Factors Influencing the Immunization of Rh - negative Mothers

According to Nevanlinna and Vainio (1962) the probability that an Rh-negative mother who has had two Rh-positive children will be immunized is 4-5%. In the third and later births the probability of immunization decreases rapidly. The reason why most mothers do not become immunized is intriguing. It may depend on individual differences in immunological reactivity, and perhaps the response of some mothers to the antigenic stimulus of the foetus is exceptionally strong. However, this factor is unlikely to be the main one, as a transfusion sensitizes the majority of mothers (Nevanlinna, 1953). When small amounts, 0-20-6 ml. of foetal red cells were injected (group 0, Rh-positive), 4 out of 15 women became immunized (Zipursky, Pollock, Chown, and Israels, 1965). Levine (1943) observed that ABO compatibility* between the mother and her husband favours, and incompatibility inhibits, the development of maternal Rh-sensitization. His observation has been confirmed by many authors. The probability of sensitization is even lower if mother and foetus are incompatible than if mother and husband are incompatible (Knox, Murray, and Walker, 1960; Murray, Knox, and Walker, 1965). In complete agreement with these findings, ABO compatible Rh-positive red cells injected into Rh-negative volunteers immunized the volunteers much more readily than ABO incompatible ones (Stem, Davidsohn, and Masaitis, 1956). Most of the studies on the subject ofmaternal Rhsensitization stress the association between ABO incompatibility and lack of Rh-sensitization. The recent discovery that Rh-sensitization could be prevented by injections of anti-D y-globulin was based on an analogy with the reduction in sensitization associated with ABO incompatibility (Clarke, 1966).