The Rhesus Factor

The Rhesus Factor

The discovery of the Rhesus factor just over 3 5 years ago led to an understanding of why some babies used to become rapidly jaundiced soon after their birth and then sometimes died, while others suffered from a condition known as erythroblastosis foetalis which caused them to be still-born or to die very soon after delivery. They suffered from gross swelling of all parts of the body and severe anaemia.

The remarkable story of how the Rhesus factor was first discovered, its importance understood, treatment devised for the women suffering from Rhesus incompatibility, advanced techniques developed for the treatment of the new-born babies (including the complete changing of their blood) and then, a few years ago, the discovery of the means to prevent this condition, reads rather like a detective story in a novel. It is a remarkable feat of modern medicine. The complete elimination of a disease has taken place within a space of 25 years which is well within the working life of most people. Babies died from a strange condition, whose cause and origin were completely unknown. Slowly the pieces of a medical jig-saw have been put together until the complete picture is now available and, as a result, the condition is preventable in the majority of instances. It is a triumph of preventive medicine.

Approximately 85 per cent of European women are Rhesus positive and 15 per cent are Rhesus negative. About 94 per cent of Jews are Rhesus positive. In order to understand what is meant by Rhesus positivity it is necessary to appreciate that the blood cells of every person possess six Rhesus factors—three inherited from the mother and three inherited from the father. These factors are labelled C, D, and E, and each factor may be either positive or negative. They are represented by small letters c, d, and e when they are negative, and large letters G, D and E when they are positive. Each person, therefore, has either a small ‘c’ or a large ‘C’, a small ‘d’ or a large ‘D’ and a small £e’ or a large ‘E’ from the mother, together with a small or large C, D and E from the father.The whole is written as c, d, e/C, D, E, if a person inherited a small c, d and e from the mother and a large C, D, and E from the father. Any combination of these small or large letters can exist.

A person is stated to be Rhesus positive when he possesses the factor known as capital or big D. Big D may be present on either side or on both sides. So far as the ordinary testing of the blood is concerned the only real interest is in D and the presence of one D makes a person Rhesus positive. To help understand Rhesus incompatibility and the Rhesus factor, we can forget both types of C and E and just concentrate on the factor D. If a woman possesses D/D or d/D, or D/d she is Rhesus positive. But if she possesses d/d she is Rhesus negative. The same applies to a man. If a man possesses D/D he is stated to be homozygous because every sperm which he forms must contain D. He will therefore pass D on to all his children, so that they will all be Rhesus positive as they will all contain D. If a man is d/D or D/d then he is stated to be heterozygous, and he can pass on either D or d to his children, so that, if the mother is Rhesus negative, 50 per cent will be Rhesus positive and 50 per cent Rhesus negative.

Rhesus Sensitization

The Rhesus factor can only give rise to trouble when a Rhesus negative woman is carrying a Rhesus positive child. Throughout pregnancy, and especially at the time of delivery, blood cells from the child escape into the maternal circulation. The mother’s body reacts to these positive cells by attempting to destroy them by forming an antibody, known as Rhesus antibody, but before she can form an antibody the organs which are destined to form that antibody have to be primed, sensitized, or given, as it were, an initial warning that a foreign substance is circulating in the blood stream. They are unable to make antibody immediately, but having received the initial warning, or sensitization, will immediately form Rhesus antibody should any further Rhesus positive cells enter the circulation. A Rhesus negative woman who is carrying a Rhesus’ positive child will, therefore, not make antibodies during her first pregnancy, but the cells which escape into her circulation during the first pregnancy will provide the sensitizing dose. Similarly, transfusions of Rhesus positive cells into a Rhesus negative person, or the injection of Rhesus positive blood into the muscle of a Rhesus negative person, will cause a similar sensitization following

cde/cde = Rhesus negative; does not contain D
CdE/CdE = Rhesus negative; does not contain D
cDe/cde = Rhesus positive; Heterozygous—contains one D
cde/cDe = Rhesus positive; Heterozygous—contains one D
cDe/cDe = Rhesus positive; Homozygous—contains two D
CDE/cDe = Rhesus positive; Homozygous—contains two D

All the babies are Rhesus positive because they have all inherited D from their father.

50 per cent of babies are Rhesus negative and 50 per cent are Rhesus positive which any further injection will provoke the production of antibodies.

It is not known at precisely what stage in pregnancy a foetus is capable of sensitising its mother. It is unlikely that pregnancies terminated before the 6th week can cause sensitization. It may, however, be possible for pregnancies after the 8th week to sensitize a woman and it is almost certain that pregnancies after the ioth week can.

