Landsteiner and Wiener (1940) discovered one more type of antigen (agglutinogen) in the RBC’s of human beings. It is named after the Rhesus Moneky (Macaca rhesus) and called the Rh factor or Rh antigen. It is present in about 95% of Indians (about 85% of whites also have it). However the serum does not have the corresponding antibody in the human beings.
When a blood of rehsus monkey is injected into the rabbit, the rabbit produces antibodies against it. When this antiserum (serum with antibody) is tested against human blood agglutination takes place in about 85% cases. In about 15% no agglutination takes place. Persons whose blood agglutinates with Rh serum of monkey are called Rh positive and where no agglutination takes place they are called Rh negative.
Six groups of Rh factors have been identified in human beings C, c, D, d, E and e. Out of these D and d are commonest. D is a Mendelian gene hence is dominant over d. Almost all human beings (85%) who are Rh positive belong to D group while Rh negative people belong to d group.
The importance of Rh factors can be ascertained from the following examples.
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A) Assume that a Rh negative individual gets a blood transfusion from a Rh positive individual. At the first instance nothing will happen as there are no natural antibodies in the serum of Rh negative individual. But the presence of Rh blood will induce antibody production. Nothing will happen however as there are no sufficient antibodies. Should the same Rh negative individual require another transfusion from Rh positive blood, the donor blood “will show agglutination due to the antibodies already present. This may be harmful to the recipient.
b) A Rh negative woman marries a Rh positive man. The child of this couple obviously belongs to Rh positive as the gene D is dominant over d. In the first pregnancy however the child is not harmed even though the maternal blood and the child’s blood mix. Once again the mother’s Rh negative blood will not produce enough antibodies to haemolyse the child’s blood.
But with each succeeding conception, the antibodies in the mother’s blood accumulate, sometimes the third or even the second child itself may have its blood (Rh positive) haemolysed leading to erythroblasts from the bone marrow enter the blood (to compensate for the haemolysed RBC) This condition is called Erythroblastosis foetalis. Such a baby is generally still born or even when born highly anaemic. It can be saved with total transfusion of blood.