What is Rh incompatability?

 

One of the most amazing things about pregnancy is that we are able to grow an entire new human being along with an extra organ all without our immune system rejecting it.  This is partly due to a complex immunologic response and partly due to the fact that the blood of the fetus and the blood of the mother never mix during pregnancy.
            Most people are familiar with the ABO blood type system.  In fact, most people know their blood type.  What they don’t know much about is the positive or negative that comes after the blood type letter.  We are all either Rh+ or Rh-.   “Rh blood types were discovered in 1940 by Karl Landsteiner and Alexander Wiener.  This was 40 years after Landsteiner had discovered the ABO blood groups.  Over the last half century, we have learned far more about the processes responsible for Rh types.  This blood group may be the most complex genetically of all blood type systems since it involves 45 different antigens on the surface of red cells that are controlled by 2 closely linked genes on chromosome 1.
           The Rh system was named after rhesus monkeys, since they were initially used in the research to make the antiserum for typing blood samples.  If the antiserum agglutinates your red cells, you are Rh+.  If it doesn't, you are Rh-  Despite its actual genetic complexity, the inheritance of this trait usually can be predicted by a simple conceptual model in which there are two alleles, D and d.  Individuals who are homozygous dominant (DD) or heterozygous (Dd) are Rh+.  Those who are homo-zygous recessive (dd) are Rh- (i.e., they do not have the key Rh antigens).  Clinically, the Rh factor, like ABO factors, can lead to serious medical complications. The greatest problem with the Rh group is not so much incompatibilities following transfusions (though they can occur) as those between a mother and her developing fetus.  Mother-fetus incompatibility occurs when the mother is Rh- (dd) and the father is Rh+ (DD or Dd).  Maternal antibodies can cross the placenta and destroy fetal red blood cells.  The risk increases with each pregnancy.  Rh type mother-fetus incompatibility occurs only when an Rh+ man fathers a child with an Rh- mother.  Since an Rh+ father can have either a DD or Dd genotype, there are 2 mating combinations possible:1
 
drawings of two Punnett squares showing the possible Rh positive mates of an Rh negative woman and the probability of their children being Rh positive--100% if the father is homozygous dominant and 50% if he is heterozygous for this trait
 
                                                  
                                                                                                        Graphics courtesy of http://www.umm.edu/pregnancy/specialcare/articles/rh.html
 
The fact that maternal and fetal blood mix during delivery is well documented. It is estimated that “the smallest amount of fetal blood causing immunization is about 0.1 mL, but in some women the amount transferred is far greater”.2  This is especially significant in a first pregnancy.  It is generally accepted that the first pregnancy is a ‘sensitizing’ exposure and does not affect the baby.3  However, because even as little as 0.1mL of fetal blood will provoke an IgM response, even a first time mother should be treated for rhesus incompatibility. 
“Rh- women should be assessed as to their Rh antibody status at the onset of care, at 28 to 30 weeks and at 36 to 38 weeks.  Should an antibody titer develop, further tests should be done on a weekly basis.  A titer of 1:16 or below which develops late in pregnancy is considered to be clinically insignificant with respect to fetal risk.”4  If a woman has not been sensitized (that is she does not have antibodies to the Rh D factor) treatment consists of an injection of Rh-immune globulin, known as Rho-GAM, at about 28 weeks gestation.  The mother will be given another injection within 72 hours of the birth. 
Since blood mixing can also happen during a miscarriage, amniocentesis, ectopic pregnancy, chorionic villus sampling, or abortion, it is important for a woman to be treated with a shot of Rho-GAM after these procedures as well.  Rho-GAM provides antibodies to the Rh D factor which destroys any fetal red blood cells that enter the maternal system.  This action prevents the woman from manufacturing her own antibodies to the Rh D factor and protects the next pregnancy. 
            Effects on the fetus range from mild to severe and always include some level of jaundice due to hemolysis.  A small separation of the placenta from the uterine wall during labor is normal, so some amount of fetal blood almost always enters the mother’s system thus initiating the development of Rh+ antibodies.  As previously stated, it has been reported that as little as 0.1mL of fetal blood will stimulate a full blown antibody response from the mother.  The danger lies with the next Rh+ fetus. 
 
Rh incompatibility can cause symptoms ranging from very mild to fatal. In its mildest form, Rh incompatibility causes hemolysis (destruction of the red blood cells) with the release of free hemoglobin into the infant's circulation. Hemoglobin is converted into bilirubin, which causes an infant to become yellow (jaundiced). The jaundice of Rh incompatibility, measured by the level of bilirubin in the infant's bloodstream, may range from mild to dangerously high levels of bilirubin.  Hydrops fetalis is a complication of a severe form of Rh incompatibility in which massive fetal red blood cell destruction (a result of the Rh incompatibility) causes a severe anemia resulting in fetal heart failure, total body swelling, respiratory distress (if the infant has been delivered), and circulatory collapse. Hydrops fetalis often results in death of the infant shortly before or after delivery.Kernicterus is a neurological syndrome caused by deposition of bilirubin into the brain (CNS) tissues. Kernicterus develops in extremely jaundiced infants, especially those with severe Rh incompatibility.  It occurs several days after delivery and is characterized initially by loss of the Moro (startle) reflex, poor feeding, and decreased activity. Later, a high-pitched shrill cry may develop along with unusual posturing, a bulging fontanel, and seizures. Infants may die suddenly of kernicterus.  If they survive, they will usually later develop decreased muscle tone, movement disorders, high-pitched hearing loss, seizures, and decreased mental ability.5 
 
While there are a multitude of effects on the fetus, the woman remains unaffected.  Her body produces the necessary antibodies to rid her system of the unwanted cells without any ill effects. 
 
  
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1. http://anthro.palomar.edu/blood/Rh_system.htm
2. Beischer, N.A., Colditz, P.B., Mackay, E.V.: Obstetrics and the Newborn Third Edition W.B. Saunders Company, Ltd., Philadelphia, PA  1997. p226
3. ibid p227
4. Frye, Anne: Holistic Midwifery Volume I Labrys Press, Portland, Oregon, 1998. p914
5. http://www.nlm.nih.gov/medlineplus/ency/article/001600.htm