Many attempts have been made to desensitize women who arc already sensitized or who have circulating Rhesus antibodies. A new and exciting technique (plasmaphoresis) to remove Rhesus antibody from the maternal circulation is being developed and offers great hope in the future to affccted women.

The Rhesus FactorRhesus Antibodies

Red cells escape from the circulation of the foetus into the mother’s circulation in small quantities throughout pregnancy and in a woman who has been previously sensitized the escape of Rhesus positive cells from her baby into her circulation will provoke the production of Rhesus antibody to destroy the foreign circulating cells. Unfortunately, the Rhesus antibody so formed by the mother is capable of, and in fact does, cross the placenta into the baby’s circulation where it commences to destroy the baby’s own circulating Rhesus positive red cells.

If the mother makes a lot of Rhesus antibody it will form a high concentration in her blood and a high concentration will pass into the foetus, where it will cause massive destruction of the baby’s Rhesus positive blood cells, with the result that the baby may die in the uterus from anaemia and heart failure (erythroblastosis foetalis). If the concentration is not so high die baby may be born alive but with severe anaemia and because the antibody already in his circulation continues the destruction of his red cells after birth, the baby will rapidly become jaundiced and may soon die. Such a baby would have his blood tested immediately at birth and will need to have his blood changed by an exchange transfusion. In this way most of the Rhesus positive cells are replaced by Rhesus negative cells which cannot be destroyed by the residual antibody within his system. These transfused Rhesus negative cells survive in the baby’s circulation for approximately 40 days, during which time the baby will gradually eliminate all the Rhesus antibody from his system, and as he forms more of his own Rhesus positive cells they can no longer be destroyed. Although the baby’s blood has been changed at birth from Rhesus positive to Rhesus negative, he will grow up to be Rhesus positive because his own blood forming organs can only make a Rhesus positive blood. Once the antibody has been washed out of his system he will come to no further harm and will develop into a perfectly normal child. Some babies may be only mildly affected at birth with only slight anaemia and a mild degree of jaundice. They require frequent tests, and careful observation but do not need exchange transfusions.

Blood Groups

The statistical odds of a Rhesus negative woman marrying a Rhesus positive man and having a child with Rhesus disease are 0-2 per cent or 1 in 500. Only about 4 per cent of Rhesus negative women who arc married to Rhesus positive men do in fact develop Rhesus antibodies and have problems with their children. For many years it was not known why only 4 per cent should be so unfortunate, nor could it be predicted who would be in the unhappy group. The answer to this problem depends upon the four major blood groups: O, A, B and AB, to one of which every person must belong. It is well known that people of one blood group cannot receive transfusions from people of certain other blood groups because for some unknown reason every person has an inbuilt mechanism to destroy any blood cells which do not belong to his own, or to a compatible, group.’ Let us now reconsider the Rhesus problem.

When a group B Rhesus negative woman is carrying a group A Rhesus positive baby the shower of the baby’s red cells which enter the maternal circulation at delivery are incompatible with the mother’s blood. They will, therefore, rapidly be destroyed by the mother’s inbuilt mechanism to destroy any group A cells which’ enter her circulation, before they have had time to sensitize or even provoke the tissues of the mother into considering the production of Rhesus antibody. If on the other hand a group B Rhesus negative woman is carrying a Rhesus positive child who is also group B, the foetal cells which pass into the maternal circulation when the child is delivered are compatible with the mother’s major blood group (they are both group B) and will not be destroyed but will continue to circulate until they have sensitized the mother who may later develop Rhesus antibodies.

A Rhesus negative woman can therefore be sensitized by receiving a quantity of Rhesus positive blood of a compatible or similar blood group either by transfer from her baby at the time of delivery or by the inadvertent injection of Rhesus positive blood. If these cells can be destroyed within one or two days (as happens when the blood group is not compatible) the woman is not sensitized. The Rhesus negative group B woman mentioned above who has been delivered of a group B Rhesus positive baby now has in her circulation group B Rhesus positive cells from her baby which, unless they are destroyed, will sensitize her to the production of Rhesus antibody.

If the woman who has compatible Rhesus positive red cells circulating in her system can be given Rhesus antibody shortly after her delivery it will destroy the Rhesus positive cells before they have had an opportunity to sensitize the maternal host and she need never develop any Rhesus antibodies. It is possible to isolate Rhesus antibody from a woman who has previously developed it to a high concentration in her blood. The Rhesus antibody can be extracted and purified and, as it is specific for the Rhesus D factor against which it was originally formed, it is known as anti-D immunoglobulin (see below). Rhesus disease is therefore controllable providing an adequate amount of anti-D immunoglobulin is given to the mother within 36 hours of delivery.

