Blood has two main components: serum and cells. In 1900 Karl Landsteiner, a physician at the University of Vienna, Austria, noted that the sera of some individuals caused the red cells of others to agglutinate . This observation led to the discovery of the ABO blood group system, for which Landsteiner received the Nobel Prize. Based on the reactions between the red blood cells and the sera, he was able to divide individuals into three groups: A, B, and O. Two years later, two of his students discovered the fourth and rarest type, namely AB.
Antigens and Antibodies
To understand blood typing, it is necessary to define antigen and antibody. An antigen is a substance, usually a protein or a glycoprotein , which, when injected into a human (or other organism) that does not have the antigen,
|RELATIONSHIPS BETWEEN BLOOD TYPES AND ANTIBODIES|
|Blood Type||Antigens on Red Blood Cell||Can Donate Blood To||Antibodies in Serum||Can Receive Blood From|
|A||A||A, AB||Anti-B||A, O|
|B||B||B, AB||Anti-A||B, O|
|AB||A and B||AB||None||AB, O|
|O||None||A, B, AB, O||Anti-A and anti-B||O|
will cause an antibody to be produced. Antibodies are a specific type of immune-system proteins known as immunoglobulins, whose role is to fight infections by binding themselves to antigens. In the case of the ABO blood groups, the antigens are present on the surface of the red blood cell, while the antibodies are in the serum. These antibodies are unique to the ABO system and are termed "naturally occurring antibodies." The table shows the relationships between blood types and antibodies.
This aspect of the ABO blood group system is very important in transfusion. Blood group O individuals are said to be universal donors, because their blood can be used for transfusion in individuals who have any one of the four blood types. On the other hand, individuals with blood type A can only donate to either type A or type AB, and individuals with blood type B can only donate to B or AB types. AB individuals can only donate to type AB. However, before any transfusions, donor blood is mixed with serum from the recipient (a process called cross matching) to ensure that no agglutination will occur after transfusion.
The genetic basis of the ABO blood group system is an example of multiple alleles . There are three alleles, A, B, and O, at the ABO locus on chromosome 9. The expression of the O allele is recessive to that of A and B, which are said to be co-dominant. Thus, the genotypes AO and AA express blood type A, BO and BB express blood type B, AB expresses blood type AB, and OO expresses blood type O. In the past, ABO blood group typing was used extensively both in forensic cases as well as for paternity testing. More recently, DNA testing, which is much more informative, has superseded these tests.
The ABO blood group substances are glycoproteins, the basic molecule of which is known as the H substance. This H substance is present in unmodified form in individuals with blood type O. Adding extra sugar molecules to the H substance produces the A and B substances. The frequency of the ABO blood types varies widely across the globe. For example, blood group B has a frequency of 25 percent in Asians, 17 percent in Africans, but only 8 percent in Caucasians. The frequency of blood group O in Europe increases as one travels from southern to northern countries.
Alleles at a locus independent of the ABO blood group locus, known as the secretor locus, determine an individual's ability to secrete the ABO blood group substances in saliva and other body fluids. There are two genes, Se and se, where Se is dominant to se. In other words, an individual with at least one Se gene is a secretor. Approximately 77 percent of Europeans are secretors. This frequency is rarely less than 50 percent and sometimes as high as 100 percent in other populations.
An interesting aspect of the ABO blood groups is their association with disease. Among individuals with stomach and peptic ulcers, there is an excess of type O individuals, whereas among those with cancer of the stomach, there is an excess of type A individuals. Not all type O individuals have an increased risk for peptic or stomach ulcers, however. If type O individuals are secretors, they are protected against ulceration, whereas non-secretors have a two-fold increased risk. Thus the presence of ABO blood group substances act as a protective agent against the development of stomach and peptic ulcers.
The Rh System
The second most important blood group in humans is the Rhesus (Rh) system. Landsteiner and Wiener discovered the Rh blood group in 1940. They found that when they injected rabbits with Rhesus monkey blood; the rabbits produced antibodies against the Rhesus red cells. These antibodies reacted with red blood cells taken from 85 percent of Caucasians in New York City, who were thus said to be Rh positive, while the remaining 15 percent were Rh negative.
