PEDIATRICS Vol. 7 No. 1 January 1951, pp. 118-130
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Round Table Discussion

IMMUNOLOGY IN THE PRACTICE OF PEDIATRICS

JOHN J. MILLER JR. M.D., CHARLES E. SMITH M.D., PAUL HATFERSLEY M.D., HENRY B. BRUYN M.D., ROSEMARIE J. TURSKY M.D., and GILBERT J. LEVY M.D.

1. CLINICAL SIGNIFICANCE OF Rh SENSITIZATION

Dr. Paul Hattersley: I shall begin with an obvious oversimplification. I shall speak of the Rh factor. We all are aware that there are many Rh factors. However, it has been established that at least 99% of sensitizations within the Rh group of antigens are by the Rho or D antigen. It is therefore almost entirely among individuals negative for this factor that sensitization occurs. On this basis it is clinically correct to classify individuals as Rh-positive or Rh-negative according to the presence or absence of this one important factor.

The Rh factor is a hereditary characteristic, inherited as a simple Mendelian dominant. Being antigenic, it is capable of stimulating the formation of antibodies upon reaching the body of an Rh-negative individual. There are 2 ways in which Rh-positive cells, containing the Rh antigen, can reach the circulation of an Rh-negative person. The first and most obvious is by intravenous or intramuscular transfusion. The second is by pregnancy. Small amounts of the blood of the fetus cross the placental barrier into the circulation of the mother. If the fetus is Rh-positive, and the mother is Rh-negative, sensitization of the mother to the Rh factor may occur.

There are 3 variable factors which, in different degree, determine whether an Rh-negative individual will become sensitized to the Rh factor. They are the total dose of Rh antigen received, the duration of time in which this dose is received, and the individual's antibody producing capacity. It is not surprising, therefore, that while the over-all percentage of sensitization by pregnancy alone is around 4%, the incidence of sensitization by transfusion approaches 50%.