What is Rh factor?

Structure and Functions

Blood replacement in emergency or surgery can be critical: blood loss exceeding 40 percent can lead to a condition called shock in which the heart cannot pump efficiently, resulting in death. In the search for human blood replacements, scientists have found that animal blood is not compatible. More important, they have discovered that even blood from different humans does not always mix. Sometimes the red blood cells will agglutinate, or settle out of the plasma in clumps. Consequently, these red blood cells are destroyed by the body, and jaundice and death may follow. To prevent this reaction, human blood must be classified into types and cross-matched. The two most important general groupings are the ABO and Rh types.

Human blood is classified into types according to the antigens that might be present on the red blood cells as a result of heredity. Antigens are usually large, complex molecules made of protein alone, protein with attached carbohydrates, or lipids with attached fatty acids and alcohol. They may be free molecules, as in the case of toxins released by invading bacteria, or they may be located on a cell’s surface and serve to label or mark the cell. The markers attached to cell surfaces identify the cell as “self” or “foreign.” Such antigens are the basis of blood types.

Karl Landsteiner
showed that there are four major blood types, based on two antigens that might be present or missing. He called the markers A and B. People with both markers on their cells are called type AB, people with one of the two are type A or type B, and those without either marker were originally called C but later changed to O. Landsteiner’s system of classifying blood according to the presence of the A and B antigens is now termed the ABO system. Landsteiner demonstrated the chemistry of the antigens and the antibodies by mixing blood from himself and coworkers in his laboratory and observing that some combinations agglutinated.

The next major breakthrough came with Philip Levine
and Rufus Stetson’s study of the blood of a woman whose fetus had died six weeks before birth. The mother’s immune system had produced antibodies against the Rh factors on the blood cells of her developing child. She was Rh negative while her child was positive, having inherited an Rh-positive gene from the father. The positive blood of the child caused the mother’s immune system to react. The importance of the discovery was twofold. It not only explained why some babies suffered from an immune reaction in their mothers but also showed that blood transfusions could be typed as ABO compatible and still fail if the Rh factor was not considered.

The term Rh factor came about because of a misunderstanding. Working independently and believing that they had found the same factor first in their laboratory animals, Landsteiner and Alexander Wiener claimed discovery and named the Rh antigen after the rhesus monkey. They injected the monkey blood into rabbits, and the rabbits developed antibodies against the foreign factor. Hypothesizing that closely related primates might share the factor, the rabbit antibodies in a serum were then mixed with samples of human blood. Further work verified that there was indeed a new important human factor, but it differed from the one in monkeys. By this time, however, it was too late to change the misleading name. To clear up the confusion, the factor in humans kept the name of Rh factor, while the monkey antigen was labeled the LW factor.

Even if it is inappropriately named, the Rh factor is an important discovery. People with the marker on their blood cells are Rh positive, while people without it are Rh negative. Rh-negative people can give blood to people who are positive if all other antigens such as those found in the ABO types are compatible. If the reverse is attempted, however, the Rh-negative person will develop antibodies against the Rh marker. Clumping of the red blood cells will occur, and illness and death are likely.

Discovery of the presence of these markers was the key to understanding both the blood types and what happens in immune responses. When a foreign protein or antigen enters the body, antibodies are produced by the immune system and released into the blood and lymph. These antibodies are specific in the sense that a particular antibody will only combine with a particular antigen. (If a particular antigen is present on a person’s own blood cells, the individual would not normally produce and carry antibodies for this molecule; otherwise, the antibodies would attack the person’s own blood cells.) The antibodies fasten the cells together in what is called agglutination. Agglutinated cells are destroyed by white blood cells.

Combining the ABO and Rh systems, a person could be A+, A-, AB+, AB-, B+, B-, O+, or O-. Because an AB+ individual has A, B, and Rh antigens and no antibodies against them, this person can receive blood from all others. An O- individual has no antigens and is a universal donor, assuming that the O- person has no other important antigen differences from other, more rare types. The importance of the ABO and Rh systems is shown by the standard practice of hospitals in typing and sorting by these systems.

Unlike the ABO types, Rh-negative blood does not normally contain antibodies for positive blood unless the person has been previously exposed (sensitized) to positive blood. The A and B antibodies are developed early in people because A and B antigens are common in the environment. They are found not only on red blood cells but also in milk, colostrum, saliva, and other body fluids. Should a transfusion of Rh-positive blood be given to an Rh-negative person, the negative blood produces the antibody, which will have a violent reaction with the next similar transfusion.

The Rh factor is inherited, as are all blood types. A person inherits one gene involving the factor from each parent. If the person inherits two genes (DD) for the factor, it will be present on the red blood cells. If a person inherits one gene for the factor and one that does not produce it (Dd), the individual will still be Rh positive. If a person inherits two recessive genes (dd), that person will be Rh negative. Consequently, the gene for production of the Rh factor is called dominant; the other gene is recessive.

