Rh system and alloimmunization during pregnancy

Rh system and alloimmunization during pregnancy

  Autor Daniela Chirila - Biochimist Data: 21.07.2005


Rh system and alloimmunization during pregnancy
Human body contains a great number of antigens which condition species, group or individual specificity. Erythrocyte antigens represent one of the most important categories of antigens. They can be classified in three groups, according to specificity: 
  • heterophile antigens, which, besides human, they are also found in other animal species; 
  • species antigens, common to all humans; 
  • group antigens (alloantigens), specific to some group of people. 

Among erythrocyte antigens, the most important are those of ABO, Rh, MN, Kell-Cellano and Lewis systems. 

Rh system 

K. Landsteiner and A. S. Wiener signaled in 1940 the fact that in rabbit serum immunized with monkey erythrocytes Macaccus Rhesus agglutinates the erythrocytes of 85% of humans. Initially, it was discovered the existence of one antigen common to monkeys and humans called factor D, but later it was discovered also the existence of other antigens (C, c, D, E, e), controlled by six organized genes in three pairs of alleles, namely: Cc, Dd, Ee. Among these the strongest is antigen D and it corresponds to the classic Rh factor. In clinic, only the presence or the absence of this antigen is determined and important for transfusions or course evolution of pregnancy. The other types, from uncertain immunologic point of view, mark useful individual differences for genetic investigations or paternity establishing. 

Antigen D determination (Rh blood groups) is useful because in the case of incompatible transfusions in this system can appear transfusion accidents which lead to acute intravascular hemolysis. 

Alloimmunization in pregnancy 

Alloimmunization in pregnancy is equivalent with alloimmunization in Rh system and it is due to some fetus-mother incompatibility in Rh blood group. 
If a woman with Rh D- is fecundated by a man with Rh D+ in her serum will appear antibodies anti-D which can cause hemolysis and serious disorders to RhD+ fetus, phenomenon which can lead even to his death. In the case of Rh system, body doesn't develop natural antibodies, so the immune sensitivity takes place after repeated exposures to antigens of certain system. That's why, first pregnancy usually evolutes without problems. 

As we already said above, human body doesn't elaborates normally antibodies anti-D; but when they exist, they have a pronounced cytotoxic character as they action over a single targe cell (Rh+ erythrocytes). 
Pregnancy immunization is determined by the entrance of some small quantity of fetal Rh+ erythrocytes in maternal blood flow during labor. Appeared antibodies persist long time, and if their concentration is great, at the next pregnancy they pass through placenta into fetal blood flow where they will destroy fetal erythrocytes. In consequence in fetal body is installed a severe anemia, known as hemolytic disease of the new born baby. In such situations, it can be administrated prophylactic antibodies anti RhD which will dissolve RhD+ cells avoiding mother's sensitivity. 



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