Prevention of Rhesus Disease

Blood Tests

Blood is taken at the beginning of pregnancy to test whether the woman is Rhesus positive or negative. If she is found to be Rhesus negative then a further test will be done to discover if any antibodies are present in her blood. Tests for Rhesus antibody will be repeated at intervals throughout pregnancy in any Rhesus negative person. It is not usual in Great Britain to perform tests on the husband’s blood unless his wife develops antibodies. It is then important to know if the husband is homozygous or heterozygous so that the likelihood of his offspring being Rhesus positive or Rhesus negative can be assessed.

If antibodies are present they arc expressed as a titre, that is the dilution of blood in which antibody can be recognized, and the titre is expressed as 1 :4 or 1:8. As the amount of antibody increases in the blood so the dilutions in which it can be detected increase and if a large amount of antibody is present it may be detected in a dilution of 1:500 or even 1:1,000. Usually the antibody titre rises as the pregnancy progresses, but the outcome of the pregnancy is not directly related to the concentration of Rhesus antibody in the bloodstream. It is for this reason that other tests have been devised to judge more accurately how severely the baby may be affected.

Amniocentesis

The amniotic fluid which surrounds a baby suffering from Rhesus incompatibility contains a substance known as bilirubin which comes from the breakdown of the baby’s destroyed red cells. The amount of bilirubin in the amniotic fluid increases as a greater number of the baby’s red cells are destroyed and is therefore an accurate indication of the condition of the baby. A small amount of fluid is examined in the laboratory for bilirubin, with the result that accurate assessment can be made of the severity of the actual disease within the baby. Amniocentesis may be repeated at two or three weekly intervals. The results obtained are carefully plotted on a chart which, together with the patient’s condition and antibody level, will determine the exact course of treatment to be adopted.

Treatment of Affected Babies

It is seldom that even mildly affected babies are allowed to proceed beyond the expected date of confinement. Moderately affected babies will almost certainly be delivered before the expected date, whereas severely affected babies may require special treatment before birth.

Mildly affected babies will have a blood test performed on a sample of blood taken from the umbilical cord at the time of delivery so that their haemoglobin level can be estimated and the degree of anaemia and jaundice assessed. The tests are repeated at frequent intervals, but it is unusual for the baby to require any specific treatment.

Moderately affected babies also have a blood test performed immediately at birth. The degree of anaemia in these babies is such that they nearly always require an exchange transfusion. The umbilical cord is carefully preserved at the time of delivery and a very fine plastic catheter is threaded along its vein. The end of this catheter passes into the main vessel which returns the blood to the baby’s heart. A small quantity of Rhesus positive blood is withdrawn from the baby and the same amount of especially prepared, fresh Rhesus negative blood is injected into the baby. This process of withdrawal and replacement is repeated taking 20 ml. at a time until approximately 25 or even 30 lots of 20 ml. have been with drawn and each carefully replaced by Rhesus negative blood. By this method approximately 90 per cent of the baby’s own blood is removed and replaced by Rhesus negative blood. It also has the advantage of removing from the baby’s circulation most of the antibody which he inherited from his mother as well as a number of the harmful products causing the jaundice. Frequent blood samples are taken during the next four or five days to check that the newly injected blood cells arc performing satisfactorily and that the level of jaundice is not increasing. Occasionally a second exchange transfusion, and in rare instances a third exchange transfusion, has to be given.

It takes approximately three days for an infant to excrete all the Rhesus antibody that he has acquired from his mother, so that generally speaking after the third day there is no further need for exchange transfusion, unless the level of bilirubin which causes the jaundice builds up within the baby’s circulation to such a high level that his liver is incapable of excreting it. If this happens yet a further exchange transfusion may be necessary to remove it. Careful and repeated tests are performed on the bilirubin level in the baby’s blood for the first six or even ten days after delivery because particularly high levels of bilirubin can cause damage to the baby’s brain. This is the only method by which Rhesus babies can be permanently affected and doctors therefore continuously monitor the bilirubin level and take prompt action before it can become dangerous.

The majority of children who fall into the category of Rhesus incompatibility do survive and are perfectly normal, healthy infants and adults.

