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Rh Blood Type & Hemolytic Disease of the Newborn

About 85% of humans have erythrocytes that express the Rh(D) antigen on their surface.This disease results from the passage of maternal IgG anti-Rh(D) antibod-ies through the placenta to the fetus, with subsequent lysis of the fetal erythrocytes.When an Rh-negative woman has an Rh-positive fetus (the D gene being inherited from the father), the Rh(D) antigen on the fetal red blood cells will sensitize the mother's adaptive immune response, leading to development of anti-Rh(D) IgG antibodies (Table 64-4).This sensitization occurs most often during delivery of the first Rh(D)-positive child, when Rh(D) erythrocytes of the fetus leak into the maternal circulation (Figure 64-14).

Rh Blood Type & Hemolytic Disease of the Newborn

About 85% of humans have erythrocytes that express the Rh(D) antigen on their surface. They are said to be Rh-positive. The remaining 15% are Rh-negative, that is, they lack the gene for the Rh(D) protein.

The Rh status of parents is clinically important because a specific combination can result in hemolytic disease of the newborn (erythroblastosis fetalis). When an Rh-negative woman has an Rh-positive fetus (the D gene being inherited from the father), the Rh(D) antigen on the fetal red blood cells will sensitize the mother's adaptive immune response, leading to development of anti-Rh(D) IgG antibodies (Table 64-4).This sensitization occurs most often during delivery of the first Rh(D)-positive child, when Rh(D) erythrocytes of the fetus leak into the maternal circulation (Figure 64-14).

If the mother does form anti-Rh(D) antibodies in this way, subsequent Rh(D) pregnancies are at risk of hemolytic disease of the newborn (erythroblastosis fetalis). This disease results from the passage of maternal IgG anti-Rh(D) antibod-ies through the placenta to the fetus, with subsequent lysis of the fetal erythrocytes. The direct antiglobulin (Coombs) test is typically positive (see earlier description of the Coombs test).

The problem can be prevented if the mother's adaptive immune system is not allowed to be sensitized to red cells car-rying Rh(D) antigens. This is achieved by administration of high-titer Rh(D) immune globulins (Rho-Gam) to an Rh(D) mother at 28 weeks of gestation and immediately upon the delivery of any Rh(D) child. These antibodies promptly attach to Rh(D) erythrocytes and prevent their acting as sensitizing antigen. This prophylaxis is widely practiced and effective.