One year earlier (1939), Levine and Stetson published a paper describing the mother of a stillborn infant who had a severe reaction when transfused with her husband's blood. They tested the woman's serum and found that it reacted with 77 percent of blood donors. They postulated that the mother had been exposed to blood from her fetus and produced an antibody that reacted with it. The same antigen was present in the baby's father, explaining the woman's reaction to his blood. Their conclusion was correct, and later they realized that they had discovered the same antigen (Rh) that was discovered in the following year. The antibody found in the mother of the stillborn child was shown to be identical to the anti-Rh antibody produced in the rabbit by Landsteiner and Wiener.
The Rh blood group system is the major cause of hemolytic anemia in the newborn. A fetus who is Rh+ and whose mother is Rh− is at high risk for this disorder, because the mother will produce antibodies against the fetal antigen. The first such fetus is usually not at risk since the fetal cells do not enter the mother's circulation until the time of birth. Only at this time does the mother produce anti-Rh+ antibodies. This complicates future pregnancies, because her antibodies will enter the fetal circulation system and react with fetal blood, causing hemolysis .
A treatment for Rh− women at risk to have an Rh+ fetus is now widely used. Anti-Rh+ antibody is injected into the mother soon after her first delivery. This antibody coats the fetal Rh+ cells in the mother's circulation, which prevents them from causing antibody production in the mother and, therefore, her next child will not be at risk for hemolytic anemia.
The precise genetics of the complex Rh system has been in dispute since the early discoveries. The Rh blood group system is, in fact, much more complex than simply Rh+ and Rh−. There are two genes, one of which has four possible alleles, giving six antigens of which five are commonly tested. The first is D, which is the dominant gene that determines whether one is Rh+ or Rh−. Individuals with genotypes DD and Dd are Rh+ and those who are dd are Rh−. The DD and Dd genotypes cannot be distinguished from one another, since there is no "anti-d" antibody. The remaining four antigens are C, c, E, and e. The Rh locus is on the short arm of chromosome 1 and consists of two tandem genes. The first, RHCE, codes for non-RhD proteins while the second codes for the RhD protein. The Rh polypeptide has been sequenced. It contains 417 amino acids. Thus the molecular genetics conferring different antigenic Rh types is now clear.
P. Michael Conneally
Cavalli-Sforza, L. L., and W. F. Bodmer. The Genetics of Human Populations. San Francisco: W. H. Freeman and Company, 1971.
Huang, Cheng-Han, Philip Z. Liu, and Jeffrey G. Cheng. "Molecular Biology and Genetics of the Rh Blood Group System." Seminars in Hematology 37, no. 2 (2000): 150-165.
Race, R. R., and Ruth Sanger. Blood Groups in Man, 6th ed. Oxford, U.K.: Blackwell Scientific Publications, 1975.
"Blood Types." Indiana State University. <http://www.indstate.edu/thcme/mwking/abo-bloodgroups.gif>.
In the ABO system, the surface antigens of the red cells are determined by three genes, A, B, and O. (The genes are referred to by an italicized character, e.g. A, whilst the gene product (phenotype) and hence the blood groups are referred to by simple uppercase letter, e.g. group A.) All red cells have on their surface a glycolipid substance called H substance. The A gene and the B gene convert H substance into substance A and B, giving rise to cells of the A and B groups respectively. The O gene has no effect on H substance and thus group O cells have only H substance on their surface. These three genes combine in pairs to give six possible genotypes, AA, AO, BB, BO, AB, and OO. Since A and B are dominant over O, this results in four phenotypes: A, B, AB and O (see table). Eighty per cent of individuals also have A, B, and H substances in secretions such as tears and saliva.
Since, under normal circumstances, individuals do not form antibodies against their own proteins and since all red cells have H substance, no naturally occurring antibodies against H substance are found. Likewise individuals with group A cells (genotypes AA, AO, or AB), do not have antibodies against A substance in their plasma and individuals with group B cells (genotype BB, BO, or AB) do not have antibodies against B substance. However, substances closely related to A and B are widely distributed in nature and absorption of these from the gut, presumably shortly after birth, is thought to give rise to antibodies against A and/or B if that individual does not possess A and/or B antigens on their red cells. Thus, individuals who are group A have antibodies against B substance and those who are group B have antibodies against A substance.
Thus, an individual who is group AB should be able to receive cells of any group, since he would not have antibodies against any blood group substance and the transfused cells would not be destroyed. Likewise, it should be possible to transfuse group O blood into any individual, since the transfused cells will contain neither A nor B substance, but only O, and any antibodies against either A or B substance in the recipient plasma will be without effect. As group O cells do not react with anti-A or anti-B antibodies, people of group O became known as universal donors. But it is not quite as simple as that.
