Rh incompatibility

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(medicine) A condition in which red blood cells of the fetus become coated with immunoglobulin G antibody (Rh antibody) of maternal origin which is directed against Rh-D antigen of paternal origin that is present on fetal cells.

A condition in which red blood cells of the fetus become coated with an immunoglobulin (IgG) antibody [Rh (rhesus) antibody] of maternal origin which is directed against an antigen (Rh D antigen) of paternal origin that is present on fetal cells. See also Antibody; Antigen; Blood groups; Immunoglobulin.

A pregnant woman may develop an antibody to a red blood cell antigen that the fetus has but that she does not possess. This occurs because the fetus has inherited the antigen from the father; fetal red cells pass into the maternal circulation and are recognized as foreign by the mother. The mother develops antibodies to the foreign protein; the antibodies pass across the placenta and attack the fetal red blood cells, producing a hemolytic anemia. This phenomenon is known as alloimmunization. Alloimmunization of the mother results in hemolytic disease of the newborn (also known as erythroblastosis fetalis). See also Anemia; Autoimmunity.

The development of Rh incompatibility is determined by the Rh types of the parents. Some Rh-incompatible pregnancies do not result in sensitization. Factors influencing the occurrence of sensitization include the timing and extent of the transplacental hemorrhage, the degree and strength of antibody development in the mother, and the ABO status of mother and fetus. Most immunizations result from transplacental hemorrhage during placental separation at delivery.

The at-risk mother is monitored for the development and progression of Rh incompatibility with serial serum antibody titers to anti-D. The presence of anti-D and its titer do not necessarily correlate with the status of the fetus. To monitor the status, amniocentesis is performed. This procedure permits measurement of bile pigment concentration and assessment of lung maturity. These tests are done serially to decide when to deliver the fetus or whether to transfuse the fetus until it is mature enough to be delivered safely. See also Prenatal diagnosis.

Following delivery, the infant may require transfusion immediately or during the postnatal period. This may involve simple replacement or exchange transfusion, in which the infant's blood volume is replaced with compatible blood. The aim of exchange is to remove bilirubin and antibody-coated red blood cells and to correct the anemia while replacing the infant's blood with blood that is antigen negative for the offending antibody. Other medical treatment for jaundice may also be required. See also Transfusion.

Rh immune globulin (RhIG) is a concentrated solution of IgG anti-D obtained from pooled human plasma. Its action is to interfere with antigen recognition. RhIg should be administered to any Rh-negative mother of an Rh-positive infant within 72 h after delivery. Administration of RhIG at the twenty-eighth week of pregnancy followed by a second dose at birth has been very successful in all but eliminating Rh hemolytic disease. See also Pregnancy.

A condition in which two groups of blood cells are antigentically different because of the presence of Rh factor in one group and the absence of the Rh factor in the other. See also Rh factor.