Severely affected babies present a most difficult problem. It is nearly always possible to foretell the severity of their affection from the result of amniocentesis. If a severely affected baby is left alone it will die in the uterus. It must therefore either be delivered before it dies or be treated while still in the uterus. It is always possible to deliver a baby prematurely but if it is too premature it will fail to survive. If, on the other hand, it is not delivered prematurely it may die inside the uterus from anaemia. The technique has therefore been devised by which a transfusion can be given to the baby while it is still in the uterus. This is a highly complicated and complex procedure achieved by passing a needle through the abdominal wall of the mother, through the wall of the uterus and into the abdomen of the baby. A small catheter is threaded , through theneedle and a carefully calculated quantity of specially prepared Rhesus negative blood is injected into the baby’s abdomen over a carefully prescribed length of time. The catheter is then withdrawn. The Rhesus negative blood cells are absorbed from the baby’s abdomen into his circulation. Since they are not destroyed by the Rhesus antibody present in his circulation they help the baby to survive. Such transabdominal transfusions may be repeated as frequently as 2-weekly intervals until the pregnancy reaches the 34th week, when delivery may be induced with a reasonable chance that, by exchange transfusion after delivery, the child, will survive and be normal.

The premature delivery of babies affected by Rhesus incompatibility is now very commonplace. There are, of course, hazards in the premature delivery of any baby, but the doctors weigh these very carefully against the hazards of leaving the baby in the uterus with the continuous passage of Rhesus antibody into his circulation. The modern methods that are available calculate scientifically the exact date at which delivery can be safely undertaken. This is not pure guesswork.

Anti-D Immunoglobulin

Anti-D immunoglobulin is purified Rhesus antibody obtained from people who have a high circulating antibody level. When injected into a person it will rapidly destroy Rhesus positive cells. Anti-D immunoglobulin is given to Rhesus negative women by injection after the delivery of a Rhesus positive child to prevent them being sensitized.

Blood is taken from the baby immediately on delivery and from the mother about 30 minutes after delivery. Both blood groups, Rhesus factor and haemoglobin levels are checked. If the two blood groups are not compatible then the mother will immediately destroy the baby’s red cells that have escaped into her circulation. During pregnancy small numbers of red cells from the baby’s circulation escape into the mother’s circulation but at the time of delivery there is usually a large shower of foetal red cells escaping into the maternal circulation. When the blood groups of the mother and baby are not compatible these red cells are rapidly destroyed The Rhesus Factorbefore the Rhesus factor which they contain can sensitize the mother. If the baby’s blood cells are compatible with the mother’s they will remain in her circulation where they can be detected by the Kleihauer test. This indicates that the Rhesus negative mother will become sensitized by the circulating Rhesus positive cells and an injection of anti-D immunoglobulin is given within 36 hours of delivery to prevent sensitization. There are no known toxic or side effects from the injection.

The administration of anti-D immunoglobulin is carefully controlled because its use prevents women becoming sensitized and, therefore, prevents them from producing Rhesus antibodies thus automatically reducing the source of supply. In order to make absolutely certain that sensitization does not occur it is now the usual practice to give anti-D immunoglobulin to all Rhesus negative women shortly after delivery or abortion.

Zygosity

If a man is heterozygous half his offspring will be Rhesus positive and half Rhesus negative. Those children who are Rhesus negative will be unaffected even if they are produced by a woman who has been previously sensitized or who has actually developed antibodies. The very presence, however, of a Rhesus negative baby may provoke a higher concentration of Rhesus antibody in the mother. The reason for this is now known. Tests performed upon the amniotic fluid of the baby will show conclusively if the baby is positive or negative. If the child is Rhesus negative then it can be allowed to go to term and be delivered in the normal way despite the presence of antibodies in the mother’s circulation.

A. B. O. Incompatibility

In very rare circumstances incompatibility occurs between the major blood groups of the mother and the baby. These are usually concerned with the mother being group A and the baby being a different group. A. B. O. maternal foetal incompatibilities are rare and mild, so that the babies are not severely affected and little or no treatment is necessary before delivery. The baby may become jaundiced and slightly anaemic and may require repeated tests during the first seven or ten days of life.

Kell Incompatibility

As well as the major A, B. O. blood groups and the Rhesus factor there are many minor blood groups and factors, one of which is called Kell. People are either Kell positive or Kell negative. A Kell negative woman may be sensitized by a Kell positive child, in a manner similar to that in which a Rhesus negative woman is sensitized, so that she develops Kell antibodies. Kell antibodies, however, are relatively weak and while they may cause some jaundice and a mild degree of anaemia in the baby after birth, they seldom affect the infant severely. The formation of Kell antibodies is extremely rare.