Red cell antigen
AA or AO
BB or BO
Although the ABO blood group system is the one which is of greatest concern in blood transfusion, approximately 400 blood group antigens have been described and, before blood transfusion is attempted, it is essential that the blood of the recipient and the blood of the donor are directly matched by a laboratory test to avoid incompatibility. The other blood grouping which is a common cause of transfusion incompatibility is the Rhesus system, and occasional reactions are encountered as a result of incompatibility in other systems.
The Rhesus (Rh) system is the usual cause of the so-called haemolytic disease of the new-born, although, rarely, the other grouping systems can be responsible. The Rh system derives its name from the discovery by Landsteiner and Wiener in 1940 that injection of red cells from a Rhesus monkey into a rabbit caused the production of antibodies, and that these antibodies reacted with the red cells of some humans (so-called Rh-positive individuals), but not others (Rh-negative individuals). Similar antibodies were found in the plasma of mothers who had given birth to children with HDN. The development of jaundice and anaemia soon after birth, and the occasional death of such infants was previously a mystery. The definition of the Rh group of an individual depends on the presence of a substance D; those whose cells have the D antigen are Rh positive, and their cells are attacked by D antibodies in blood of a person who is Rh negative. Clinically, only the D antigen and the anti-D antibody are important, although other (C and E) substances also differ between the groups.
Haemolytic disease of the new-born is the result of the passage of antibodies from the maternal circulation across the placenta into the fetal circulation, where they damage the red cells. The condition arises where the mother is Rh-negative but the fetus, and the father, are Rh-positive. At the time of birth in a first pregnancy in these circumstances, there is no damage to the infant, but fetal red cells leak into the maternal circulation, immunizing the mother and causing the production of antibodies. These antibodies are small enough in size to cross the placenta and enter the fetal circulation during subsequent pregnancies, causing HDN if the fetus is again Rh-positive. Often this is confined to mild anaemia, but in more serious cases the baby is severely anaemic and jaundiced because of the accumulation of bilirubin released from damaged red cells. In the 1940s complete ‘exchange transfusion’ — replacing the whole of the infant's blood via the umbilical cord soon after birth — started to be employed as a life saving measure. Fortunately, the risk of HDN caused by Rhesus incompatibility is now reduced enormously by the administration of an anti-D antibody to the mother at the time of the birth of the first and any subsequent Rh-positive child, thus removing fetal red cells from the maternal circulation before they can stimulate permanent production of antibody.
D. E. Bowyer
blood groups, differentiation of blood by type, classified according to immunological (antigenic) properties, which are determined by specific substances on the surface of red blood cells. Blood groups are genetically determined and each is characterized by the presence of a specific complex carbohydrate. About 200 different blood group substances have been identified and placed within 19 known blood group systems. The most commonly encountered blood group system is the ABO, or Landsteiner, system. Individuals may contain the A, B, or both A and B antigenic substances, or else lack these substances (type O). In the ABO system an individual who lacks one or more of these antigens will spontaneously develop the corresponding antibodies (agglutinins) shortly after birth. Thus a person with A type blood will naturally produce anti-B agglutinins, a person with B blood will produce anti-A agglutinins, and a person with O blood will produce anti-A and anti-B agglutinins; but a person with AB blood will not produce any agglutinins in this blood group system. Since these agglutinins are always present in the blood, in blood transfusion the donor blood must be compatible with the recipient's blood, i.e., the donor's blood must not contain antigen corresponding to the recipient's antibody. Other blood group systems, such as the MNSs, Lewis, Lutheran, and P systems, are not as important in transfusion because they act like true antigen-antibody systems, i.e., antibodies do not appear in blood plasma until the individual has been immunized by exposure to the other blood group antigens as in previous transfusions. In general, blood group substances are weak antigens, and antibody formation after transfusion occurs less than 3% of the time. Immunization can occur by pregnancy as well as by transfusion. Thus, in the Rh factor blood group system, an Rh-negative mother carrying an Rh-positive fetus produces anti-Rh antibodies against fetal red blood cells that cross the placenta. Since blood type is a genetic trait that is easy to test and the blood type of an individual is related to his or her parent's blood types by the laws of Mendelism (see under Mendel, Gregor), blood group typing is used legally to establish paternity. Anthropologists use the frequency of occurrence of various blood groups as tools to study racial or tribal origins.
Donor's blood group
Blood group of people donor can receive blood from
Blood group of people donor can give blood to
A, B, AB, O
A, B, AB, O
blood type: see blood groups.