60 INTRODUCTION TO IMMUNOCHEMICAL SPECIFICITY 



Fetal antigen-containing erythrocytes, or perhaps in some cases 

 merely antigen from disintegrating fetal erythrocytes, passes the 

 placental barrier or crosses minute breaks in the placental capillaries 

 and starts an immune response in the mother. This results in the ap- 

 pearance of anti-Rh antibodies in the maternal circulation. These 

 diffuse through the placental barrier back into the fetal circulation 

 (Fig. 4-2) where they combine with the fetal erythrocytes causing 

 red cell destruction and, in severe cases, anemia, jaudice, edema, 

 and death. 



It should be mentioned that erythroblastosis fetalis, although seri- 

 ous when it occurs, is rare. In the United States it occurs in about 

 one out of 400 births and rarely affects the first pregnancy, even 

 when Rh incompatibility exists. 



The Rh blood groups are much more complicated than would the 

 case be if there were simply two possibilities : Rh-positive (the 

 erythrocytes contain the Rh antigen) or Rh-negative individuals 

 (the erythrocytes do not contain the Rh antigen). Our knowledge 

 of the Rh blood groups, which extends now to five basic Rh antigens, 

 is the result of the work of many able and industrious researchers. 

 Only a simplified outline of it can be given here. 



Although it does not have priority, the notation devised for the 

 Rh antigens by Fisher and Race (see Race, 1944) has proven clearer 

 and more convenient than Wiener's notations. The five basic Rh 

 antigens are, according to Race, designated as C, c, E, e, and D. 

 Genetically C and c are alleles, and so are E and e. It is not yet cer- 

 tain whether this means that three closely linked loci are involved in 

 their mechanism of inheritance, as the British workers believed, or 

 a series of alleles at one locus, as Wiener continues to maintain. In 

 any case, the allelic behavior of C and c and E and e is an important 

 phenomenon for their application in legal medicine. The allele cor- 

 responding to D is d, but no d antigen has been demonstrated with 

 certainty. 



The number of Rh blood groups that can l^e distinguished de- 

 pends on the number of anti-Rh agglutinins available. Anti-e is seldom 

 available and anti-c is not always easy to get for routine determina- 

 tions. If all five agglutinins are at hand, three different blood groups 

 can be distinguished with respect to the C locus, namely C-positive 

 c-negative, C-negative c-positive, and C-positive c-positive. (Since 



