Round Table Discussion

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Abstract
Chairman Zuelzer: Anemia happens to be one of the manifestations of erythroblastosis, but we intend to keep the purely hematologic aspects to a minimum and shall consider the disease as a whole, paying particular attention to its clinical features. The Rh factor which appears in the title of this discussion is likewise only of incidental interest to us. In recent years we have heard too much about Rh and too little about the clinical entity erythroblastosis fetalis which is our topic for today. We have agreed for our purpose to define erythroblastosis as a disorder of fetal and neonatal life resulting from maternal sensitization against antigens of the Rh group present in the fetal cells. We exclude deliberately the still controversial cases of so-called erythroblastosis due to incompatibility in the O-A-B system of blood groups and will touch on this problem only in so far as it has a bearing on diagnosis. Only a few words are needed in regard to the Rh antigens. The emphasis on the subtypes of Rh which are of interest mainly to the geneticist and immunologist have made the subject unpalatable to the clinician and the unnecessarily complex nomenclature has further beclouded a basically simple issue. For the clinician the original Rh antigen with a distribution of 85% in the white population is still by far the most important. In the simple language of the Fisher-Race theory we are chiefly concerned with the genes D and d, familiar to some of you as Rho and rh. The genetic background of maternal isoimmunization can be summarized by the simple statement that sensitization is possible only when a homozypous mother carries a heterozygous fetus. An heterozybous mother (Dd) cannot be sensitied since both antigens are present in her own cells. The homozygous fetus (DD or dd) cannot sensitize its mother because by definition it receives the same type of gene from both parents and must therefore be compatible with the mother. In the usual situation of erythroblastosis the mother is Rh negative (dd), i.e. homozygous, the Rh positive fetus is ipso facto heterozygous (Dd). Rarely a homozygous Rh positive mother may become sensitized to the c-d-e genes of a heterozygous fetus. While the basic relationship of erythroblastosis fetalis to antigenic differences between mother and fetus is well established it seems only fair to say that we know as yet very little about the reasons why certain women become sensitized and others not.

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