relation with ABO blood groups. That is a person of A – blood group might belong to any of the three (M, N or MN) MN blood groups. The gene responsible for the production of M and N antigens are dominant one and are alleles.
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The heterozygous for M and N gene showed co-dominance. However, these three classes (M, N and MN) do not occur in simple Mendelian ratio in the general population and the percentage of each class varies from one race to other. The MN blood group have no importance in the blood transfusion but have got medicolegal importance e.g. paternity test. The following table (Table 13.2) shows the paternity test for MN blood group.
3. Rh Factor:
An important agglutinogen has been demonstrated (1940) in human red corpuscles also by Landsteiner and Wiener. It is agglutinogen of the Rhesus monkey and is present in 85% of white people. Although information is limited, yet it is found that amongst Indians and Ceylonese, the proportion is even larger (about 95% or more). There is no corresponding agglutinin in the human plasma.
Recent studies indicate the Rh factor is not a single entity. There are six Rh agglutinogens – C, c; D, d; E, e; of these, D and dare the commonest. These two will provide three sub-groups – D, Dd and d. D is Mendelian dominant, while d is recessive. Hence, groups D and Dd (collectively called D group) will be Rh positive (Rh+) and d will be rh negative (Rh~). Practically all Rh positive people belong to D group and rh negative people to group d.
Clinical Importance:
1. If Rh+ blood be transfused to a Rh” patient, an Anti-Rh factor will develop in the patient’s blood in about 12 days. If a second transfusion of the same blood is given to such a patient after this period, haemoagglutination of the donor’s corpuscles will take place. In other words, blood which was compatible before has become incompatible now. So that, before transfusion, the test for Rh factor should be carefully done.
2. During pregnancy the fetus may be Rh+ whereas the mother Rh–. The Rh agglutinogen (slightly present also in the plasma) from the foetus passes into the maternal blood and stimulates the formation of Anti-Rh factor. This antibody enters the foetal blood and destroys the red cells of the foetus. The foetus may die (causing miscarriage) or, if born alive, suffers from severe anemia. This disease is known as erythro-blastosis foetalis.
3. Such a mother becomes sensitised to Rh factor. In future, if she gets a transfusion of otherwise compatible blood but containing Rh factor, agglutination will take place.
4. For the same reason, a Rh” woman, before menopause, should not be given transfusion of Rh+ blood. Because in case she becomes pregnant with Rh positive foetus, the problem as described under no. (2) will become all the more acute.
The specific agglutinins are not present in the foetal plasma. But maternal agglutinins, being filtered through the placenta, are found in the foetal plasma. Only 50% of new-born infants show an appreciable amount of this agglutinin.
Specific agglutinins start appearing from about the 10th day after birth and rises to the maximum at about the 10th year. Agglutinins, like other antibodies, are found in the globulin fraction of the serum. They are also present in low dilutions in body-fluids which are rich in proteins, such as milk, lymph exudates and transudates. They are not found in urine and cerebrospinal fluid. Haemoagglutinins increase temporarily during serum sickness and are reduced in leukemia.
Like other antibodies, the concentration of specific agglutinin varies at all ages from man to man and even in the same individual under different conditions. They act best at a lower temperature.
The blood group of a particular subject is of fixed character and does not vary with age, or disease.
Non-specific agglutinins may sometimes appear in the blood which act in cold (at 0°-5°C or F) and not at body temperature. These cold agglutinins may at times be sufficiently high to cause auto-agglutination at body temperature. For this reason there may be intravascular haemolysis leading to haemoglobinuria (Paroxysomal haemoglobinuria).
Details about Rh Factor:
1. Rh Agglutinogens:
There are three pairs of Rh agglutinogens C, c; D, d; and E, e; C, D and E are Mendelian dominants and c, d and e are recessives.
2. Human red cells (RBC):
R.B.C. will always carry three agglutinogens – one from each pair, but they will never carry both the members of any pair. Thus ODE, CDe and cDE are possible but cDC and CDd are not.
3. Rh groups (genotypes):
It follows, therefore, that there are 8 possible combinations, any one of which may be carried by both the parents. Hence, mathematically, there are 64 possible combinations (Genotypes). Of these 28 being identical, 36 sub-groups are biologically available. Of these again, only 5 are commonly found, viz, CDe/CDe, CDe/cDe, CDe/cde, cDe/cde and cde/cde. Others are rare.
