The author establishes that there is no reason to think that the isoimmunization must be limited to the Rh factor, especially in pregnancy cases, because such a condition brings the ideal means for its presence.

A brief historical review is presented, establishing some evidence for the antigenic capacity of the A and B blood factors, such as the experimental production of serum in animals, the increase of agglutinin titre following an incompatible transfusion or intramuscular blood injection and heterospecific pregnancies. Some fetal antigens are analyzed, as well as the possible pathogenic process of sensitization. Some factors of the antigen-antibody conflict in the fetal organism are studied. The pregnancies are also analyzed from the viewpoint of the fetal maternal incompatibility explaining the low frequency of the observed cases, due to nonpermeable placentas, the anergy of the mother, and the fetal characteristic of being secretor or nonsecretor, considering that the clinical form of the fetal alteration depends upon the presence or absence of A and B substances in fetal tissues. The writer gives as a basis to consider a true isoimmunization the presence in the blood stream of erythroblasts with nuclear alterations, and a positive "developing test."

A clinical case of twins in which the sick twin was of incompatible blood type in respect to the mother is presented.

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