The original Rh factor can also be called the D factor. Additional investigation has shown that the entire Rh factor is not a single factor caused by one pair of genes; rather, at least three pairs of genes may be involved. Antisera have been found not only for the most reactive D antigen but also for four other factors. The situation can be explained by imagining Rh to be determined by a combination of three genes, which are probably closely linked on the same chromosome. Ronald Fisher labeled the genes C, c, D, d, E, and e. An Rh gene complex could then be any of these combinations: CDE, CDe, CdE, Cde, cdE, cDe, cde, or cDE. An individual would have two of these complexes, one from each parent. The number of different Rh types then reaches sixty-four.

To illustrate, a person with CDe/cdE would test Rh positive using standard anti-D sera because of the D gene; so would people with any combination of C, c, E, and e with at least one D. Nevertheless, the other nearby genes can cause agglutination problems. Each of them, except d, produces an antigen on the red blood cells. The antigens cause antisera to form in human blood that recognizes them as foreign. One can also choose to think of the situation as having eight different alleles for Rh. Then a single symbol can stand for each combination: r = cde, r′ = Cde, r″ = CdE, ry = cdE, R0 = cDe, R1= CDe, R2 = cDE, and Rz = CDE. Any r is an Rh-negative combination in the classic sense, and any R is Rh positive. Additional discoveries of new Rh antisera have caused some investigators to hypothesize about the possibility of more than thirty antigens, some that are variant forms of the above and some that require more genes.

A 1948 paper by R. R. Race, A. E. Mourant, Sylvia D. Lawler, and Ruth Sanger reported that R1r (or CDe/cde) is the most common Rh blood gene combination in England, at about 33 percent of those tested. The R1R1 (or CDe/CDe) combination follows at 16.6 percent, rr (or cde/cde) at 15.8 percent, R1R2 (or CDe/cDE) at 12.9 percent, R2r (or cDE/cde) at 12.8 percent, and R2R2 (or cDE/cDE) at 2.7 percent. All the other combinations total about 6 percent.

Rh has turned out to be quite complex. Nevertheless, the system can be understood and applied at a very basic and useful level of Rh positive or Rh negative, which involves consideration of the very reactive D antigen on the blood cells. In that case, the Rh symbol is often labeled Rh0.

Disorders and Diseases

The discovery of the ABO system allowed transfusions to proceed with some confidence of success during World War I. Still, some transfusions produced problems, and some minor independent blood-type systems (MNS, P) were discovered. Clearly, people were members of more than one blood-type system. The additional discovery of the highly reactive Rh factor or D antigen was critical for safe transfusions.

Another immediate application of the discovery was in the area of childbirth. Rh incompatibility explained why some babies either died at birth or were born in serious trouble. The attack of the mother’s antibodies on the fetal blood cells can lead to various forms of hemolytic disease of newborns, or erythroblastosis fetalis. Incompatibility between mother and child is also one of the causes of spontaneous miscarriage early in pregnancy. Knowing the existence of the Rh factor has saved countless infants.

Recall that the Rh factor is inherited. If an Rh-negative woman (dd) marries an Rh-positive man (DD or Dd), the child may be Rh positive. During pregnancy, there is no direct blood flow from mother to child because red blood cells cannot cross the placenta. At some time during the pregnancy or at birth, however, blood will probably mix, and the mother will then be sensitized. She then will form antibodies against the Rh factor. Many of these antibodies are of the IgG type and are smaller than A or B antibodies (IgM). The small IgG antibodies can cross the placenta into the blood of the fetus. The first Rh-positive child usually escapes harm by being born, but a second positive child will be in great danger, as the mother’s preformed antibodies will cross the placenta and attack the red blood cells of the fetus. Blood cells are likely to be broken open, releasing hemoglobin. The fetus will become anemic and jaundiced and may suffer brain damage or be stillborn.

The occurrence of erythroblastosis fetalis can be prevented if an Rh-negative mother is given an injection of rhesus gamma globulin (RhoGAM) within seventy-two hours of the delivery of her first Rh-positive child. This approach was developed by C. A. Clark, P. M. Sheppard, and others working at Liverpool University. The gamma globulin destroys the fetal blood cells in the mother and prevents the production of antibodies that would affect the next positive child. Miscarriages or abortions of Rh-positive pregnancies count as an exposure to the antigen and can cause the mother’s immune system to react. Therefore, these events also require the injection to protect future children. Also, any Rh-negative woman accidentally given a transfusion of positive blood would be in danger herself, as would the fetuses in any of her future pregnancies.