4. Rh+ and Rh- groups:
These groups containing the dominant agglutinogens—i. e., C, D, E—will be Rh+. But since C and E seldom remain without D, practically all Rh+ cases contain D, i.e., belong to
group D. The Rh- cases will contain the recessive agglutinogens – c, d and e and due to similar reasons state4 above belong to group d. Every man carries some Rh agglutinogen. Majority have D and are of Rh+. The rest carry d and are of Rh- . All Rh incompatible reactions are due to interactions between group D (donor) and group d (recipient).
5. Rh Antibody:
a) Each of the six agglutinogens has antigenic property, i.e. they can stimulate antibody formation. The corresponding antibodies are known as Anti-C, Anti-D, etc. D is strongly antigenic, others are very feeble.
b) If D cells are repeatedly injected into a Rh” subject, Anti-D will develop. This antibody may be of two types — “early” and “late”. The early Anti-D is formed first and is called complete antibody. It can agglutinate D cells in vitro, when they are suspended either in saline or albumin solution. Hence, it is also known as saline agglutinin. The late Anti-D is formed later and is called incomplete antibody.
It can agglutinate D cells in vitro, when they are suspended in albumin solutions only and not in saline solutions. Hence it is also called albumin agglutinin. But in the latter case, though the D cells are not agglutinated, yet they are somewhat modified. Because, these cells, once treated in this way, will not be agglutinated by early Anti-D serum, even when they are suspended in albumin solution. Hence, the late Anti-D is also known as the blocking antibody.
c) As mentioned above, D is very strongly antigenic. It causes Anti-D formation even by intramuscular injection; so that repeated intramuscular injections of whole blood — as often done in medical practice without matching the blood groups — is not necessarily a safe procedure. Hence, direct cross-matching before each such undertaking is the only surest safeguard.
6. Racial distribution:
Write people – 85% Rh+, of which D – 35%, Dd – 48% and the remaining 2% also contain D along with some other agglutinogen. Indians, Ceylonese – 95% Rh+, Japanese about 100% Rh+ Hence, in the latter, Rh incompatibility reactions are extremely rare.
7. Haemolytic Disease of the New-born:
This disease is due to destruction of the Rh+ R.B.C. in the foetus by an Anti-Rh agglutinin, present, in the mother’s serum, which has filtered through the placenta during pregnancy. The incompatibility between the blood of mother and child is caused by the inheritance of the Rh factor. The following Table (Table 13.3) indicates the probabilities of Rh group in child.
In this disease destruction of the normal R.B.C. leads to the presence of abnormal nucleated R.B.C. in circulation. A few hours after birth there is anemia, acute jaundice and related symptoms.
Importance of blood group:
1. Blood transfusion.
2. Certain blood diseases.
3. Paternity test.
4. In forensic medicine.
5. Ethnological studies.
6. Anthropological studies.
7. Various experimental purpose.
Incompatibility of the blood might arise only in cases marked asterisk (*) — as in these two groups the mother is capable of producing an Anti-Rh agglutinin to destroy the Rh+ R.B.C. present in the foetus.
Genetic Control of Antigenic Structure:
The Rh antigens:
The two independent sets of allelic blood group genes discussed so far are relatively simple examples of the genetic control of blood grouping substances. One final case will be presented in some detail in order to illustrate the most complex situation in humans that has been made intelligible through an understanding -of the relationships of genes and antigens.
This case is that of the rhesus substances, which represent a series of antigens that are inherited independently of the MN and ABO antigens and which are determined by genes that occur on yet another pair of chromosomes. The series of antigens derive its name, Rh, from the rhesus monkey (Macaca mulatta), in which the first member of the series was discovered by Landsteiner and Winner in 1940.
Levine and Stetson (1939) had established that the hemolytic disease of newborn infants termed erythroblastosis foetalis was due to the isoimmunization of mothers to an unknown antigen on the red cells of their children. Shortly after the description of the Rh antigens, Levine, Katsin, and Burnham (1941) found that this was the antigen responsible for the disease that they were studying.
These discoveries initiate an intensive investigation of the Rh antigens that has continued ever since. This investigation not only has provided a solution to many problems associated with the disease but has greatly advanced concepts of the nature of the inheritance of blood grouping substances in general.
Two major hypotheses have been advanced to explain the genetic mechanism that controls the Rh antigens. One of these, proposed by Wiener, postulates a series of alleles at a single locus m a pair of chromosomes different from those carrying any other genes for blood grouping antigens.
