Rh - factor is discovered by Landsteiner and Wiener in 1940 in the blood of rabbit immunised with the blood of Rhesus monkey (Macaca rhesus). The red blood corpuscles of Rhesus monkey has an antigen which is called Rh factor. This antigen is found in the blood of about 85% human population. The people having this antigen in their blood are known as Rh+, while the people do not have this antigen are called Rh-.There is no natural antibody of Rh antigen is found in human blood but Rh antibodies are produced in the blood plasma of a Rh- person after the exposure to Rh+ antigen. Hence, if such person receives the blood of Rh+ person second time, Rh antibody produced by earlier cause agglutination of the red blood corpuscles leading to the death of recipient. Fisher told that three pseudoallelic genes found close to each other on a chromosome are responsible for the determination of presence or absence of Rh antigen in human blood. The dominant and recessive alleles of the genes are represented as CDE and cde respectively. Therefore, the genotype of a Rh- person would be CDE/cde. If any of the allele of these gene is present in dominant form, the person would be Rh+. Erythroblastosis Foetalis This is a special type of incompatibility found between a pregnant women (Rh -ve) and its foetus (Rh +ve). This is a lethal disorder and the child may die in foetal stage or just after the birth. These children are always Rh positive (Rh+). They have Rh+ father and Rh- mother. They obtain this character from their father only. During the late gestation period, the placenta becomes slightly weak and then some of the RBCs of the foetus reach into - mother�s blood circulation. Therefore, production of Rh antibodies starts in mother�s blood. As the mother is Rh-, the Rh antibodies do not affect the mother adversly. But during development when these antibodies reach into foetal blood or foetal fluid through placenta, they cause agglutination of red blood cells of foetus. This leads to death of the foetus. Generally, the first child is not seriously affected by this disorder because sufficient Rh antibodies are not produced till the birth of child. But the severity of this disorder increases in the fact uses of subsequent pregnancy.