Amniocentesis, a sampling of fluid from the sac around the developing fetus, can reveal such difficulties as Rh incompatibility. An Rh-negative woman can also be given a series of blood tests (Rh titers) during her pregnancy. If the tests show that the antibodies are increasing in number, intrauterine transfusion of negative blood may be attempted. Moreover, if the child is nearing full term, delivery may be induced to prevent the blood of the fetus from being completely destroyed. If the child is born with signs of circulatory problems, a blood transfusion can help.

Anthony Smith notes that the ABO type has an effect on trouble with Rh during pregnancy. If the mother is Rh negative, the child is positive, and their ABO types are also incompatible, then the Rh reaction is diminished. The reason may be that the mother already has antibodies against incompatible ABO types. When the red blood cells leak into the mother’s circulatory system, they are immediately destroyed by already-existing maternal ABO antibodies before any antibodies against Rh factor can be formed.

Some interesting associations with Rh have been discovered but are not yet understood. Typhoid, mumps, mononucleosis, and viral meningitis are more common in Rh-negative people. Viral diseases tend to be more common in the nonantigenic types of both the ABO and Rh systems (O and Rh negative).

Perspective and Prospects

Few successful blood transfusions took place before 1900. In that year, Karl Landsteiner discovered that there are different types of blood. Some would mix, while others would clump. He and his coworkers identified four major human blood groups: A, B, AB, and O. Even so, eight years passed before the first transfusion using Landsteiner’s ABO types was attempted. Transfusions became more likely to succeed. People could be typed by the antigens on their blood cells, and donors could be matched with the patient. Yet sometimes the transfusions still did not work as predicted. In 1930, Landsteiner won the Nobel Prize in Physiology or Medicine for his discovery of ABO blood types.

Landsteiner and Philip Levine discovered the MNS types in 1927; these are not important in transfusions but are of great help in cases of doubtful paternity. When beginning his own work, Levine agreed with Landsteiner not to study new blood groups, as Landsteiner had reserved that project for himself. Nevertheless, in 1939, Levine and Rufus E. Stetson published a report showing that the blood of a mother with a stillborn child was able to react hemolytically with 80 out of 104 ABO-compatible donors. They correctly concluded that the mother’s blood lacked an antigen that many others had: an unknown marker that was independent of the known ABO, MNS, and P blood groups. Levine and Stetson had correctly analyzed the problem but did not name their new antigen. Clearly, they had discovered what would be called the Rh factor.

Less than a year later, Landsteiner and Alexander Wiener immunized rabbits and guinea pigs with the blood of the monkey Macacus rhesus. They found that the resulting rabbit serum agglutinated not only the rhesus monkey blood but also about 85 percent of blood samples from people in New York City. They called these people Rh positive and the remaining 15 percent Rh negative. Wiener and H. R. Peters attempted to show that the Rh antibody in the rabbits was the same as that found in the serum of people who had suffered incompatible transfusion reactions not explained by ABO blood typing.

A bitter exchange ensued between Levine and Wiener about who had discovered the Rh factor. This was resolved when it was shown that the antigen on the rhesus monkey cells was not the same as the human Rh factor. Unfortunately, the name Rh was too well established to be changed by this time. To avoid further confusion, Levine suggested that the factor in the monkeys be called the LW factor after Landsteiner and Wiener. Despite this controversy, R. R. Race and Ruth Sanger called the discovery of the Rh factor the most important event in blood-group science since the discovery of the ABO system forty years before.

Soon, different investigators were able to derive sera with different antibodies for the Rh factor. Clearly, the Rh factor was not simply a single antigen. In 1943, Ronald Fisher studied the different antisera that had been developed and proposed that eight different Rh gene complexes were involved.

The existence of the Rh factor is useful in other ways. In addition to transfusions, another application of blood typing (including the presence or absence of the Rh factor) is in criminology. Blood left at the scene of a crime can be powerful evidence against a suspect. Blood types can also eliminate individuals as possible fathers in paternity suits. For example, if both parents are Rh negative, the child cannot be Rh positive. A more complete typing of the blood would allow further strong evidence. Such typing does not prove paternity, however; it only shows whether paternity is possible.

Blood typing also allows anthropologists to develop theories about the relationships among various human groups and how people may have migrated. The breakdown between being Rh positive or negative varies among the races. The Basques, a group of people near the Bay of Biscay between Spain and France, are only 64 percent positive, while about 85 percent of the Caucasian population in general is Rh positive. Races other than Caucasian are generally nearly 100 percent Rh positive. According to Sir Peter Medawar, however, the advantages of being one type or another are obscure. Why there are so many different blood types remains an interesting question.

The discoveries of the ABO types and the Rh factor stand as fundamental achievements in medical science. Even though many other types continue to be uncovered, ABO and Rh determinations remain the most basic steps in matching blood for many purposes, especially for safe transfusions.

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