The other of these, advanced by Fisher and Race, agrees with the foregoing in stating that the genes involved are on their own chromosome pair, but disagrees in that it postulates three pairs of closely linked alleles at three separate loci.
The linkage involved is held to be so closely that cross-overs occur with such low frequencies as never to have been observed. Unfortunately, the genetic predictions of these two hypotheses are alive in so many of their aspects that it has not yet been possible to establish with finality which one is correct.
A Schematic Comparison of the Wiener and Fisher-Race Concepts:
One of the fundamental points at issue is whether or not there is a one-to-one relationship between the number of kinds of Rh antibody that a cell will combine with and the number of kinds of genes determining the antigenic specificities responsible for this combination.
This point is illustrated by considering cells (of an individual genetically homozygous) capable of combining with three different kinds of antibodies, anti-1, anti-2 and anti-3. Weiner’s hypothesis would permit the concept that all three antibodies were combining with different portions of a single molecule of antiges, the complex specificities of which were determined by a single kind of gene.
The Fisher- Race hypothesis would not permit this concept, but visualizes each antibody combining with a molecule of antigen with only a single specificity, determined by a single gene. The accompanying diagram outlines the nature of the contrast between these two concepts.
Careful attention should be paid to the point that the Wiener concept does not conflict with the one gene-one antigen relationship referred to at the start of this chapter. Rather, it is readily conceivable that the antigen determined by a single gene can have a complex topographical structure that will induce, and combine with, more than one kind of antibody in a manner analogous to that observed in the study of “artificial” antigens; In other words, the concept of a one-to-one relationship between a gene and the antigenic specificity that is its product does not at all necessitate a one-to-one relationship between this antigenic specificity and the antibodies that it engenders.
The Weiner Concept of Rh:
Weiner’s concept postulates a basic series of 8 allelic genes (additional members have been added to this series, but these need not be considered here), any two of which can occur in a single heterozygous individuals. Each of these genes determines an antigen able to induce and combine with one to three (and more) kinds of antibodies.
The antigenic specificities involved occur in various combinations, determined by the particular allele responsible for any given antigen. (The antibodies used in this research are generally obtained from isoimmunised humans, either volunteers or mothers who have a child suffering from hemolytic disease; Wiener’s symbols for the various genes, the antigens that they determine, and the reactions of these antigens to selected antiserums will be found in Table 13.4. Such gene is written as single letter, followed by a superscript, while the antigen that each determines is written as two letters followed by a subscript or superscript. The various antigens will now be considered.
The symbol Rho is capitalized because it represents the first Rh antigen that was discovered and which still remains the most significant in hemolytic disease. The symbols rh’ and rh” stand for additional antigens subsequently found. The symbols Rh1and Rh2 stand for complex antigens consisting of two specificities. Rhj is composed of the units Rho and rh’; Rh2 is composed of the units Rho and rh”. The additional symbols Rhz and rhy stand for antigens with multiple specificities as indicated. The symbol rh requires special comment.
Originally this symbol stood for the absence of any known antigenic specificities (i.e., Rh0, rh’, and rh”). However, the discovery of two new kinds of antiserums has disclosed the existence of two additional kinds of antigenic specificities. These occur in various combinations with the other, specificities just described.
The first of these antiserums, originally found by Levine and his collaborators, identifies a specificity now termed hr’ which occurs on all cells lacking the rh’ specificity. The second of these identifies a specificity termed hr” which occurs on all cells lacking the rh” specificity. (The antigenic counterpart of the Rh0 antigen, Hr0, has yet to be identified with certainty.) This historically complicated situation has led to the recognition of the rh symbol as representing a complex antigen with both the hr’ and hr” specificities.
In addition, the two new antiserums have extended the description of the other Rh symbols. These relationships are shown in Table 13.5. In order to understand these (and those already presented in Table 13.4) the student should prepare some diagrams similar to the ones shown previously, substituting the Wiener symbols for the numbers used.
To summerise the Wiener scheme, the five antiserums considered here permit the detection of variable clusters of a series of antigenic specificities (individually termed blood factors) that together from the Rh blood group of any given person. These clusters pass from generation to generation, their specific and structural continuity being determined by the particular allele of which they are the product. Further consideration of the inheritance of these factors Is given in a later section.
The Fischer-Race Concept of Rh:
The Fischer-Race concept has its origin in the analytical insight of the British geneticist and mathematician R. A. Fischer. He proposed, in a-suggestion presented by Race in 1944, that the Rh antigens then known could be considered as the products of the action of a series of three pairs of very closely linked alleles, each gene in each pair producing a single antigen with the ability to induce and react with one kind of antibody only.
The allelic pairs of genes proposed were symbolized as C, c; D, d; and E, e. Each was held to produce a distinct antigen designated by the same letter. No dominance is implied by the use of capital and small letters, these being selected merely to show their allelism.
The formalized relationships of these several genes on the chromosomes of an individual heterozygous for all of them is:
CDE/cde
Other combinations of three alleles on particular chromosomes of course occur, e.g., C D e, c D E, C d e, etc. (some authorities write the sequence of letters involved as D. C. E in recognition of genetic considerations of linkage and possible deletion; these however, are considerations not pertinent here.)
At the time that the Fisher-Race concept was established, antiserums were known for the C, c; D; and E antigens. The additional antiserums, for antigens d and e, were predicted, of which anti-e has now been established with certainty.
In addition, the existence of the then unknown chromosome c (d) E was predicted and subsequently found. The success of these predictions, as well as the relative simplicity of the terminology and concepts involved, led to wide acceptance of the Fisher-Race scheme, especially among clinicians and European research workers.
In summary, the British concept recognizes a series of chromosomes carrying different combination of the very closely linked C, D, E allelles. These combinations are held to arise as the result of crossing over, so infrequent as not to have been detected.
The symbol D corresponds to that of Rho, and further parallels in the two terminologies are shown in Table 13.6. Similarly, the two sets of symbols for the five kinds of Rh-antibodies can be related as follows:
Inheritance of the Rh Blood Factors:
It is apparent that, in the absence of dominance, mutation crossing over, and epistosis (none of which has yet been found to occur during the course of genetic studies on Rh antigens), the Rh blood factors will reappear from generation to generation as characteristic clusters.
For example, a cross between a father of genotype R2r (CDE/cde) and a mother of genotype ϒr’’ (Cde/cdE) can potentially produce four kinds of children, as is readily shown through the use of Punnett’s square:
Two, among the children shown, would possess the antigen RhofD) which their mother lacks. In the classical usage of the Rh terms, their mother would be “Rh negative” while they would be “Rh positive”. This example also shows that the definition of Rh positiveness and negativeness is a relative one that must be made in terms of the antigens involved.
In theory, any child possessing Rh antigens that its mother lacks is positive with respect to these antigens, while its mother is negative with respect, to them. In practice, however, the Rho(D) antigen has been found to be the most frequently involved in hemolytic disease, with rh'(C) the next, the other blood factors being much less frequently implicated.
Significance of Developing the Correct Concept of the Genetic Relationships of the Rh Antigens:
The foregoing sections have shown that either the Weiner or the Fisher-Race systems of nomenclature can be used to describe the Rh antigens and antibodies. This point is recognised by the National Institute of Health which require that both systems be applied to the labelling of commercially produced antiserums.
However this should not detract attention from the need for determining the validity of one or the other concept underlying this nomenclature, even though this may appear to be “academic” and not of direct concern in clinical work.
One reason alone for the need of continued efforts to resolve this problem is that, as often noted already, antigens appear to be the direct products of the genes that produce them. The antibodies that they induce therefore become, because of their fine specificities, the most sensitive indicators of variations in gene action that are known.
This makes if essential that an accurate conceptual scheme be arrived at which will relate the production of antigens to those schemes which are being evolved concerning the relation of genes to enzymes and of nucleic acid structure to the “genetic code” used in the hereditary transmission of-messages”.
Detailed consideration of these relationships lies far beyond the scope of this text, the interested reader being referred to the ‘ popular accounts of Crick, Gamow and Beadle for an introduction to the stories involved.
Stomont has summarised reasons for a growing tendency on the part of several leading geneticists to favour the Wiener concept, in spite of its more difficult terminology. His summary, too advanced to present here, is based upon parallels between the behaviour of the Rh antigens in humans and the B and C series of alleles that determine blood types in cattle.
These series of alleles control by far the most complex array of blood factors known to exist, an array the relationships of which can reasonably be explained only in terms of multiple alleles rather than in series of linked genes. Additional details concerning cattle blood types are given later in this chapter. Race and Sanger and Levine present discussions and further references of the Fisher point of view.
The student should realize that the leading proponents of either scheme have been conducting research that has seldom been excelled in the annals of biology and that experimental resolutions of their differences will be neither an easy or trivial matter.
